Help with a 3-Level photoresistor smoke detector circuit?

Thread Starter


Joined Oct 19, 2012
Hello everyone! New to the forum here! I'm quite novice at circuit building and designing, but what I have on this project so far is about how far my knowledge goes on electronic circuitry.

So me and a small team of engineering university students are attempting to build a 3-tier smoke detector.

I designed a circuit that uses three switches in place of the detector components. When the first switch is opened, or disconnected, one light goes on. Then when the second switch is opened, a second light goes on, along with a buzzer. And when the third switch is opened, a light goes on, a buzzer sounds, and it opens a transistor gate to activate another device. I used a capacitor on the third one to allow the light to fade off as well, and a reset button to discharge the capacitor afterwards.

WHAT I NEED, is a way to replace the switches with photoresistors. The idea would be that when a laser's path is blocked by smoke, it acts as opening the switch. I tried paralleling the photoresistor with another resistor just below the photoresistors highest resistance, but this didn't work.

(FYI it's a 9 volt battery)

Here's a schematic from a circuit simulator app at

(the circles are LEDs)

and if you would like to simulate my circuit yourself, I'll reply to this message with the code you can use in the file > import menu.

Thank you so much for taking your time, I really appreciate it!
Last edited:

Ron H

Joined Apr 14, 2005
I think you are laboring under a misconception.
When smoke gets in the path between the laser and the photoresistor, the photoresistor does not act as a switch. The resistance of the photoresistor goes up as the incident light goes down. Smoke will never get dense enough to block the laser, unless the photoresistor is many feet from the laser.
I think you would be better to make a resistive voltage divider between Vcc and ground, with an LDR as one leg of the divider. Set up 3 comparators (LM339 is a nice quad comparator), each with a different threshold voltage, and let them monitor the LDR. You can take the outputs of the comparators to drive transistors which in turn drive your buzzers and LEDs, and also to logic gates and/or other digital and analog processing.
If I understand your system, you should only need one LDR.