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Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Why do you need the 555? What kind of coilgun is this for? You can use a phototransistor and an LED to make a photo gate. Use this schematic but instead of an MOSFET use your SCR.
Registered Member #4034
Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138
ldr's (light dependent resistors have a terrible response time, so i am inclined to think this would not work. just go for the typical phototransistor circuit. you could also use electromagnetic feedback coils, but id have no idea for a curuit for them.
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Like mattster_mac said, those sensors have crappy response time. You can find phototransistors inside VCR's quite often. Something else that might work is using an LED as the sensor. An LED will produce voltage when a light of the same frequency is shone on it. If you have two identical LEDs, you can use one to illuminate the other. This will produce a voltage and you could feed that to the base of a transistor to amplify it. When the projectile blocks the light, the transistor will turn off.
Registered Member #3792
Joined: Sun Mar 27 2011, 06:07PM
Location:
Posts: 136
If you like, you can use external interrupt type optocouplers (you can find them mostly in printers): seperate the two parts (photodiode - phototransistor and infrared LED), wire them with the correct polarity and you will have your trigger.
I have used this approach in my coilgun with excellent results.
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
comparators/opamps are very common. if you take a couple of things apart you'll probably find atleast one. Also, texas instruments and others will sample IC's for free. just make sure you get the dip or pdip packages unless you want to solder smt components.
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Optical 555 timer trigger
