i think you're better off with a pir sensor like the unconventionnal airsoft one , an infrared one would require a mirror on the other side to reflect the ir beam to a phototransistor, and i dont think infrared would work well on a sunny day! you could also do it using a laser and a phototransistor but you will have to align the laser very carefully with a mirror so the beam reflects on the phototransisor, then you rig a relay and a mechanical actuator to trigger the shell.
member of fire team NOM. my other regular forum: www.toyotanation.com
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