Security has been a major issue in the present day world. And security gadgets of various nature are available today. Here I introduce a simple touch alarm that can be used on doors with metallic handles (This alarm works only on metallic handles) . This is a simple and easy to make circuit.
The main part of this circuit is the first 555 timer IC which is set as a monostable multi vibrator , The metallic loop is attached to PIN2 of 555 IC and it is active low, so when someone touches the door handle to which the alarm loop is attached the voltage to get grounded or the voltage at PIN2 gets reduced to less than 1/3 Vcc, so a pulse is formed with t=1.1RC. here the value of R2 and C2 can be made small just sufficient to generate a small pulse to set the latch. (Sometimes the IC may remain triggered even when not touched in such case ground PIN2 with a capacitor of 100nF).
The second part of the Circuit is a transistor that has been set as a not gate. For a note gate output is low when input is high. The input is given to transisitor base and output is taken from the collector, this output is given to the third part of the circuit.
The third part of the circuit is another 555 IC set as a latch, when the PIN2 is grounded the latch output will be set to 1 and when PIN4 is grounded the latch is reset, the switch is provided to reset the latch, the output of the latch is given to a buzzer.