I need a pulse generator that will output a adjustable length pulse(between 1ms to 50ms). I have built a 555 monostable circuit using a 1uf cap for C1 and a 50K pot for R1, this will give me the range I need. I am using this to fire my multistage coilgun. I am using IGBTs for switching the coils. I have a few problems with my circuit though. When I power it up, a few moments later it fires. I have built a photodiode/IRled gate to provide a trigger pulse to the 555. I know that the 555's stable state is when pin2(trigger) is held high. I tried to use a NPN transistor conecting pin 2 to ground, with the pulse from the photodetector gate going to its base. This way when the opto sensor detects the projectile it will send a pulse to the NPN and inturn cause pin 2 to go low. Thats what I thought. The thing fires as soon as I turn it on. Any tips or comments? I am using the basic monostable setup for the 555.