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The Final Hardware
And here is the final part of the entry, starting with the schematic that I finally used. The core is the same as before. But now there is a linear voltage regulator for the controller. Also shown here is the serial interface and most importantly, the sensor circuit.
The sensor circuit consists of two parts. First, the reference voltage on PB1 is generated with a variable voltage generator. The voltage can be varied between 2 and 3 volts. The second part is the signal input, also a voltage divider at 2.5 volts and the transducer. I adjusted the reference voltage so that the LED just stops flashing without any input activity.
There is no voltage regulator on the LED. The circuit has to be operated with 7.2 volts.
Everything works. The main improvement would probably be on the sensor. It has to be very sensitive, and this leads to false triggers. E.g. switching the room light on or off trigger it. Perhaps an amplifier would help. As a work-around I set the dead time parameter to four seconds, so it would only trigger once per photo.
Other sensors could be interesting, like a sound trigger.
To improve the brightness you could use several LEDs in series. You'd have to increase the operating voltage of course.
I was able to take a few really interesting pictures. It is quite amazing how water behaves and the stroboscope makes it possible to explore that. I put a few pictures on flickr and there will be more. It is quite useful to look at some howtos on water drop photography. And of course you can experiment quite a lot.
The software is written in AVR studio assembler and can be downloaded here: Source