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.
Fig. 1: Door-opening alarm circuit
Fig. 2: Proposed door mounting arrangement
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