Audio Camera Flash Trigger help

[newcube]

Hi All,

I'm an absolute rank amateur at electronics (although I did A Level physics 20 years ago!) but a keen photographer and thought it'd be good fun to build an audio flash trigger to capture balloons bursting, etc.

I started with this:

direct_piezo.png (8.69 KiB) Viewed 13192 times

but the piezo disc was nowhere near sensitive enough, only rapping it with a pen would cause the flash to fire.

My second attempt added an 741 op amp to try to boost the piezo signal, but the same problem remains..

piezo_opamp_trigger.png (12.69 KiB) Viewed 13192 times

I'm awaiting a delivery of an SCR that I'm intending to build this with:
http://hiviz.com/tools/triggers/makeown.htm#st

I assume the SCR is more sensitive to the piezo disc.

Can someone cast an eye over my circuits and let me know where I might make improvements to the sensitivity of the disc, or will I need to wait for the SCR?

Thanks in advance,
Nic

[pebe]

Hi newcube,

Looking at your 3 circuits in turn,

1. The piezo is open cvircuit as far as DC is concerned, so the 2N2222 has no forward bias. So base current would not flow unless the piezo volts exceeded 600mV. If the piezo output was high enough, the transistor would even then only turn on during spikes. It is likely they would not be on long enough to operate the circuit. It may improve things to fit a 100nF from collector to emitter to hold the collector low between spikes.

2. The outout of the 741 (pin6) needs to be normally at less than 0.6V so it does not trigger the 2N2222. As pin2 will follow pin6, it also needs to be <0.6V. But the 741 needs pins2 & 3 to be at least +1.5V relative to pin4 - so the circuit cannot work.

3. In the circuit from your link, the 2N2222 is normally held in conduction by base current through the 68K. So its collector is low and the SCR is not triggered.

When the piezo is struck, negative spikes will turn the 2N2222 off, allowing the SCR to be triggered via current through the 5K pot.

Start of with the 5K to max resistance, and reduce it till you get the trigger level you want.

I think it should be OK.

[pebe]

Hi newcube,

Looking at your 3 circuits in turn,

1. The piezo is open cvircuit as far as DC is concerned, so the 2N2222 has no forward bias. So base current would not flow unless the piezo volts exceeded 600mV. If the piezo output was high enough, the transistor would even then only turn on during spikes. It is likely they would not be on long enough to operate the circuit. It may improve things to fit a 100nF from collector to emitter to hold the collector low.

2. The outout of the 741 (pin6) needs to be normally at less than 0.6V so it does not trigger the 2N2222. As pin2 will follow pin6, it also needs to be <0.6V. But the 741 needs pins2 & 3 to be at least +1.5V relative to pin4 - so the circuit cannot work.

3. In the circuit from your link, the 2N2222 is normally held in conduction by base current through the 68K. So its collector is low and the SCR is not triggered.

When the piezo is struck, negative spikes will turn the 2N2222 off, allowing the SCR to be triggered via current through the 5K pot.

Start of with the 5K to max resistance, and reduce it till you get the trigger level you want.

I think it should be OK.

[newcube]

Thanks for the info. Knowing why components are used is extremely useful. Do you know any place on the web that explains circuits in this way (you should teach this stuff!)

I managed to get the third circuit working http://www.flickr.com/photos/newcube/6766304151/

[pebe]

I'm glad the circuit worked for you.

Regarding help understanding circuits, this site may be a good place to start.
http://www.allaboutcircuits.com/
It starts with basics and progresses from there. Some parts you may want to gloss over - for instance the bit about treatment after an electric shock, although important, does not really belong in a book about circuits.

Also, personally I don't think you need to have mastered quantum physics or know how a semiconductor is doped, to understand how semiconductor devices behave. They are best treated as black boxes - as long as you know what they can do and what their limitaions are, you don't really need to know what goes on inside them.