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Simple light detector

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Macwarrior

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Hey there,

I'm trying to build a simple optical control for my camera's cable release. Because of the way the release is set up, all it has to do is close a circuit - no power or anything. Light detection is ludicrously easy; turns out most photoresistors will do the job with nothing else added.

However, I'd rather have it detect darkness, so that I can trigger it with a laser or similar beam being broken. I'm sure this is also an incredibly simple circuit that you have requests for all the time...but I'm a designer, not an engineer. :). I've dabbled in electronics and built a few simple projects, but I've never had any formal training.

I think that something like is pretty close to what I'm looking for, except that I don't want any power running through the "buzzer" area for fear of blowing up my camera. AFAIK, a transistor to close the circuit is all that will be necessary.

I'm pretty sure that a PNP transistor is the only thing I'm looking for - they open the gate with low voltage (= dark photoresistor) on the base, right? - but I've never done enough work with transistors or ICs to know for sure. Perhaps a phototransistor connected to a PNP transistor would allow it to activate with no external power source?

Can anyone help me out?
Thanks a bunch.
MacW
 
hi macwarior, nice to see someone liked my circuit. i designed it (with some help) so im sure that id be qualified to help u with it :lol: . In the projects section, i have posted an updated version that works very well, and some more info on it. its called laser or IR beam alarm system.

to make it really simple, id just use a relay. ive never used any relays before but i know wut they do, and i dont have a clue how to draw one on the schematic, but im sure someone else will help u with that. an emitter resistor might be required (i dont know diddly squat about relays and power consumption). i think a relay would be best suited for wut u want to do, but i dont fully understand wut u r looking to do. i dont know wut a cable release is, but are u just aiming to have the camera snap a pic if the beam is crosed or somethin?

As for the resistor that says 300k to 1M, you will have to play with that value to find the sensitivity that you want. also, you can use a voltage other than 9v, 3v would probably work fine also, but you would have to play with that resistor value. i only used 9 volts to make the buzzer loud enuf. the higher that resistance, the less sensitive, and vica versa.

I'm pretty sure that a PNP transistor is the only thing I'm looking for - they open the gate with low voltage (= dark photoresistor) on the base, right?

nope. a PNP transistor turns on wen there is a negative voltage @ the base. and an NPN turns on wen there is a positive voltage @ the base. they're just oposite of each other. and there HAS to be a power source.
 

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zachtheterrible said:
i think a relay would be best suited for wut u want to do, but i dont fully understand wut u r looking to do. i dont know wut a cable release is, but are u just aiming to have the camera snap a pic if the beam is crosed or somethin?

Yep, that's exactly what I want to do - have the camera take a picture when then beam is broken. I can use the cable release to avoid mucking around with solenoids or opening my camera's case.

Basically, a cable release is just a connection (electrical in this case) to the camera, with a button that operates the shutter release (the button that takes a picture). It's on a cable so that you can trigger it without touching the camera, avoiding shake. Since the cable release for my camera is electrical (most are mechanical) I figured there's all sorts of remote-operation things I can do - optical, acoustic, piezo, computer-controlled, whatever.

Thanks for the circuit. I've been wanting a simple circuit that activates in the dark for quite a while now...this will be perfect. I'm a bit unfamiliar with the sumbols...the three lines in this case are the negative terminal, not ground, and the arrow is positive? It seems I can also replace the relay with an SCR or transistor for more flexibility?

In order to simplify the system, I think I'll end up buying a cheap laser diode pointer and disassembling it, then running it off the same circuit as the detector. I'll put it all in a box and put a mirror on the opposite wall...that ought to allow pictures to be taken in a room up to about 50 feet across.


Thanks again.
MacW
 
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