enter image description here Hello!

I am working on a clap relay project and I want to do my own clap relay using timer 555 but I am not sure how the circuit works in details. I am familiar with the 555 timer and I know that the trigger voltage has to be lower than 1/3 of Vcc (in this circuit 4.3/3=1.43, so lower than that). Electret microphone M1 is also included.

I have posted a part of a clap relay circuit which I found it on the internet and this circuit makes the led turn on(not on the picture). I marked points A and B because I am want to know the potential in those two points.

My assumption how it works(please correct me, because I am not sure!):

  1. before any sound is made:
    • electret mic is biased with 4.3V voltage and it acts like a capacitor because of its internal structure
    • current flows through 470k resistor and base of NPN, so that the potential in point B is around 0.7V and the transistor conducts current
    • if the transistor is in active/linear mode and Ic=beta*Ib is valid in active region, therefore potential is calculated to be around 2.6V

But that means that 2.6V>1.43V and the 555timer is not triggered.

  1. When sound is produced
    • the 4.3V biased electret mic vibrates which changes the voltage on a microphone and therefore changes current through 220n capacitor and at the same time also changes base current.
    • because the base current is changed and the transistor is in linear/active region, that also changes the collector current and also potential in point A.

Please correct me if I am wrong. I would like to make this circuit but before I make it I would like to know if I am understanding it correctly!

Thank you in advance.

EDIT: I have made a circuit on a breadboard and measured voltage across microphone to be around 2.9V. Apparently my knowledge about electret microphone is a problem :) Potential at point A was 0.2V and in point B around 0.7V. I will experiment further and post what I found out, maybe it will help someone else :)


2 Answers 2


Your assumptions on how it works are correct. However this is a poor design because it is critically dependent on the current gain of T1.

The BC847's HFE can be as low as 110 or as high as 800. The 'A' version is more tightly specified at 110 to 220, but even this narrower range could produce wildly varying results. With HFE = 110 the steady state trigger voltage will be about 1.8V (above the threshold), but at HFE = 220 it is about 0.2V so the 555 will be continuously triggered.

To ensure that it worked with any BC847 you would have to either choose a higher value for R2 that covers the full range of possible HFE values, select the required value on test, or make it a trim pot that is adjusted to suit each individual transistor.

The other alternative is to change the circuit to make it less sensitive to HFE variations. If R2 is connected between the Base and Collector of T1 then the Collector voltage will tend to stabilize at around half the supply voltage, and the circuit should work with a wide range of transistors.

  • \$\begingroup\$ You are correct, HFE is specified in a wide range.. based on my calculation it was around 300 at few miliamps collector current. \$\endgroup\$ Aug 15, 2016 at 19:56

I have increased resistance to 2Mohms and is working correctly as planned. When no sound is produced the potential in point A is 3.4V (it is a great match with the calculated result which was 3.5V, active region). When I clap the potential drops and triggers the 555 timer and turns on the LED.


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