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Registered Member #1694
Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108
Hi,
I've made myself an infa red switch with a LED and a photodiode. It uses an LM311n to detect the change in voltage across the photo diode as the projectile passes through. It works well, except . . .
When the projectile goes through, the 311 puts out a pulse of negative potential . . . electrons flowing out. Ya can't turn on an SCR with negative potential. I can't find any LM311ps, so thats not really and option, and I have half a million 311ns floating around so I would prefer to use them if I could.
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
Pretend the out put is positive. How would you wire the SCR? Now just swap the gate and cathode leads. Positive will connect to the gate lead, and the lm311 out connects to cathode.
Registered Member #1819
Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137
big5824 wrote ...
could just use a NOT gate
It doesn't matter. What if the NOT gate puts out a negative voltage on a high output? You can use a PNP transistor, with the collector tied to the gate of the SCR.
As for the gate current, you should limit it to a short pulse of about 5 to 10A.
Registered Member #1694
Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108
Nah, the output is hi when the + input is highest, and its low when - is higher. Swapping the two inputs around would the output hi until the projectile passes through the detector.
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Infa red trigger
