This article is about LM311 comparator IC (integrated circuit) touch buzzer circuit:

You can see the circuit working in this video:

Step 1: Design the Circuit

I have drawn the circuit in PSpice version 9.1 student edition:

The output of LM311 comparator is an NPN transistor collector. The current in pin 7 starts flowing when the positive input (pin 2) is below the negative input (pin 3). The output is inverted because the output of the comparator is an inverter logic gate.

Comparator is designed to drive coils. A buzzer coil turns ON and OFF similarly to a relay turning ON and OFF. The current in the coil continues to flow in the same direction when the current to the coil is discontinued. This is why I connected the diode.

The buzzer current is 40 mA. The 9 V battery can supply a current of 100 mA. The remaining 60 mA current is for the comparator IC (integrated circuit) and the resistor circuits.

The circuit will work without the 10 Megohm resistor. However, it is bad practice to leave pin 2 disconnected.

Step 2: Simulations

Simulations show that when the human resistance falls before about 8 Megohms high current begins to flow.

Step 3: Make the Circuit

I used a wire wrap socket and wire wrap tool to connect the circuit. I tried Rtouch resistor of 1 Megohms, 4.7 Megohms. However, the circuit was not turning ON well with those resistors (1 Megohm and 4.7 Megohm) when compared to Rtouch of 10 Megohms.

Conclusion

This circuit is useful for toy applications.