I am interfacing my Pi via I2C to an analogue to digital IC on the Quick2Wire analogue board (pity Q2W isn't being supported as well as it was but that's another topic for discussion...). I have the board working fine, it all tests OK and detects voltages as it should.
The real issue is what I am connecting it to. I am intending to automate a tennis-ball launcher that works like a potato cannon. In particular I am using a camera flash connected to a car ignition coil pack to produce a spark to ignite the fuel, see:
http://www.spudfiles.com/spud_wiki/inde ... mera_flash
to get an idea of what it is I am doing.
The flash I am using at the moment produces a good spark but what would be good is to be able to detect when the unit is fully charged. Since the unit uses an LED to show when it is charged in its normal use I would like to detect when the voltage across the LED reaches a threshold so that I can tell the Pi that it is ready. Seemed to make sense to me. I found that the voltage is AC (not much use) so I have rectified it to produce a DC voltage that the ADC can detect. The problem is that using my multimeter I found that there is a voltage spike when I trigger the flash to spark. Normally the rectified voltage is about 1.5V when the unit is charged (which is well within the ADC range) but leaps to 100+V when the unit is discharged
Now obviously I shouldn't attach this rectified voltage to my Pi even with the diode protection on the break-out boards. So... I have tried placing an inductance coil in series with the output of the rectifier to block the spike and also put in a zener diode in parallel to clip the voltage at 3.3v but still to no avail. I can only assume that the diode simply doesn't switch fast enough. Is there any way to make sure I don't get these huge spikes so I don't fry the Pi? The only route I can think of is to ditch the attempt and use an LDR to detect when the LED is fully lit and read off that, or to cut off the connection to the LED using a relay so the voltage spike doesn't reach the ADC at all.
Any help and tips would be gratefully received
P.S. I teach physics and like electronics so I do have a good knowledge of circuits but I am by no means an expert!