Photocell circuit

If it is a cadmium sulfide cell, the cell and a fixed resistor form a voltage divider with perhaps 5 or 12 volts applied. Then, one just sets up the trigger points (UTP and LTP) accordingly.

Hello Jack,

I have a circuit and PDF PC board layout that could be modified into what you need, especially triggering off a low level lamp output. This will be using a photo-transistor which is more sensitive. Also using a low powered TLC555 gives you a CMOS Schmidt Trigger.

If you leave everything as I designed, it would pulse/buzz a small speaker every flash of the dim lamp.

Modifications for what you indicated:

Leave off C1 so it does not output a tone (oscillate), then adjust the value lower of R2 a 1Meg resistor to give you the trip point you need.

Remove and "jump wire" C2 for a DC rail to rail voltage swing on the output. Leave Pin 7 of the TLC555 disconnected or cut the copper foil. Better yet use a IC socket and leave this IC pin folded upward.

Pin 7 is just above the "1" of the U1 label on the IC graphic.

Pin 1 has a black dot above it and the pins number counter clockwise.

• * * Christopher

Temecula CA.USA

most photocells are not much faster than that

basically you want a resistor and the photocell in series across the supply with the mid point connected to the schmidt input. try different resistors until you get the switching point where you want it. 1/4 of the dark resistance of the photocell may be a good starting point.

Bye. Jasen

Thanks very much. This looks very useful.

After a quick scan, I didn't see the photocell number. Do you have it?

I'm going to drive an LED counter with it. I need to see when the input lamp lights when I'm away and a rough count.

Thanks again.

Jack

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I'm not familiar with the terms but if photoresistive indicates a cell that acts as a resistor, that's what I'm thinking that I need.

If there is a generator cell that will trigger a CMOS Schmidt, then maybe I could use that. I don't think that I could adapt it to the input lamp since I don't have any way to measure the lamp intensity (not smart enough in optics).

Thanks.

And thanks to all other the other responders.

Jack

I found it. Sorry.

Jack