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Old 05-27-2013, 04:24 PM
wvmarle wvmarle is offline
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Default Light activated sound

Light activated sound circuit - it's been done before, I know. What I have in mind is not that easy, and I'd like to hear suggestions on what to look for.

What I want to build is a contraption that produces a sound (preferably something like an icy scream, as if from a ghost) that lasts maybe 2-5 seconds, staring the moment the light of a torch hits the sensor.

It has to work in the dark only. It is supposed to be hidden (part of a geocache), giving away its location when people shine on it with a reasonably bright torch.

Further restrictions: it's got to be battery powered, preferably 3V (I am thinking of using C or maybe D cells - AA batteries would be preferred for size), no more than 6V. Low power use, particularly when sleeping (it'll be sleeping all the time - except that rare occasion of someone coming by at night shining a torch on it). Batteries should last at least a year, maybe two, when just sleeping. Circuit should be as simple (and robust) as possible.

The first problem: how do I prevent it from screaming during the day? Sunlight should not activate it. It has to work only at night. When it's dark out there, and the torch is the only light.

I am thinking using two LDRs - one pointing towards where the torch light will come from, a second one pointing in a different direction, to switch off the circuit when the sun is shining, but not sure how to go about it.

The second problem: how to produce a sound lasting 2-5 seconds, when the light sensor may be lit for far longer or shorter duration.

Timing can be done with a 555 I suppose. Any simple circuit (or IC) that can produce a screaming sound?
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Old 05-27-2013, 04:52 PM
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GopherT GopherT is online now
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Use a mono stable 555 circuit (3-5 seconds) to turn on/off an astable multivibrator 555 circuit that creates the scream frequency.

Your idea of two LDRs is good. another way would be to use a capacitor to couple the LDR to the monostable LDR circuit - that way, only a sudden change in brightness will trigger the monostable 555.

Instead of using two 555 chips, one 556 has two 555s inside (although they are not always immediately available.

If you really want to go low power, you should use a Microcontroller. Getting a loud enough scream from a 3 volt source may not happen. That may be the tough part, unless another AAC member knows of a buzzer/ horn device that make reasonable volume - the just add power type.
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Old 05-28-2013, 03:39 AM
wvmarle wvmarle is offline
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The low power part is primarily when sleeping: it should switch off and use as little current as possible at that moment. The power use when producing sound is less important.

It is supposed to be installed in a dead silent environment, so volume shouldn't be too high anyway.

Do you have an example diagram of the capacitor-coupled LDR? And this should also only work in the dark; not that when during the day say the sun shines on it but due to wind movement it is occasionally in the shadow of a leaf, triggering the circuit. So need some kind of sensitivity to set.
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Old 05-29-2013, 03:36 PM
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If you cannot find a scream chip, program a SN76477N, complex sound generator.
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Old 05-29-2013, 03:48 PM
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Dodgydave Dodgydave is offline
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Make your own recording on Ebay.

just add a photo switch circuit.
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Old 05-29-2013, 04:06 PM
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The, "sudden change" idea is a very good one. It would be a much simpler circuit than, "When the light is on, but not that light".

As usual, this should be designed backwards, from the output to the input. You come up with the sound circuit and then design the driver that responds to light and turns on the sound circuit. I can do a sudden change circuit but it will only be a concept. It will have to be modified to drive the sound circuit.
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Last edited by #12; 05-29-2013 at 04:13 PM.
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Old 06-13-2013, 03:21 PM
wvmarle wvmarle is offline
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Thanks for the replies.
I've finally gone a different direction: having it react to a remote control, using an RC receiver. It was just too tricky to do with light; sudden changes happen in nature (it's supposed to be an outdoor installation) and the low light intensity of flashlights just wouldn't trigger reliably.
So one bistable 555 to switch the rest on and off; one monostable 555 that listens to the TSOP31238 IR receiver and in turn switches on a sound circuit (which I'm now going to redesign as I am not happy with what I got first).
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