Electronic – If I drive a piezo sensor with switched DC, will I get back over-voltage

You clearly do not have enough gain.

In first attempt, you used MOSFET with V_gs(th) of 0.85 volt. This means the FET will not open at all as long as voltage is less than 0.85 volt.

In your second attempt, you have connected high-impedance piezo output to input LM386, which has input resistance of 50K. This put way too much load on the piezo -- they like the loads of 100K to 10M.

I advice you either get a real jfet (mot a mostfet!), or put an op-amp such as TL081/741 in front of your audio amp.

And I recommend to add two diode protection circuit -- see http://www.openmusiclabs.com/learning/sensors/piezos/ . Otherwise, if you drop the device on the floor, piezo might generate so much voltage it will burn out the amplifier input.

Duncan, rather than try to answer the question, I'm going to go on a tangent, inspired by my overwhelming belief that you are going about this the hard way. I speak as one who, many years ago, took a course in just this sort of thing under "Doc" Edgerton.

The first thing you need to do is replace the straw/dart combination with something with a consistent muzzle velocity. A pellet gun or BB gun is ideal. Trigger the electronics by shooting through two pieces of aluminum foil about a millimeter apart, and look for the two being shorted.

If you insist on using a battery rather than a power supply, start experimenting with the CD4000 series logic family. Otherwise, switch to a 5 volt supply and go with 74HC logic.

For logic, you can do something very simple, for instance

^{simulate this circuit – Schematic created using CircuitLab}

With a consistent muzzle velocity, you don't need a vibration sensor. You just need a variable delay from detecting the short. And you don't need to automatically trigger the camera, either. What you do is first, arm the circuit, then turn out the lights, then trigger the camera (on bulb, as you are doing), then pull the trigger, then release the camera shutter, and finally turn on the lights. You adjust the amount of balloon deflation by adjusting the delay from short detection to flash trigger. You can put the foil assembly close to the balloon, but just outside the field of view of the camera. This way you need a short delay, and this will have less jitter than a long one.

Trust me, this works fine.

EDITED TO ADD CIRCUIT DESCRIPTION - The first two gates form a set-reset flip-flop. Grounding either input for more than about 20 nsec will cause the output of that gate to go high, and force the other side to go low. When the ground is removed, the state persists. Just for fun, I added an "armed" LED to show that the circuit is ready to go. It will turn on when the circuit is armed, and will turn off after the projectile triggers the flash. A one-shot is just another name for a monostable (or monostable multivibrator, to use full terminology). Assuming you do the sensible thing and go to a 5-volt supply, the one-shot of choice is the 74HC4538, http://www.nxp.com/documents/data_sheet/74HC_HCT4538.pdf What my block shows can be produced by one half of a 4538 configured to trigger on a rising edge. Upon receipt of a rising edge the output will go high (or low, depending on which output you use) after no more than 50 nsec, and will remain high (or low) for a duration which is selected an RC pair. Depending on choice of components the delay can range from about 150 nsec to 10s of milliseconds. What I did not show on the block diagram, but which I suspect you'd use, is a second one-shot of fixed pulse width, triggered by the end of the first pulse, which would actually go to strobe. Maybe not, since I don't know how your strobe is triggered. If the strobe is edge-triggered the second one-shot would not be necessary.

If you do decide to go this route, be aware that, although you can use a panel-mounted pot (variable resistor) to vary the delay, the connections from the pot to the one-shot should be as short as possible.