Night Alert

Idea of this circuit
came to me at midnight when my pet dog started barking continuously on
sensing a moving shadow, perhaps that of an intruder. Dogs have a night
adaptation capability to maximise the sensitivity of vision in low
light. They are well adapted to see moving objects rather than
stationary ones in darkness. This circuit turns a lamp ‘on’ for a short
duration when the dog barks, giving an impression that the occupants
have been alerted.

The condenser microphone fitted in the dog’s cage senses barking
sound and generates AC signals, which pass through DC blocking capacitor
C1 to the base of transistor BC549 (T1). Transistor T1 along with
transistor T2 amplifies the sound signals and provides current pulses
from the collector of T2. The input trigger pulse is applied to the
collector of transistor T3 and coupled by capacitor C3 to the base of
transistor T4 causing T4 to cut off. The collector voltage of transistor
T4 forward biases transistor T3 via resistor R8.

Circuit diagram:

Night Alert Circuit

Night Alert Circuit Diagram

Transistor T1 conducts and capacitor C3 discharges to keep
transistor T4 cut-off. Transistor T4 remains cut-off until capacitor C3
charges enough to enable it to conduct. When transistor T4 conducts, its
collector voltage goes low to drive transistor T3 into cut-off state.
Resistor R9 and capacitor C3 are timing components. When fully charged,
capacitor C3 takes about two minutes to discharge. So when sound is
produced in front of the condenser mic, TRiAC1 (BT136) fires and the
bulb (B1) glows for about two minutes.

Assembly instruction:

Assemble the circuit on a general-purpose PCB
and enclose in a plastic cabinet. Power to the circuit can be derived
from a 12V, 500mA step-down transformer with rectifier and smoothing
capacitor. Solder the triac ensuring sufficient spacing between the pins
to avoid short circuit. Fix the unit in the dog’s cage, with the lamp
inside or outside as desired. Connect the microphone to the circuit
using a short length of shielded wire. Enclose the microphone in a tube
to increase its sensitivity.


Since the circuit uses 230V AC, many of its points are at AC mains
voltage. it could give you lethal shock if you are not careful. So if
you don’t know much about working with line voltages, do not attempt to
construct this circuit. EFY will not be responsible for any kind of resulting loss or damage.

Author: D. Mohan Kumar – Copyright: Electronics For You Mag

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