Momentary Contact Circuit

Something like this?

http://mycomsats.com/blogs/photo-transistors-applications/

G.A. That will turn ON the relay on dark, the O.P. needs to build one more, swapping R and Q1 to turn ON the relay on light.

If he wants just a brief pulse from each circuit, he will have to use a dual mono-astable multi-vibrator, 74LS123 is a very friendly one.

Well, I got some 74LS123 chips, and they are very nice, put out a nice pulse. I'm using this circuit as an example of what I need to do. Instead of the phone ring input, I need to use another Phototransistor to sense when another light comes on. That makes for one pt to sense dark and trigger a pulse, and one pt to sense light and trigger another pulse. (This is for a model train crossing. First pt dark when train covers it, cycle starts. Second pt light after train passes, cycle ends.)

I haven't figured out how to do that. I sure would be nice to use only one 123 IC. Since they have two separate circuits in them, and trigger on falling edge and rising edge, I'm hoping I'm close to my goal. Any help appreciated. Mike

Do a search for what it called a "one shot relay". That's exactly what they are for.

Relay contacts (well, any switch contacts) bounce, so you will need to add some de-bounce CR.

Del

Here is the circuit you need

Working in 5 volts, that's TTL range, you can just build an edge trigger:

Output B provides a pulse every time input A goes from Low to High.

You can adjust the width of the pulse by changing the number of inverters in the loop, it has to stay an odd number, though, or the circuit won't work.

If you want a positive pulse when A goes from High to Low, then just put an inverter on the A input.

This can all be built with basic 74LS Logic chips, you could even build it using only 74LS00, the quad 2-input NAND gate, if you remember your Digital Logic and Boolean Algebra classes.

Hope this helps.

I'm going to breadboard this idea, sorry for the bad drawing. The phototransistor holds the pnp open until it goes dark, while charging the cap. When the pt goes dark, the cap discharges through the pnp to give a momentary pulse to the ground, which makes the circuit I need. Hopefully...

Yup, can't beat breadboarding it.
My circuits always work on paper.

I have a saying...

"Circuits seldom work first time, software never does!"

Del

I have already given you the simplest circuit possible.

The last circuit I posted didn't work, but it helped me understand the problem. I'm using a 1000uf cap and LEDs as amp meters. The LEDs show the cap charging and discharging, very helpful for a visual-oriented guy like me.

What I'm realizing is phototransistors don't drain the cap completely, or charge it well. So I will have to use comparators or transistors to make those connections, so much for simplicity.