Hello,

I started a technology club in our local community and am doing a motion analysis workshop. Basically, a microphone picks up a sound from an event (balloon popping, firecracker, etc), triggers a flash and a camera with the aperture gets painted the scene. It makes for some very interesting pictures of things exploding, coming apart, etc.

I purchased a kit but for the life of me cannot get the circuit to work. The circuit basically shorts a disposable camera flash. I can get that to work by shorting to wires to the disposable camera.

I have attached the circuit diagram that came with the kit along with my breadboard.

Can anyone help me in figuring out why this will not short the disposable camera flash when it picks up a sounds?

I am new to electronics (software background), so any information on the circuit or advise would be greatly appreciated. Thank you in advance.

-Marty

Pictures Below

 

I don't see any attached circuit.

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They are up there now..

 

The SCR is not an ideal switch, as it is polarity sensitive and has a voltage drop. If you do not know the Camera circuitry you may have trouble. Have you tried swapping the leads from the cam to your circuit?

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There are two cameras involved, one is the camera taking the picture. This has nothing to do with the circuit because it sits with the aperture open waiting for the scene to get painted on.

The other camera is a disposable camera, you basically are just using the flash and that's it. This flashes camera can be trigger by simplifying completing a circuit.

I have two wires coming off the camera and when I put them together the camera flashes. So thats part good...

The goal is to have this circuit basically connect those two wires coming from the disposable camera flash when a sound is picked up.

 

I got that. Problem is putting the two wires together is just making a direct short with no concern to polarity. An SCR is not a direct short and requires current flow in one direction.

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Your objective as I see it is to emulate the shorting of two wires. An SCR may work, but it will depend on the direction that it is connected to the camera. If you try connecting both ways, one way may work and other way will not work. If you find that connecting either way does not work, then you might need another type of switch such as a relay or SSR.
Does that make sense?

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I think I get it..

Its because the SCR is a diode and if its the wrong way the connection will not be made?

Ill give that a shot.

Thanks!

 

Hmmm.. That did not work. Tried the SCR output both ways.

Marty

 

The circuit won't work if the piezo has built in circuitry. Connect the piezo directly to the 9V (Red to + & Black to -) and if it beeps then it is the WRONG kind. If it just clicks, then it is the correct type. The blue wire between the transistor and the SCR goes off the picture; is there anything connected to that wire off screen?

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To be honest this circuit has me a bit puzzled. The way the scr works is it needs a plus voltage applied to the gate in order to trigger the voltage. Once the scr is triggered, it will remain on until current is removed from the anode/cathode. So it would seem that after each flash you would have to unplug the circuit. Not very handy.

So considering that the scr needs a plus volt trigger, I then assume the transistor is biased to always be on. What puzzles me is to turn off the transistor the base needs to be pulled low. I am not familiar with piezo devices so only thing I can think of is that the piezo part has Piezoresistive properties and sound will cause it to pull base low. Not really sure though. Do you have part number for piezo part? Also do you have a meter?

I am hoping one of the analog experts will step in and bail me out here as I just think this circuit looks funky.

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hi Marty,
Looking at the picture of the piezo, it looks like a piezo sounder, as kchriste has suggested. If so it wont drive the transistor.

The SCR is triggered on its Gate by a +Vpulse/current, once in conduction it will remain conducting until there is insufficent holding current flowing thru the SCR or the flash voltage drops to near zero. At that time it will stop conducting until the next gate pulse.

You require a piezo element that produces an output voltage when sound vibrates the element, NOT a sounder where you apply a dc voltage.

Do you follow OK.?

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Eg and Kc, does not the piezo have to pull the Q1 base low in order to pulse the scr gate? If this is correct, do piezo's pull current? I only ask because I never thought of a piezo as being a sound active resistor. Also does this piezo property exist in all non sounder piezo devices?

Thanks

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Mike, a piezo element will generate AC voltage when stressed by sound waves. The "negative" peaks will turn off the transistor. The piezo in a BBQ igniter is basically the same thing except it is more rugged, you hit it harder, and it generates way more voltage.

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Thanks for clarifying. Permit me one more question.

What is the expected output of a piezo element? I am thinking millivolts, and if this is so, I would think the adjustment would be very persnickety and a multi turn pot would be needed. Based on the OP's photo, he is using a single turn pot. Would you not agree then (if my original assumption was correct)that this pot would make it difficult to acheive the right adjustment?

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