After reading Robert`s article, I immediately wanted to build this project so I ordered an HMC 1001 sensor and started breadboarding the circuit. As it turns out, there were a couple errors in the schematic that appeared in Sport Rocketry. I took Robert`s design apart and did a bit of redesigning to make it work. There has since been a correction
magnet - schematic

published, but I had already come up with a much simpler circuit that works perfectly. Instead of trying to amplify then compare the outputs, I simply used a single op-amp as a comparator right off the magnetic sensor. Like the original, this drives a Field Effect Transisor to fire a flashbulb (or e-match) circuit. I also found that the voltage regulator really isn`t neccessary. The components are rated for up to 12 volts, so a 9 volt battery could be used to power everything without having the 5 volt power supply from the regulator. Be cautious as the HMC 1001 can be damaged if it sees more than 12 volts. According to Honeywell, the circuit performs better with a 5 volt supply, so I have left the regulator in this schematic. Another option on the power supply is to use the 544 battery. This is a small 6 volt battery found in the photo section. The battery is a little smaller than an "N" cell and measures about 1/2" diameter and 1" long. They are also available in Lithium. I built a circuit without a voltage regulator using a single Energizer 544 battery. It was able to fire the smaller flashbulbs taken from flash cubes very well, but didn`t have the power to handle the larger AG-1 flashbulbs. Of course it was easy to simply replace that battery with a standard 9 volt and it will fire the AG-1 bulbs just fine. I haven`t done any testing with electric matches or any other type of ignitor. For the serious weight conscious...

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