When touched the touch metal then the Alarm is switched ON and when Leave the finger from the touch plate then the Alarm is Automatically Switch OFF . This is very simple and easy because only three transistors and some other components are used , No Any IC required for this Project.
Using this circuit you can change the Old Experience of Doorbell Switching method and can Create new hightech Method With Automatic Touch Doorbell Alarm Switch System .
Assemble the circuit as given in fig. Capacitor value can between 2.2uf to 100uf. Careful with transistor terminal when constructing the circuit.
This Automatic Touch Doorbell Alarm Switch is very sensitive with human finger and this is very simple to construct. No any ic is used in this circuit. At first when Power is on to circuit then switch remain OFF, When any touch the touch plate then transistor Q1 triggered and as Q1 get on, the transistor Q2 is also turned ON.
Q2 is PNP transistor and Emitter of this transistor is directly connected with Positive supply . As this transistor turned ON the Positive supply voltage is passed through Emitter to Collector of Q2. Therefore the voltage goes to Base of Q3 transistor through R4, and transistor Q3 turn ON and connected load with this transistor Collector get negative supply and complete the circuit and Get Turn ON.
You can connect a 9v Relay with transistor Q3 for More Power Load Like AC bulb. This circuit is tested on breadboard as given diagram .
If you want to make this circuit to Remain connected Alarm ON for a fix duration of some second then change the capacitor of 220uf Elase use 10uf capacitor. With the 10uf capacitor the time duration to automatic swith OFF is very less about 1 sec. So with Less value capacitor this touch alarm switch is work as astable mode like Push buttom with no delay to return in original state.
This is very interesting and much effective Project for beginner . Use any small piece of any metal for making of touch plate.
Note- Pin Of BC 558 is the same as BC547