Working on a project that responds to presence of smoke. Need advice.

[neu_ee]

So I am working on designing a circuit that responds to the presence of smoke between two sensors. I have the circuit designed and a good idea of what to use for the sensors, but I'm looking for a little input into options I may or may not have.

What I have been thinking is if I used a laser diode to emit a weak light onto a photocell resistor from about 2-4 inches away, my circuit will result in an LED being off. When smoke appears between the light and the resistor, the LED will turn on(hopefully), until the smoke disappears. (This is just a basic overview of what I'm trying to do.) I know that my back end circuitry of the light turning on/off will work, but I was wondering what do you guys think about the possible success of the light sensing circuit.

The problem I have is that I have to order the parts for that idea. I have in my possession a few IR photointerrupters that I was considering using, but from what I've read online, smoke does not reflect or block infrared light(?). Would there be any possible way to implement these interrupters so that my design would work?

If this makes sense, let me know if anybody has any ideas.

Thanks.

EDIT: The interrupters i have are the LTH-1550-01, so I'm thinking if I don't utilize the IR transmitter, but use an LED to manipulate the phototransistor, I might be able to accomplish the task.

 

[4pyros]

I would use a regular incandescent lamp and an LDR or CDS sensor.
I think a laser diode will be to narrow and brite.

 

[Diver300]

You would be far better to look for scatter from the smoke.

If you do not align the sensors, the light from the LED or laser diode will shine straight past the photocell, which won't see much light at all. When there is smoke, the light from the LED or laser will scatter and go everywhere, so more light will enter the photocell.

 

[neu_ee]

If you do not align the sensors, the light from the LED or laser diode will shine straight past the photocell, which won't see much light at all. When there is smoke, the light from the LED or laser will scatter and go everywhere, so more light will enter the photocell.

I was looking into how smoke detectors work, and saw that they operate similarly to this idea. However, they usually set up the sensors perpendicular to each other but that would not be optimal for me. I'll have to experiment to see if I get enough scatter with different angles of the LED(+/- 30 degrees from the horizontal). As I think about it, this approach seems more ideal. Thanks.

 

[jpanhalt]

However, they usually set up the sensors perpendicular to each other but that would not be optimal for me.

Why?

That is the usual way to set up scatter and fluorescent sensors for decades. The reason is obvious, enhanced S/N.

John

 

[alec_t]

Another possibility is to use a junction FET (2N3819) with its gate floating but adjacent a supply rail. Smoke particles are conducting (or charged) and will change the gate potential, in turn changing the FET drain-source current. I've not built this system but have seen it demonstrated (many years ago!). The circuit would need to be screened to prevent unwanted electrical noise pick-up.