SENSITIVE  TOUCH  SECURITY ALARM 

 101％ working  project hello  friends  today I am gonna  telling  you  about  some  easy  and valuable  electronics projects for  school and  college  practical , 

This alarm circuit is frequency- operated. It uses an NE567 phase-locked loop to sense a particular frequency band. We all know that AC mains line frequency is 50 Hz. Radiation signals from mains wiring can travel a few meters distance. These can induce an electromagnetic field in the human body also.

 So when you touch a metallic object, 50HZ signals from your body (induced from mains wiring) will be injected into it. These can be amplified and fed to a PLL circuit tuned to 50 Hz.

Fig1 -SENSITIVE  TOUCH SECURITY ALARM Circuit Diagram 

Fig. 1 shows the circuit of the hum- sensitive touch alarm, while Fig. 2 shows the power supply circuit required to operate the touch alarm. Here, popular operational amplifier HA741 is used as the signal amplifier. The amplified signals are fed to NE567 PLL circuit.

 If the input frequency is 50 Hz, the output of PLL goes low and activates the electrical circuit of LED1 and buzzer. LED1 glows and at the same time the piezobuzzer sounds. Adjust preset VR1 for correct operation. Use a small touch plate at the input of op-amp IC1.

 Connect the touch plate to the input circuit of op-IC1 using a shielded cable. Keep amp the length of the shielded cable as short as possible to avoid losses to already feeble induced signal.

Fig2- power supply circuit diagram

 If you use a battery, keep the unit near AC mains wiring for generating 50HZ signal through induction. Use of a half-wave rectifier power supply is recommended for this circuit. The reason: The output of the half-wave rectifier is 50 Hz as against the 100HZ pulsating ripple of the full-wave rectifier and it will also serve the purpose of inducing 50HZ signal.

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