somke sensor ckt..problem

Thread Starter


Joined Mar 17, 2008
We are using a smoke sensor in our project.

The circuit diagram has been attatched

We borrowed the circuit diagram from somewhere,but it didnt give a proper explaination to it.

we would like you to give a breif explanation of ckt , including flow of current especially from the transistors.

from the circuit ....what we understood is when light falls on the phototransistor it does not trigger the 555 timer, so the LED does not glow

and when light path is obstructed the 555 timer is triggered and the LED glows indicating smoke sensed



Joined Dec 20, 2007
Both of your LEDs are connected backwards.

When light falls on the phototransistor do you know what it does?
When the phototransistor stops conducting then do you know what the first transistor does?
When the first transistor turns off then do you know what the second transistor does?
When the second transistor turns on then what happens to the voltage at pin 2 of the 555 IC?
When pin 2 of the 555 IC goes low then do you know what its output does?

Your 555 monostable circuit is missing a timing capacitor.