I need this circuit to close a relay to stop a stop watch. Would this be the best route to go? Or does anyone know of a better way?
I am using an IR sensor to do the job now. Its kind of difficult to set it up outdoors. The laser would make set up easier. Cost is also a concern if I end up needing a large quantity.





http://home.cogeco.ca/~rpaisley4/ATLaserDetctor.GIF

 

Without looking at the circuit in too much detail or checking the components.....A relay could be made to work with this and it might be the easiest way to go, depending on how your stopwatch works.

Using a 12VDC relay, connect the positive terminal of the coil to +12V and the negative terminal to the output of the comparator. Then, rather than the led illuminating as the train goes by, the relay will switch.

That is what you want, right?

 

Why not use a digital stopwatch and parallel the Start / Stop button switch with a Cds (Cadium Sulfite Cell). At such short distances the illumination could be a bright LED shining through a short tube, no laser needed.
I know the Cds works because I have used them to remotely control one of my digital cameras.

 

Thanks for the reply about the relay. I'm going to give it a try.
This schematic is for a model railroad but I am using it in a different application. The stop watch just needs a closing switch to stop it, so a relay will be fine.
I do need the laser because I intend to use it outdoors and the IR sensor I am using is difficult to set up. The visible laser would be bright enough to direct at the "target" (phototransistor), and work, as long as it closes the relay when the beam is broken.

 

Don't neglect to connect a diode across the relay coil with the cathode connected to the +12V end, otherwise it will blow your comparitor the first time the relay is released due to the back EMF that occurs when the coil circuit is opened.

__________________
Len

 

I found a much simpler way to do this usin a photo transistor and a transistor to energize the realy. It works here on the bench. Hope to try it in real time this weekend.

 

Tried it today outside and I am now faced with the sun and all of its output that gets into my photo transistor! Any suggestions? I am going to try mounting the PT in a length of tubing as a type of filter.

 

The usual way to avoid the effects of ambient light is to modulate the signal so the receiver can ignore "DC" and only respond to AC.

This is how remote controls for TVs etc. work. They modulate the signal at either 36 kHz or 40 kHz.

You can buy ICs for IR receivers that do the demodulation.

For an example of a receiver, see www.jaycar.com.au and search for catalogue number ZD-1952.

I don't know if there are transmitters in an IC, so you may have to modulate an IR LED, see cat numbers ZD-1945 and ZD-1946 at the same site.

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Len

 

I guess I have some learning to do, thanks for the help.

 

I found antother thread that says the modulation would have to be "pulsed" to avoid being mistaken for interference. How would I do that? Could this be done with a 555 or another IC?

 

That's what I meant by "modulate". The light is switched on/off at either 36 kHz or 40 kHz depending on what frequency the receiver is designed to detect.

A 555 driving an IR LED is one way it could be done.

If necessary, you could mount them in tubes and even use lens to form the light into a beam.

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Len

 

or just grab an old remote for some gizmo you don't use anymore (ive got lots of vcr remotes (but where did my tapes go?))

the receiver doesn't need to decode what the remote is sending, only that it is transmitting the modulated signal.

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