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.

Cellphone-Based Remote Controller for Water Pump

Cellphone-Based Remote Controller for Water Pump

Inconvenience in switching on a water pump installed in a remote farm is a common problem faced by farmers. Many circuits have been developed to solve this problem. Most of them are expensive and microcontroller-based. Here we present a cellphone-based remote controller for water pump. By calling the cellphone attached to the controller, the water pump can be directly activated.

Circuit and working
Fig. 1 shows the block diagram of cellphone-based remote controller for water pump. Fig. 2 shows the circuit. The circuit is built around DTMF decoder IC MT8870 (IC1), timer NE555 (IC2) wired as monostable multivibrator and a few discrete components. The main component of the circuit is IC MT8870. This DTMF decoder has band-split filter and digital decoder functions. It offers the advantages of small size, low power consumption and high performance.

Fig. 1: Block diagram of cellphone-based remote controller for water pump

Fig. 2: Circuit of cellphone-based remote controller for water pump

Once monostable timer IC2 is triggered, its output goes high for the preset time period. The time period depends on the values of resistor R7 and capacitor C4. It can be adjusted between 8 and 50 minutes using pot-meter VR1. The high output at pin 3 of IC2 energises relay RL1 to switch on the water pump.

The triggering pulse for IC2 is generated by DTMF decoder IC1 and the arrangement of diodes D1 through D5. Std pin of IC1 provides a high pulse when a valid tone-pair is received. Transistor T1 conducts only when outputs Q0 through Q2 and Std are high simultaneously. This can be achieved by sending digit ‘7’ through DTMF.

The water pump controller is connected to a dedicated cellphone through connector J1 with auto-answering mode enabled. The DTMF signal sent from the user end is decoded by the DTMF decoder and the corresponding binary-coded decimal (BCD) value appears on outputs Q0 through Q3. In this circuit only three of them are used.

Working of the circuit is simple. To switch ‘on’ the water pump, call the cellphone connected to the controller circuit and press ‘7’ once the ring stops. LED1 will glow to indicate that the water pump is switched on. The water pump turns off automatically after the preset time. LED1 turns off simultaneously.

Construction and testing
An actual-size, single-side PCB for cell-phone-based remote controller is shown in Fig. 3 and its component layout in Fig. 4. Suitable connector is provided on the PCB to connect the cellphone. Assemble the circuit on a PCB to minimise time and assembly errors. Carefully assemble the components and double-check for any overlooked error. Use suitable IC socket for MT887 and NE555 ICs.

Fig. 3: An actual-size, single-side PCB for cellphone-based remote controller

Fig. 4: Component layout for the PCB

Use relay RL1 with contact current rating capable of carrying the water pump’s current.

To test the circuit for proper functioning, press switch S1 and verify 5V at TP1 with respect to TP0. Connect the cellphone to the controller using connector J1. Call this cellphone and press ‘7’ once the ring stops. At the same time, verify high-to-low triggering pulse at TP2. TP3 now should be high for the preset time period.

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.

 

USB-Powered PC Lamp

USB-Powered PC Lamp

This white LED lamp lets you use your PC or laptop at night without disturbing others’ sleep. It produces a soft white light just enough to see the keyboard in darkness as well as when the ambient light is poor during daytime. The circuit is powered by regulated 5V DC available from the USB socket of the PC.

LDR1 acts as a light-dependent switch to turn on the lamp (LED2 through LED7) when the ambient light in the room drops below the preset level. Transistors T1 and T2 (BC547) are used to switch on the lamp. The base of transistor T1 is connected to the voltage divider comprising LDR1 and preset VR1.

When light in the room is sufficient, the resistance of LDR1 is low. This results in a high voltage at the base of T1, driving it into saturation. When transistor T1 conducts, transistor T2 is cut off. This disconnects the power supply to all the white LEDs (LED2 through LED7). LED1 (green LED) glows as it is forward-biased, indicating the standby mode.

When it is dark, or the ambient light in the room is lesser than the pre-determined level set by VR1, transistor T1 is cut off and T2 conducts. All the white LEDs glow with sufficient brightness as these are connected to the power supply through series dropper resistors R2 through R7. These resistors are used to limit the current through white LEDs to a safe level.
White LEDs are arranged in parallel as each white LED requires a minimum of 2V. Preset VR2 is used to set the base voltage of transistor T2. Adjust preset VR2 until the white LEDs turn off in the preset light intensity level during the day.

Assemble the circuit on a general-purpose PCB and enclose in a suitable case. Power to the circuit is obtained from the USB socket using a USB cable. You can use an old USB cable for the purpose. Cut the ends of the USB cable to get the red and black wires for positive and negative supplies of the circuit. Cut the green and white wires of the USB cable and solder the red and black wires to the PCB. Use the USB plug at the other end to draw power from the USB socket. Fix the unit near the keyboard so you can see the key buttons easily.

 

Sound Sensor Alarm

Sound Sensor Alarm  

This 6V battery-operated circuit triggers an acoustic piezobuzzer when a sound is detected. It can also be used as a cheap acoustic-type glass break detector and/or ambient sound level monitor.

The circuit has an ordinary condenser microphone MIC1 as a sound sensor. Sensitivity of this microphone can be changed to some extent by changing the value of the bias resistor R1. When the circuit is powered by a 6V battery through switch S1, it goes into standby mode and the red LED1 lights up to indicate that the circuit is ready for use.

When microphone (MIC1) detects a sound, electrical signal from the microphone is amplified and processed by a small-signal amplifier wired around transistor T1 (BC547). Amplified signal from the collector of T1 is passed to electrolytic capacitor C4 through diode D1 (1N4148). Transistor T2 (BC547) conducts and triggers the monostable built around the timer IC NE555 (IC1). As a result, piezobuzzer (PZ1) at the output of IC1 starts sounding for a fixed duration, determined by the values of resistor R7 and capacitor C5. PZ1 can be replaced with an electromagnetic relay to drive heavy external electrical loads such as power sirens.

Assemble the circuit on a general-purpose PCB and enclose it (including battery) in a tamper-proof cabinet. Glue the condenser microphone at the rear side of the window/door glass to be protected, and connect the microphone to the sensor circuit using a short length of transparent screened cable.
note. Using a glass break sensor may cause false alarms by confusing the breaking of glass such as cookware, or the sound of bells, with the sound of breaking windows.

 

Electronic Electroscope

Electronic Electroscope

An electroscope is the instrument used to detect charged bodies. Here is an electronic version of the scope that is sensitive and, unlike conventional scopes, indicates the polarity of charge as well. This circuit consumes very low quiescent power and reliably indicates charge induction and detection. The polarity of charge is indicated through LEDs (green LED indicates positive and red LED negative).
As shown in Fig. 1, the circuit is built around popular dual timer 556 (IC1), quad 2-input NAND gate CD4011 (IC2), transistors BC547 and BC557, and a few discrete components. The CD4011 has four NAND gates with very high input impedance. This high impedance is used in detecting the potential difference produced by an external charge distribution.
When the positively charged object is brought near the positive-detecting plate, it attracts electrons from the detector plate and hence capacitor C6 becomes positively charged.

Fig. 1: Circuit of electronic electroscope

Diodes D3 and D4 (1N4148) help determine the direction of current flow, removing certain shortcomings of the circuit. (These prevent the capacitor of the opposite arm from charging when the object is withdrawn quickly.) Even though diode D4 in series with capacitor C6 is reverse biased, the current through it is adequate to charge capacitor C6. Thus capacitor C6 gets charged with respect to the ground and the input voltage of gate N1 goes high. This makes the gate output low and transistor T3 conducts to light up the green LED.

The process is similar during negative charge detection. Here transistor T4 conducts to light up the red LED.
When a negatively charged object is brought near the detecting plate, it also affects the positive-detecting plate. The negatively charged object pushes the electrons in the positive-detecting plate towards its input capacitor and the plate becomes negatively charged with respect to the ground. These electrons leak through the gate input into the other parts of the circuit.
 
When the negatively charged object is withdrawn, the positive-detecting side having lost electrons (detecting plates and the capacitor) appears to be positively charged. Since this voltage may not exceed the supply voltage (V+0.6), the gate will not conduct heavily to return the electrons immediately. In this case, the charge developed across the capacitor takes a long time to decay and the gate becomes locked.
To overcome this problem, two monostable multivibrators built around IC1(A) and IC1(B) have been incorporated for each arm. The high output of monostable IC1(B) shorts capacitor C6 to the ground for a specific time as soon as the output of the negative-sensing gate (N4) goes from high to low. Similarly, the high output of IC1(A) charges capacitor C1 for a specific time as soon as the output of the positive-sensing gate (N1) goes from low to high.
A similar arrangement is provided for both the positive- and negative-detecting sides. So when a positively charged object is withdrawn, the negative-charge-detecting input capacitor is shorted. The monostables prevent locking of any of the two gates and allow the scope to function smoothly.
The electroscope can be designed for bench-mounting or more compactly inside a gluestick tube with identical metallic detector plates on the cap. Two CR2032 (3V) cells are used to power the circuit. Take the tube slowly towards the charged object or bring the object slowly towards the scope. Either of the two LEDs will glow depending on the polarity of the charge on the object. The author's prototype is shown in Fig. 2.To test the circuit after construction, whack your hair with a comb and bring the comb near the scope slowly. The red LED should glow to indicate a negative charge. This glow should be constant as long as the comb is near the detecting plates.
It is also possible to use the scope as a direct charge detector within limits by bringing the corresponding plates (or just the lead) in direct contact with the point of charge accumulation in a circuit. The sensitivity can be increased to some extent by decreasing the values of capacitors C1 and C6.

Fig. 2: Author's prototype

not:-external reset can be provided using an SPDT switch (not shown in the circuit) between the output of the monostable and the 1-mega-ohm resistor, with its pole connected to the 1-mega-ohm end. Connect one throw to the monostable and the other to +V or 0, as per the transistor. 

 

Automatic Headlights Switcher

Automatic Headlights Switcher

Automatic headlamps are the latest convenience in today’s cars. These eliminate the need for the driver to manually switch on or switch off the headlamps in most driving situations. The automatic headlight system reacts like the human eye to outside light levels and independently turns the lights on and off when needed. Such a system offers both safety and convenience.

This circuit can be particularly helpful when driving on roads with many tunnels, at twilight or sunset, and even in foggy, icy, stormy and rainy conditions. For example, when the car enters a dark tunnel, the driver will not have to fumble for the headlights switch. The car’s headlights will automatically switch on after sensing the poorly lit tunnel. When the car comes out of the tunnel, the headlights will switch off.
The circuit is built around timer NE555 (IC1), light-dependent resistor LDR1 and some discrete components. Potmeter VR1 is used to set the light sensitivity of LDR1. On sensing the darkness, LDR1 turns the headlights ‘on’.

Basically, an LDR is a resistor whose resistance decreases with increase in the intensity of the incident light. Usually, an LDR exhibits very high resistance in darkness and low resistance in the presence of ambient light. Thus a varying voltage drop can be obtained across it with changing ambient light conditions.

The LDR1 is connected to the trigger input (pin 2) of IC1.The output of IC1 is connected to the base of relay-driver transistor T1. The 12V supply voltage is connected to the circuit through switch S1. LDR1 and the 100-kilo-ohm preset constitute a voltage divider arrangement at pin 2 of IC1.

Working of the circuit is simple. Enable the circuit using switch S1. When there is sufficient ambient light, the resistance of LDR1 remains low (a few hundred ohms). The voltage at pin 2 is greater than two-third of 12V. The output at pin 3 of IC1 remains low—stable state for monostable mode of operation—and the headlights of the vehicle connected to the normally-open (N/O) contacts of relay RL1 remain off.

When the ambient light decreases, the resistance of LDR1 shoots up to a few mega-ohms and the voltage at the trigger input (pin 2) of IC1 decreases to less than one-third of 12V. The output at pin 3 of IC1 goes high to energise relay RL1 and turn the headlights ‘on’. Switch S2 can be used to manually operate the headlights.

Assemble the circuit on a general-purpose PCB and enclose in a small suitable cabinet such that the LDR sensor receives ambient light. Connect power supply switch S1 on the rear side of the cabinet to connect/disconnect the 12V car battery. Connect potmeter VR1 at the front side of the cabinet for varying the sensitivity to light as desired. Now your headlight circuit is ready for use.