Security Alarm

Security Alarm 

Thwart any attempt of burglary in your house using this alarm circuit. When someone opens the door of your room, it sounds an alarm intermittently and flashes light as well. The circuit can also be used as an audio/visual alarm in case of fire or other emergency by momentarily pressing switch S3.

The circuit (refer Fig.1) is built around transformer X1, a standard bar magnet, reed switch S2, timer IC NE555 (IC1), opto-coupler IC MOC3020 (IC2), TRIAC BT136 and a few discrete components. Timer IC1 is wired as an astable multi-vibrator whose reset pin 4 is controlled by the reed switch. The reed switch fitted in the door frame acts as the sensor. A magnet is fixed on the door panel close to the reed switch.

Fig.1: Security alarm circuit

The reed switch consists of a pair of contacts on ferrous metal sealed in a glass envelope. The contacts may be normally-open (which close when a magnetic field is present) or normally closed type (which open when a magnetic field is applied). A normally open- type reed switch is used here.

When the door is closed, reed switch S2 is in open state. When the door is opened, the bar magnet moves away from reed switch S2. As a result, reset pin 4 of IC NE555 goes high. The high output at pin 3 of IC1 enables IC2. Pin 4 of IC2 is connected to the gate of TRIAC1.

When the door is opened, bulb B1 flashes and the bell sounds (provided switch S4 is closed) indicating that the door has been opened. Flashing of the bulb and the alarm continue until the door is closed.

Assemble the circuit on a general purpose PCB and enclose in a suitable cabinet. Connect the call bell at the back side and the bulb at the front side of the cabinet. Install the unit on the door of the room as shown in Fig.2.

Fig.2: Reed switch fitting in door

The circuit is powered by mains supply.

 

1-30 Minute Timer

1-30 Minute Timer 

 Using this circuit you can switch on an appliance for a desired time. The circuit provides selectable time settings of 1, 2, 5, 10, 15 and 30 minutes, and can be used for domestic as well as industrial applications.

The circuit can be divided into two sections—power supply and timer. The power supply is built around transformer X1, bridge rectifier BR1, capacitor C1 and 12V voltage regulator IC LM7812 (IC1). The 230V AC mains supply is stepped down by transformer X1 to deliver the secondary output of 12V, 250 mA. The transformer output is rectified by full-wave bridge rectifier BR1, filtered by capacitor C1 and regulated by IC1. The circuit can also be powered by a 12V battery. The power source can be selected by using switch S2.
The timer section is built around IC NE555 (IC2) along with resistors R1 through R6, capacitor C2, transistor BC548 (T1) and a 12V relay. IC2 is configured in monostable mode to provide the different time settings ranging from 1 to 30 minutes. The desired time is selected by rotary switch S1 as shown in the table. Pressing switch S3 starts the operation. Once triggered, the timer returns to its original state after the preset time.
Working of the circuit is simple. Capacitor C2 charges through a resistor or combination of resistors R1 through R6. When start switch S3 is pressed, the monostable triggers and its output pin 3 goes high for the time selected by the position of rotary switch S1. The output time (T) of the monostable in seconds is T =1.1RC.

Assemble the circuit on a general-purpose PCB and enclose in a suitable case. Fix the unit near the appliance that has to be controlled. Use a 12V relay (RL1) with contact current rating suited for the the appliance.

DC Motor Control Using PWM

DC Motor Control Using PWM 

Small DC motors are efficiently controlled using pulse-width modulation (PWM) method. The circuit described here is built around an LM324 low-power quad-operational amplifier. Of the four op-amps (operational amplifiers) available in this IC, two are used for triangular wave generator and one for comparator.

Op-amp N2 generates a 1.6kHz square wave, while op-amp N1 is configured as an integrator. The square wave output of N2 at its pin 14 is fed to the inverting input (pin 2) of N1 through resistor R1. As N1 is configured as an integrator, it outputs a triangular wave of the same frequency as the square wave. The triangular wave is fed to pin 5 of op-amp N3, which is configured as a comparator.
The reference voltage at pin 6 of the comparator is fixed through the potential divider arrangement formed by potmeter VR1 and resistors R4 and R5. It can be set from –6V (lowermost position of VR1) to +6V (uppermost position of VR1).

The triangular wave applied at pin 5 of N3 is compared with the reference voltage at its pin 6. The output at pin 7 is about +12V when the voltage at pin 5 is greater than the voltage at pin 6. Similarly, the output at pin 7 is about -12V when the voltage at pin 5 is lower than the voltage at pin 6.

The output from comparator N3 is the gate voltage for n-channel MOSFET (T1). T1 switches on when the gate voltage is positive and switches off when the gate voltage is negative. Setting of the reference voltage therefore controls the pulse-width of the motor.

When T1 is switched on for a longer period, the pulse width will be wider, which means more average DC component and faster speed of the motor. Speed will be low when the pulse width is small. Thus potmeter VR1 controls the speed of the motor.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. The circuit requires ±12V power supply for its working. It can also be modified to control the speed of a 6V or 24V DC motor. 

Automatic Fan Controller For Air-conditioners

Automatic Fan Controller For Air-conditioners

Many central air-conditioning systems (ACs) have the option for cooling and heating both. These have a fan for blowing hot or cold air drawn from a central unit, often with no automatic speed control for the fan. The speed of the AC fan has to be manually controlled to maintain a comfortable temperature throughout the room.
Manually adjusting the AC temperature is not very convenient. So we present here an automatic control system for the AC fan which could also help control the room temperature to some extent. It allows the air-conditioning system to automatically blow warm air in winters and cool air in summers without requiring manual intervention.

Circuit description Fig. 1 shows the circuit of the automatic fan controller for the AC. It comprises regulator IC 7805 (IC1), bar-graph driver IC LM3914 (IC2), comparator IC LM339 (IC3), temperature sensor IC LM335 (IC4) and some discrete components. Pin configurations of 7805, BC337 and LM335 are shown in Fig. 2.

Fig. 1: Circuit of the automatic fan controller for ACs

Fig. 2: Pin configurations of 7805, BC337 and LM335

Power for the circuit is derived from mains supply. The 230V, 50Hz AC mains is stepped down by transformer X1 to deliver a secondary output of 12V, 500 mA. The transformer output is rectified by a full-wave rectifier comprising diodes D1 through D4, filtered by capacitor C6 and regulated by IC 7805. Capacitor C7 bypasses ripples in the regulated supply.
IC LM3914 is configured for bar-graph mode by connecting its pin 9 to 5V supply. It functions both as the temperature and set-point indicator, depending on whether switch S1 is in RUN or SET position, respectively. A highly stable internal reference voltage is generated at pin 7 using resistor combination R1-R2 and presets VR1 and VR2. This voltage is directly fed to the upper end of the internal voltage divider chain at pin 6. Pin 4, the lower end of the internal voltage divider chain, is connected to the wiper of preset VR1. The voltage difference between pins 4 and 6 determines the range of temperature control.
Potentiometer VR3 connected between pins 4 and 6 of IC2 provides temperature setting depending on the position of switch S1. The comparator inside IC2 compares the voltage at pin 5 with the voltage difference across pins 4 and 6, and incrementally turns on LED1 through LED10 at every tenth of the temperature range. Current driven through the LEDs is regulated and programmable, thus eliminating the need for resistors.
The temperature control function is performed by comparator IC LM339. This IC uses only three of the four independent precision comparators operating off a single power supply. Comparator A1 is wired as a non-inverting comparator with hysteresis. R6 is used as a pull-up resistor for comparator A1, while resistors R7 and R8 provide a hysteresis voltage. The inverting input of A1 at pin 4 is connected to the wiper of potentiometer VR3.
The output of comparator A1 goes high when the voltage at its non-inverting input is greater than the voltage at the inverting input. Comparators A2 and A3 act as inverting and non-inverting buffers, respectively. Resistors R4 and R5 form a voltage divider which provides reference voltage at pins 7 and 8 for comparators A2 and A3, respectively.
Mode switch S2 is used to select the output of A2 (pin 1) or A3 (pin 14). Pole of switch S2 is connected to the base of transistor T1. The base of transistor T1 is driven into saturation via resistor R9, which is connected to unregulated 12V supply. Relay RL1 is connected to the collector of T1. Therefore T1 acts as a switch for relay RL1. Diode D5 across the coil of relay acts as a free-wheeling diode. The motor of the AC fan is connected to the circuit through relay contacts. Thus relay switches the fan on or off.
Temperature sensor IC LM335 acts as a zener diode. Its breakdown voltage is directly proportional to the absolute temperature at 10 mV/ºK. Resistor R3 limits the current through IC4. Capacitor C4 bypasses any external noise.

Fig. 3: An actual-size, single-side PCB for the automatic fan ACs

Fig. 4: Component layout for the PCB

Construction
An actual-size, single-side PCB for the automatic fan controller for ACs is shown in Fig. 3 and its component layout in Fig. 4. Assembling the circuit on a PCB minimises time and assembly errors.
Use bases for ICs LM3914 and IC LM339. Enclose the assembled circuit in a suitable cabinet. On the PCB, provide suitable connectors for switches S1 and S2 and potentiometer VR3 to extend these out from the cabinet through cable.
The sensor is brought out from the cabinet with a two-core cable. LED1 through LED10, switches S1 and S2, and potentiometer VR3 are mounted on the front panel of the cabinet. LED1 through LED10 are marked with calibrated temperature values.
Calibration. Calibrate the automatic temperature controller before putting it into the AC’s circuit. Calibration is done for temperature control between 20ºC and 29ºC and temperature indication by LED1 through LED10.
First, with the help of a thermometer and digital voltmeter, set the temperature range to be indicated and controlled. Make sure that temperature sensor IC4 is free-standing in air, away from any source of heat, such as soldering iron or a heat-emitting lamp in the room.
Apply AC power to the circuit after finishing the construction. Do not connect motor to the unit yet. Using the thermometer, take the room temperature reading. Adjust preset VR2 such that the LEDs indicate a corresponding temperature. For instance, if the temperature is 24ºC, LED1 through LED5 should glow, while LED6 through LED10 remain off.
Next, connect the digital voltmeter across potentiometer VR3 and adjust preset VR1 such that the voltage reading is exactly 0.111V. This sets the temperature range by potentiometer VR3 between 20ºC and 29ºC.

 

Solid State Voltage Stabiliser

Solid State Voltage Stabiliser

In India, we have a large power distribution system with heavy distribution losses and variations in industrial/ domestic load. This results in voltage variations that may damage electrical/ electronic appliances like light, fan, television, mixer-grinder, air-conditioner, heater, water pump, toaster, etc.

Here, we describe how to make a solid-state voltage stabiliser that does not use electromechanical relays and is suitable for most purposes. Key features of the solid-state voltage stabiliser are:

1. Wide range of voltage variation from 120 V to 280 V
2. Only two settings are required low voltage and high voltage
3. Stabilised output of 220V4. Compact size
5. Silent operation and no relay chattering sound
6. Bar graph LED voltage indicator
7. Low/high voltage indicator and cut-off protection

The block diagram of a solid-state voltage stabiliser is shown in Fig.1.

Fig.1:Block diagram of solid-state voltage stabiliser

The circuit diagram comprises following four sections:

1. Analogue voltage to digital step changer
2. Isolated solid-state power relay
3. Control power supply unit
4. Mains transformer

Fig.2:Circuit diagram of voltage stabiliser

Analogue voltage to digital step changer. The circuit diagram of a solidstate voltage stabiliser is shown in Fig.2. The heart of the stabiliser is IC1 ( LM3914) bar display driver. It is used as LED-type bar graph voltmeter with lower voltage and upper voltage settings through presets VR1 and VR2. IC1 senses mains voltage. The difference between the lower voltage and upper voltage is divided into 10 steps. every LED indicates one step or one voltage level and is lit depending on the level of voltage received.

All the 10 outputs of IC1 that are used to lit the LEDs are also fed as inputs to dual decoder/demultiplexer CD4556. CD4556 is used for converting analogue voltage to digital steps to ensure that, at a given time, only one tapping of mains transformer gets input supply voltage from mains. In all conditions only one step can be active as per analogue input voltage.

Assume the first condition when the mains voltage is less than the lower set value. All the output pins (1, 18, 17, 16, 15, 14, 13, 12, 11, 10) of IC1 will be high. IC3(A) will be disabled and no step will be selected (means low volt 16, 15, 14, 13, 12, 11, 10) of IC1 will be high. IC3(A) will be disabled and no step will be selected (means low voltage cut-off).

As the mains voltage increases to more than the lower set value, LED1 of the bar graph voltmeter glows as pin1 of IC1 is low and all other outputs pins are high. In this condition IC2(A) is enabled because input E (pin 1) is low. As inputs A0 and A1 of IC2(A) are high, out put Q3 goes low. This is step 1 of step charger.

When voltage increases, input A0 of IC2(A) goes low and its output Q2 also goes low. This is Step 2 of step changer.

Both these outputs are combined with 1N4148 diodes and given to cathode pin of internal LED of IC7 (MOC3011). As internal LED of IC7 glows, TRIAC1 conducts and provides AC mains to tapping ‘A’ of mains transformer X2.

When voltage increases further, both inputs A0 and A1 of IC2(A) go low, while both of its outputs go high , and TRIAC1 goes off. Input A1 and output Q2 of IC2(A) generate enable input E for IC2(B) with the help of set and reset input pins (S and R) of flip-flop IC5(A) (CD4013). Pin 1 of IC5(A) provides low signal to enable input E of IC2(B) and output Q3 of IC2(B) goes low. This is Step 3 of step changer. Similarly, other conditions work in the same manner (see Table).

The number of tappings for transformer X2 and the number of solid-stat relays to be used depend on the voltage range to be covered. If the minimum voltage can drop to 100 volts and the maximum could rise to 300 volts, we need to cover 200 volts deviation. This can be managed either through ten tappings with 20V difference or just five tappings with 40V difference between each.

Isolated solid state power relay. Isolated solid-state power relay comprises an opto-isolatortriac driver MOC3011, bridge rectifier (5A) and triac BT136. The opto-isolator triac driver MOC3011 is used for controlling the steps and connecting AC mains power supply to correct tapping of mains transformer X2 via solid-state relay. The capacity of solid-state relay depends on both the components traic and bridge rectifier. Here triac BT136 and 5A bridge rectifier are used for 1kW load. Triac BT139 with 10A bridge rectifier can be used for a solid state relay of more than 1 kVA and less than 3 kVA. You can use up to 3 kVA solid-state voltage stabiliser with 3 kVA transformer.

Control power supply. Circuit diagram of control supply circuit is shown in Fig. 3. The 230V, 50Hz AC mains is stepped down by transformer X1 to deliver a secondary output of 24V, 500 mA. The transformer output is rectified by full-wave rectifier BR6, filtered by capacitor C9 and regulated by IC 7812 (IC12), which provides a 12V DC output. C10 and C11 provide further filtering. LED1 acts as the power indicator. Resistor R23 acts as a current limiter.

When capacitors used in the output are more than 10 μF, it is necessary to protect the regulator IC using diode (in this case, D11), in case their input is short to ground. Unregulated DC supply voltage is used for input sensing by IC1 for controlling the steps of mains transformer through solid-state relays.

Fig.3: Power Supply

Mains transformer. The mains transformer used here is an auto-transformer with tappings of 120V, 152V, 184V, 216V, 248V and 281V, respectively (as shown in Fig. 2). All the tappings are connected with the voltage control solid-state relays to provide respective voltages. The tap at 216 volts is connected directly to the output.

Construction

An actual-size, single-side PCB for the solid-state voltage stabiliser is shown in  and its component layout in . The total circuit of solid-state voltage stabiliser can be assembled on a PCB. All the BT136 triacs need to be fixed on suitable heat-sinks with mica and insulated nut-bolts to isolate them. To begin with, the setting of solid-state voltage stabiliser is done without connecting mains transformer as described below.

1. Use a variable voltage transformer having 0 to 300 volts range and a digital voltmeter (3½-digit) for measurement of mains power supply.

2. Connect this solid-state voltage stabiliser with variable transformer starting from zero volts setting. Increase voltage slowly from 0 to 120 volts

3. Set variable voltage transformer output of 120 volts with the help of digital voltmeter

4. Set low voltage setting preset VR2 of IC-LM3914 so that only LED1 glows

5. Now set the transformer at 281 volts with the help of digital voltmeter

6. Set high voltage setting preset VR1 of IC1 so that all LEDs from LED1 through LED10 can glow

7. Set the transformer at 184 volts and check the LEDs

8. Set the transformer at 248 volts and check the LEDs

9. Move variable transformer from 120 to upside and check solid-state relay output one by one with the help of a test lamp of 220V, 40W. Also see table of CD4556.

10. Connect mains transformer tappings to solid-state relays with great care at proper tappings.

11. Now connect digital voltmeter to output socket and check the voltage with variable transformer from 120V ~ 281. the output should remain 220V

12. Connect test lamp as a load and check voltage variation. Now the solid-state voltage stabiliser is ready for use with a load of 1KVA.

note:-Male and female pin type power connectors may be used to connect mains transformer tapping with solid-state relays on PCB. Bar graph LED voltmeter connection also  provides male and female pin connection from PCB to front panel of the stabiliser.

 

Metal Detector Using Difference Resonator

Metal Detector Using Difference Resonator

Described here is a simple circuit that can detect metallic conductors in its vicinity up to a range of 25 to 30 millimetres. Concealed metallic objects such as metal foils enclosed in a plastic cover, e.g., toothpaste tubes, and small objects like refill tips made of magnetic materials too can be detected using this circuit. However, very thin metallic foils may go undetected due to a large resistance.

The circuit is based on the principle of a difference resonator and consists of inverters, detector coils, capacitors and transistors as shown in Fig. 1.

Fig. 1: Metal detector circuit

You can design a longer-range model on similar principles by using higher power and larger dimensions of detector coils.

The working of the circuit is based on detecting the magnetic field produced by Eddy currents generated in a conductor when it is placed in a varying magnetic field. The detector circuit is formed by coils L1, L2 and L3. Coils L1 and L2, each having 200 turns of 44SWG (0.08mm diametre) enameled copper wire, are wound on a gel-pen refill. Two small ferrite rods are inserted into the gel-pen refill and fixed at both ends using glue as shown in Fig. 2.

Fig. 2: Detector coil assembly

Fix the refill on a base (support), such as a small general-purpose PCB, using glue (refer Figs 2 and 3). Fix the gel-pen refill PCB on one end of a 50gm solder wire bobbin such that the refill is in the centre of the bobbin. Coil L3, having 200 turns of 25SWG (0.5mm diametre) enameled copper wire, is wound on the solder wire bobbin. Varying magnetic field produced in coil L3 induces current in coils L1 and L2. Coils L1 and L2 in series form a difference resonator along with capacitor C1. Coil L3 itself is made to resonate by driving it with a square-wave signal at a frequency approximately equal to the resonance frequency of the L-C circuit formed by outer coil L3 and capacitor C2. The square wave is generated by the oscillator formed by gates N1 and N2 (IC CD4069). Gates N3 through N6 act as buffers to drive outer coil L3. This produces sinusoidal current in coil L3, producing sinusoidal magnetic field mutually coupling the inner two coils.

When a metal (conductor) is brought near one of the inner coils, say L1, the Eddy currents in the conductor reduce the magnetic flux in coil L1, reducing the induced electromotive force (emf). This means a difference-signal is produced by the two coils due to the presence of a conducting object (metal) near coil L1 as shown in the circuit. Coils L1 and L2 are connected such that the difference of the induced emf is fed to transistor T1 through capacitor C4. Transistor T1 is configured as a small signal amplifier.

The amplifier is biased using a large base resistor of 1 mega-ohm. The AC-difference signal directly appears across the base-emitter junction of transistor T1 producing changes in the emitter current. This results in a voltage change in the collector of T1, which drives transistor T2 to glow LED1.

A small signal produced due to the magnetic field of Eddy currents in a small piece of metal like a screw or nut is sufficient to trigger T2 through T1.

Normally, the ferrite rods within coils L1 and L2 are adjusted such that the difference-signal from them is minimum. In this particular design, it is possible to adjust the signal to a voltage as small as 5 mV of sine wave. Transistor T2 plays the role of an electronic switch to drive LED1, which acts as a visual indicator whenever the metal is detected.

Thus when the detector assembly is brought close to a conductor, LED1 glows. You can change capacitors C1 and C2 on trial-and-error basis and fix the value for maximum sensitivity to select a resonant frequency and drive the oscillator (N1 and N2) at that frequency. Here a frequency of 55 kHz has been selected.

Make sure that the two resonators (one formed by L1 and L2 in series and other by L3) have approximately the same frequency of resonance.

The frequency of resonance of an L-C circuit is given by:



The frequency of an R-C oscillator using gates is given by:




You can vary R5 and C3 values using variable resistors and capacitors to fine-tune the frequency.

Assemble the circuit on a general-purpose PCB and enclose in a suitable small cabinet.

Terminate all the four terminals of the coils on the PCB base for connecting the coils to the main circuit (refer Fig. 3). The gel-pen refill should be sufficiently rugged. Secure it firmly inside the bobbin using non-magnetic, non-conducting materials. The ferrite rods too must be sufficiently secured in their positions using a synthetic enamel. Even a slight unintentional displacement can upset the balance of the resonator drastically. So use of a screw-type ferrite rod is recommended.

Fig. 3: Author’s prototype

For detector coil assembly, first insert one of the ferrite rods in the front end of the detector such that it is just inside the gel refill tube. Now insert the second ferrite rod in the other end of the tube. At this point, LED1 should glow brightly. Push the ferrite rod very slowly inside the tube while watching LED1. As soon as LED1 goes off, stop pushing the rod, mark the position of the rod and fix it in the tube using glue. Now the detector is well adjusted and ready for use.

 

Single Control Switch For Fan And AC

Single Control Switch For Fan And AC

Here is an electronic switch that can be used to switch on both the air-conditioner as well as fan of your room, one by one.

The circuit consists of power supply and control sections. The power supply section is built around transformer X1, bridge rectifier BR1 and filter capacitor C1. The 50Hz, 230V AC mains is stepped down by transformer X1 to deliver a secondary output of 9V, 300 mA. The transformer output is rectified by the bridge rectifier and filtered by capacitor C1.

When the mains is switched on for the first time, pin 3 of IC CD4017 (IC1) goes high and relay RL1 energises to switch on the fan. When mains is briefly switched off using S1 and then switched on, the power to IC1 is maintained by the charge on capacitor C1. At the same time, there is a trigger pulse on the clock input (pin 14) of IC1, which advances the decade counter and relay RL2 energises to switch-on the air-conditioner. Both the air-conditioner and the fan will be turned off if the switch is in the ‘off’ position.

Assemble the circuit on a general-purpose PCB and enclose in a suitable case. Fix the unit onto the switchboard. Use relays RL1 and RL2 with proper contact ratings. The current rating depends on the load that you are going to control.

 

LED-Based Running Display

LED-Based Running Display

Here is a circuit that creates an eye-catching running display effect using LEDs. It can be used to light up borders of animations, pictures, etc, and also for short word displays.
The circuit is built around an NE555 timer (IC1), a decade counter IC CD4017 (IC2) and a few discrete components. IC1 is wired as an astable multivibrator whose output is fed to the clock input (pin 14) of the counter.

When power to the circuit is switched on, NE555 oscillates to produce clock pulses. These pulses are fed to the clock input (pin 14) of the counter, which starts counting from Q0 to Q5. When output Q0 of the counter goes high, line L1 (LED1 through LED6) glows. When output Q1 of the counter goes high, lines L1 and L2 (LED7 through LED12) glow sequentially. When output Q2 goes high, lines L1, L2 and L3 (LED13 through LED18) glow sequentially. When output Q3 goes high, L1, L2, L3 and L4 (LED19 through LED24) glow sequentially.
When outputs Q4 and Q5 of the counter go high, L1, L2, L3 and L4 (LED1 through LED24) remain lit-up. Further, when output Q6 goes high, the counter automatically resets and the process repeats.
The LED status based on the counter output is shown in the table. Using this table, you can display any word with a running effect. Suppose you want to display ‘STAR.’ Build letter S with L1, T with L2, A with L3 and R with L4. When the circuit is switched on, letter S glows first. Then S and T glow sequentially, followed by S, T and A, and then S, T, A and R. When Q4 and Q5 outputs of the counter go high, the complete word ‘STAR’ glows. Thereafter, letter ‘S’ glows again and the cycle repeats.
Assemble the circuit on a general-purpose PCB and enclose in a suitable case. Arrange the group of five LEDs as desired—for word display or the border of a pictorial animation.

 

Solar Lamp

Solar Lamp With Variable Power Supply And Charger

A simple circuit of a solar lamp with variable power supply and charger is described here. It is useful where mains power failure is frequent. The power supply can be used for testing electronics projects and also charging your mobile phone battery. You can save on your electricity bills by switching to an alternative source of power using this circuit. The circuit works with the solar panel (without mains power supply) and can be used as a solar lamp during daytime (with the solar panel), night lamp (with external 12V battery), variable power supply (1.5V to 15V) and mobile battery (3.6V) charger.

The photovoltaic module or solar panel used here is capable of delivering a power of 5 watts. At full sunlight, it outputs 16.5V. The solar panel can deliver a current of 300-350 mA. Using it you can charge Li-ion batteries common in mobile phones.
The working of the circuit is simple. The output of the solar panel is fed to the circuit via diode 1N4007 (D3), which acts as a polarity guard to protect the solar panel.

For using the small LED-based lamp in daytime, just flip switch S1 to ‘on’ position. All the 5mm white LEDs (LED1 through LED22) glow. To get a higher light intensity, you can use a reflector. Similarly, if you want to use the lamp at night, connect a 12V battery to the anode of diode D1.

For using the variable power sup-ply, flip switch S2 to ‘on’ position. Connect a multimeter across CON1 terminals and set the required voltage (say, 9V) using potmeter VR1. The 9V output will be available across CON1 terminals. As mentioned earlier, the range of the power supply is 1.5V to 15V.

For charging a Li-ion battery (used in mobile phones), flip switch S3 to ‘on’ position and use connector CON2. Zener diode ZD1 provides 4.7V to charge the Li-ion battery. Using this circuit, you can charge a 3.6V Li-ion cell very easily. Resistor R13 limits the charging current.

Assemble the circuit on a general-purpose PCB and enclose in a small box. Mount RCA socket on the front panel of the box. Use RCA plug with cable for connecting the battery and LEDs. Also mount all the switches on the front panel of the cabinet.

Electronic Water Alarm

Electronic Water Alarm

 Aburst water-supply hose of the washing machine, a bathroom tap that you forgot to close, or a broken aquarium wall may turn your house into a pond. You can avoid this mess by using an electronic water alarm that warns you of the water leakage as soon as possible.

The acoustic water alarm circuit presented here takes advantage of the fact that the tap water is always slightly contaminated (or has salts and minerals) and thus conducts electricity to a certain extent. It is built around IC LMC555 (IC1), which is a CMOS version of the bipolar 555 timer chip. IC1 is followed by a complementary pair of emitter followers (T1 and T2) to drive a standard 8-ohm speaker (LS1). Power is supplied by a compact 9V PP3 battery.

Power is applied when power switch S1 is closed. The reset input (pin 4) of IC1 is held low by resistor R1 (2.2-kilo-ohm). The astable oscillator wired around IC1 is in disabled mode. When probes P1 and P2 become wet, these conduct to reverse the state of IC1’s reset terminal. As a result, the astable multivibrator starts oscillating at a frequency determined by resistor R2 and capacitor C3. The output of IC1 drives the complementary pair of transistors T1 and T2.
Although this combination causes significant crossover distortion, it doesn’t have any adverse effect on the square-wave audio signal processing. A 10-kilo-ohm potentiometer (VRI) is inserted between output pin 3 of IC1 and the bases of transistors T1 and T2 for volume control.

The probes can be made using two suitable copper needles or small pieces of circuit board with the copper surface coated with solder. Fit these at the lowest point where water will accumulate. After construction, place the alarm circuit well away from the point of possible leakage. Use a pair of thin twisted flexible wires to connect the probes to the circuit.

Capacitor C1 connected across IC1 input (pin 4 and GND) keeps the alarm circuit from responding to stray electrostatic fields. Similarly, twisting the wires together makes the relatively long connection between the probes and the circuit less sensitive to false alarms due to external electromagnetic interference. Finally, if you want to lower the probe sensitivity, reduce the value of grounding resistor R1.

Automatic Bathroom Lamp

Automatic Bathroom Lamp

This simple circuit can be used as an automatic bathroom lamp controller. It disables the bathroom lamp at daytime and enables it at night. The circuit is built around a light-dependant resistor (LDR1), reed switch (S1), two transistors BC547 (T1) and SK100 (T2), a 12V 1-change over (C/O) relay (RL1), a step-down transformer X1 (12V-0-12V, 250mA secondary) along with some discrete components.

The working of the circuit is based on the opening/closing of the bathroom door. When the bathroom door is closed, magnet comes near the reed switch and shorts its terminals. Both transistors T1 and T2 stop conducting, and neither relay RL1 energises nor CFL lamp (B1) glows.

When bathroom door is opened, magnet moves away from the reed switch and opens its terminals. Both transistors T1 and T2 conduct, relay RL1 energises and CFL lamp (B1) glows.
When bathroom door is opened, CFL lamp (B1) remains on. After entering the bathroom do not close the door completely. If the bathroom door is completely closed, lamp (B1) is switched off.

During daytime, lamp operation is not necessary due to the presence of sunlight so LDR1 is used. Fit LDR1 near the bathroom window where it can receive sunlight (reflected sunlight is enough). At daytime if you open the bathroom door, resistance of LDR1 reduces and keeps both transistors T1 and T2 in cut-off state. Relay RL1 does not energise and lamp remains off.

At night, if you open the bathroom door, resistance of LDR1 is high, which keeps both the transistors T1 and T2 in conducting state. Relay RL1 energises and lamp (B1) is switched on. The power supply required to operate the circuit is derived from transformer X1.

Assemble the circuit on a general purpose PCB and enclose in a suitable cabinet. Fix LDR1 near the bathroom window in such a way that maximum light falls on it at daytime. Fix the reed switch on the frame of bathroom door and magnet on the door. Keep the transformer inside the cabinet and place the unit above/near the bathroom door.

Door-Opening Alarm

Door-Opening Alarm

Children are at a high risk of drowning in a swimming pool. Majority of accidents occur when children get into a pool unsupervised. Such situations can be avoided by attaching a suitable alarm device to the door of the swimming pool. Here we describe the circuit of a 9V battery-operated electronic alarm driver that can be attached to the door leading directly to the pool. When the door is opened, the alarm sounds.

The circuit is built around resistor R2, a standard bar magnet, reed switch S2, IC CD4093 (N1 through N4), transistor T1 and some discrete components (refer Fig. 1). NAND gates N1 and N2 are used as an inverter. An oscillator is built around gate N3, resistor R3 and capacitor C2. Gate N4 with resistor R4 and transistor T1 works as a buffer-cum-electromagnetic relay or buzzer driver.

Fig. 1: Door-opening alarm circuit

When the door is closed, reed switch S2 is in closed state. When the door is opened, the bar magnet moves away from reed switch S2. As a result, the input to gate N1 is high. The low output (pin 3) of gate N1 makes the output of gate N2 high to enable the oscillator.

The output of the oscillator circuit is fed to piezobuzzer-driver transistor T1. When the door is opened, the piezobuzzer sounds an alarm alerting that someone is entering the swimming pool area.

Optionally, you can use electromagnatic relay RL1 (as shown with dotted line in Fig. 1) to operate a 230V AC mains operated call-bell or hooter. RL1 energises/de-energises at a frequency equal to that of the oscillator.

Fig. 2: Proposed door mounting arrangement

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Connect the piezobuzzer at the back side of the cabinet. Install the unit on the swimming pool door as shown in Fig. 2.

9V Converter Using Two AA Cells

9V Converter Using Two AA Cells

Normally, 9V PP3 batteries last less than a couple of hours if used continuously. These cannot be used if the voltage is below 7.8V. Say goodbye to these expensive 9V batteries having a short life. Two AA-size alkaline cells in conjunction with this DC-DC converter give 9V at a low current and last much longer.
The power supply is designed using a boost converter with fixed ‘on’ time and variable ‘off’ time. The variable ‘off’ time regulates power to the load. The converter consists of transistor T2, inductor L1 and capacitor C2. The conductance of transistor T1 controls ‘off’ time of the oscillator in conjunction with capacitor C2. IC TL431 (IC1) monitors the voltage across capacitor C4. When the voltage exceeds 2.5V at the reference pin (Ref) of IC1, the opto-coupler conducts more and reduces the conduction of transistor T1.

The frequency of oscillations mainly depends on the time constant (R-C) of feedback capacitor C3 and the input stage impedance (R1 plus VR1). Adjust preset VR1 to tweak the circuit for efficiency. The converter works with a single cell also. In that case, keep the output current-drain minimal.

Pull-Pin Security Alarm

Pull-Pin Security Alarm

Here is a simple and low-cost circuit of pull-pin security alarm. While travelling, rig the alarm unit to your luggage using a home-made security cable. When somebody tries to cut or remove the security cable loop to steal the luggage, the internal circuitry immediately detects and sounds an audio alarm.

Fig. 1: Circuit for pull-pin security alarm

Fig. 1 shows the pull-pin security alarm circuit. The control part is built around MOSFET T1, relay RL1 and a few discrete components. The alarm sound generator is built around transistors T2 and T3, speaker LS1 and a few discrete components. The circuit is powered by a compact 12V battery. If possible, try using a 12V, 2.8Ah rechargeable battery pack (see Fig. 2). The security cable is shown in Fig. 3.

Working of the circuit is very simple. The ends of the security cable are linked to the circuit through RCA sockets J1 and J2. When key-lock type power switch S1 (shown in Fig. 4) is turned to ‘on’ position, 12V supply from the battery is provided to the circuit. As J1 and J2 are shorted by the security loop, MOSFET T1 (BS170) is cut off and relay RL1 de-energises. During this sleep mode, LED1 lights up and there is no alarm sound. This means your luggage is safe.

Fig. 2: 12V, 2.8Ah rechargeable battery

 Fig. 3: Security cable

To test whether the alarm is working or not, simply press switch S2 momentarily. If the speaker sounds, the alarm is functioning perfectly and is ready to use.

In the absence of a security loop, J1 and J2 don't short. MOSFET T1 (BS170) conducts to energise relay RL1. The ground supply is routed to the alarm sound generator via normally-opened (N/O) contacts of the relay. During this changeover time, the relay becomes latched and the alarm sounds until it is reset by power switch S1. During this alarm mode, LED2 lights up. R1 and ZD1 ensure proper conduction of MOSFET T1.

Fig. 4: Key-lock type switch 

The alarm sound generator provides loud acoustic power output to an 8-ohm, 1W loudspeaker with a 12V supply. Transistors T2 and T3 form a complementary amplifier pair with positive (regenerative) feedback provided to the base of T2 via R6 and C3. The circuit oscillates on a frequency set by the C3-R6 combination and the base bias voltage of T2. The base bias of T2 is determined by potential-divider resistors R4 and R3. You can experiment with different R-C values to get the output tone of your choice.

Fig. 5: Proposed alarm unit

Assemble the circuit on a general-purpose PCB. After testing it for proper working, house it in a convenient, tamper-proof metal box of proper size. The proposed alarm unit (including LEDs and switches) is shown in Fig. 5. Fig. 6 shows the security alarm rigged to a briefcase using the security cable.
EFY note. The alarm generator part can be replaced with any 12V-powered transistor/IC alarm circuit without much difficulty. 
 

Fig. 6: Luggage security

 

Remote Phone-Bell Ringer

Remote Phone-Bell Ringer

   Did you ever miss an important telephone call simply because you were away where you couldn’t hear the phone ringing? This should not happen again with this inexpensive remote telephone bell. It is a battery-operated device that can be installed anywhere inside or outside your home. Since it is self-powered, it requires virtually no power from the telephone line. The input impedance of the circuit, as seen from the telephone line, is about 100,000 ohms and the input resistance is infinite. When connected across the telephone line, it has no effect on the telephone’s performance.

The circuit derives its power from four rechargeable Ni-Cd cells connected in series to provide 6 volts. Since the power is drawn from these cells only when the telephone rings, the battery will last several months. Besides, a built-in-battery charger is included in the circuit so that the cells remain charged at any time.
 

The circuit is divided into two sections: the ringer and the charger. The ringer section is built around transistors T1 and T2 along with a few discrete components. The charger section is built around step-down transformer X1, bridge rectifier comprising diodes D1 through D4, and resistor R5.

When the telephone rings, a 20Hz AC voltage of about 80-90Vrms is superimposed across the telephone line. The resistor-capacitor series circuit (comprising resistors R1, R2, R3 and capacitors C1 and C2) connected across the telephone line provides DC isolation; there is normally a DC voltage of about 48 volts across the telephone line when the line is not in use. Transistor T1 responds to the 20Hz ringing signal by conducting current during each positive half cycle applied to its base. Diode D5 prevents T1 from reverse-bias during the negative half cycle of the ringing signal.

The emitter current of transistor T1 is applied to the base of transistor T2, causing it to saturate and act as a switch. This applies full battery voltage to the bell (piezobuzzer), causing it to ring. The voltage applied to the bell is essentially a 20Hz square wave, which produces a slightly different sound from that produced by pure DC. Diode D6 and capacitor C3 protect transistor T2 from any reverse voltage spikes produced by a collapsing magnetic field of the bell.

In the battery charger circuit, step-down transformer X1 provides isolation from the AC power line while reducing the voltage to about 6Vrms. The output of the bridge rectifier is pulsating DC with about 9V peak, which is applied to the Ni-Cd rechargeable cells through current-limiting resistor R5.

This Ni-Cd charger circuit provides a constant charge current regardless of the battery charge or power line voltage. By limiting the current to not more than one-tenth of the ampere-hour rating of the cells, the charger may be operated for any length of time without damage to the battery due to over-charge.

Working of the circuit is simple. Install the device at a place where you normally cannot hear the telephone bell, such as the lawn. Press switch S2 to enable the device. When someone calls on your telephone, the buzzer produces a ringing sound in parallel with the telephone bell.

The charging circuit can be enabled using switch S1 to charge Ni-Cd rechargeable cells.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Fix two terminal connectors on the front side of the cabinet for the telephone line and another two connectors on the rear side of the cabinet for connecting mains power to the primary winding of the transformer. Place the four 1.5V Ni-Cd chargeable cells inside the cabinet. Fix both switches on the front panel of the cabinet to enable/disable the ringer and charger circuit.

Remote Control for Home Appliances

Remote Control for Home Appliances

Using this circuit, you can remotely control the switch-on and switch-off operation of your AC mains operated home appliances. The working range of the circuit depends on the orientation and the intensity of the IR beam.

The circuit consists of a transmitter and a receiver.

Fig. 1: Transmitter circuit

Fig. 1 shows the transmitter circuit. It is built around timer IC NE555 wired as an astable multivibrator. The multivibrator produces a pulsed output waveform with ‘on’ time of about 57 µs and ‘off’ time of about 326 µs, which means it generates about 2.6 kHz. The output of IC1 is fed to IR LED1 through current-limiting resistor R3. The IR LED1 used here is the same as in TV remotes. The circuit operates off a 9V battery, which is connected to the circuit through switch S1.

Fig. 2 shows the receiver circuit. It consists of phototransistor L14F1 (T1), voltage regulator 7805 (IC2), three 2N2222 transistors (T2, T3 and T4), dual voltage converter LM319 (IC3), dual J-K flip-flop 74109 (IC4) and some discrete components. The circuit operates off a 9V battery, which can be connected to the circuit through switch S2.

The Darlington pair built around transistors T2 and T3 amplifies the photo-current generated by the photo-transistor (T1). The equivalent photo-voltage appears across resistor R4. So across resistor R4 you get a replica (in term of wave shape but not in amplipude) of what you produce at the output of IC1 in the transmitter. The amplitude would vary with distance and other factors such as the angle of arrival of the IR beam at sensor L14F1.

Fig. 2: Receiver circuit

The low-pass filter constituted by resistor R7 and capacitor C4 produces about 3V DC. This DC voltage is fed to the junction of the inverting input of N1 and the non-inverting input of N2. The window comparator (IC3) is designed such that whenever the input voltage is between 2 and 4 volts (greater than 2V but less than 4V), its output goes high. If the input voltage is less than or equal to 2V, or more than or equal to 4V, the output goes low.

The window output is fed to the clock input of J-K flip-flop CD74109 (IC4). IC4 is wired in toggle mode. That means its output goes high if it is initially low and vice versa every time it is clocked. The output of IC4 is fed to the base of relay-driver transistor T5. Relay RL1 energises to light up the bulb when transistor T5 conducts.

Working of the circuit is simple. Initially, when no IR beam is falling on sensor photo-transistor T1, the DC voltage appearing at the input of the window comparator is nearly zero. The window output remains low. Transistor T5 is cut-off and the relay remains de-energised.

When switch S1 is pressed momentarily, the IR beam falls on the photo-transistor for this short period of time and a postive-going pulse appears at the input of the window comparator. The output of the comparator goes low, which toggles the flip-flop (IC4) and transitor T5 conducts. Relay RL1 energises to switch on bulb B1.

Assemble both the circuits on separate PCBs and house in suitable cabinets. In the transmitter unit, fix IR LED1 on the front side and switch S1 on the back side of the cabinet. Keep the 9V battery inside the cabinet.

Similarly, in the receiver unit, fix the photo-transistor (L14F) on the rear side such that the IR beam falls on it. To avoid circuit malfunction, cover the phototransistor (T1) with a suitable contraption so that the phototransistor is not exposed to unwanted light sources. Fix switch S2 on the front panel and the relay on the back side. Keep the 9V battery inside the cabinet.

Solar Battery Charging Indicator

Solar Battery Charging Indicator 

Here is the circuit of a simple charging monitor that indicates whether the storage battery of a solar power unit is being charged or not. It, however, does not tell the state of the solar panel.

Fig. 1: Block diagram of solar battery charging indicator

The circuit consists of two common ICs, an npn transistor, ten 5mm red LEDs and a few discrete components. It can be divided into two parts: voltmeter and display controller.

The voltmeter, built around IC LM3914 (IC1), is a low-power, expanded-scale type LED voltmeter that indicates small voltage steps over the 7-16V range for 12V solar panels. The meter saves power by operating in a low-duty-cycle 'flashing' mode where the LED indicators are on (and hence consuming power) briefly. The circuit may be switched to steady mode where the active indicator remains on at all times.
The input for IC1 (LM3914) is derived from the solar panel voltage via a potential divider network comprising preset VR1 and resistors R1 and R2. This variable input is about 3V for a DC potential of 12V.
The display range depends on the internal voltage reference and resistors R3-VR2-R4. The lowest LED (LED1) glows when the input voltage at pin 5 of IC1 is 1.8V and the top most LED (LED10) glows when the voltage exceeds 4V. as the input signal is divided by 4, the display ranges should be multiplied by this figure. So the actual display range is 7-16V, i.e., 1V per LED.
The display controller is built around IC LM555 (IC2) that is wired in astable (free-running) mode with a narrow-pulse output. The duty-cycle of IC2 is controlled by the ratio of resistors R6 and R7. If you want faster blinking, use a smaller value of resistor R7. A preset may be substituted for R7 if a rate adjustment is desired. Increase the value of resistor R6 to get a longer 'on' time for LED indicators. The frequency of oscillations is determined by the combination of capacitor C4 and resistors R6 and R7.
The output of timer IC2 is fed (through current-limiting resistor R5) to transistor T1 ,which, in turn, controls the power to IC1. Capacitor C1 filters the control voltage input to IC1 and capacitor C3 provides DC filtering for the entire circuit. When you press switch S1 across capacitor C4, the output of IC2 remains high, and the display switches to steady mode from flashing mode. Switch S2 is the master power-on/off switch.
Assemble the circuit on a small, general-purpose printed-circuit board (PCB) and enclose in a suitable plastic box. After necessary calibration, connect the circuit to the output cable of the charge controller unit with correct polarity.
For calibration, lock preset VR1 at the centre position and then set VR2 to its maximum resistance with the help of a digital multimeter. Now close both the switches (S1 and S2) and connect the circuit to a variable-voltage DC power supply unit with its output level set to 12V (1%). Adjust VR1 until LED6 (at pin 14 of IC1) lights up. Finally, lock presets VR1 and VR2 using glue. 

Fig. 2: Circuit of solar battery charging indicator