Sound Sensor Alarm

Sound Sensor Alarm  

T. K. HAREENDRAN

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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.

EFY 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. 

Automatic Bathroom Lamp

Automatic Bathroom Lamp

Pradeep G.

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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.