# Clap relay using 555timer - not sure about voltages around transistor

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!

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 :)