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Registered Member #1397
Joined: Mon Mar 17 2008, 12:47PM
Location: Finland
Posts: 43
Hi all and I`m new here as you can see and first I want to say that I might speak a bad english because I`m from Finland. So my question is that what is the simplest way to use an optical sensor to detect the bullet and give to signal to SCR? I`d like to see somekind of scetch, one picture tells more than thousand of words Thanks!
Registered Member #514
Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295
Woah, there seems to be quite a lot of finnish people posting here!
Anyway, I think the simplest way would be to use a phototransistor like shown here:
When the gate is open the phototransistor pulls the base on the 2n2222 down, but when the photogate is blocked, the 2n2222 turns on and triggers the SCR. Nice, clean and simple.
Registered Member #1397
Joined: Mon Mar 17 2008, 12:47PM
Location: Finland
Posts: 43
OK, But I just remember that photodiode is faster than phototransistor? Anyway, Is the SCR in picture that SCR that is used to trigger capacitors to coil?Thanks!
Registered Member #1386
Joined: Tue Mar 11 2008, 08:16AM
Location:
Posts: 13
Common concern and kind of misconception for beginners. Yes, a photodiode is faster than a phototransistor. The difference is not in the scope of hobby coilguns. A phototransitor can react within 15 microseconds. React as in starting from ground and reaching to full applied voltage usually +5v. This speed howerver is based on the biasing resistor for either photodiode or phototransistor. That schematic shows a +9v and a transistor buffer meant to trigger at a far lower voltage than 9v but still a 100K resistor does make the responce time slow. Use a low value from 1k to 2k to attain that 15 microsecond responce time. Phototransitor datasheets spec out the responce times based on biase voltage and resistors. That and a scope measuring delay and rise times will show you that there is no need to question this again.
Your margins will be in the 100+ microseconds range even for fast 50 meters/second velocity. Your are covered.
Supporting image to post above. Its animated, so open in new browser.
[Edit: Double posting is not allowed, read this: ]
Registered Member #1386
Joined: Tue Mar 11 2008, 08:16AM
Location:
Posts: 13
Gunner wrote ...
So, does the "response" go to the thyristor`s gate and ground goes to cathode?
Yes and no. The diagram is not a completed circuit but more a concept and test harness for comparing and measuring responce times. You may connect directly but the SCR does not like low power control. The response should be connected to an amplifier of current and possible voltage. Amplifier would be a power transistor or mosfet driver.
Registered Member #1397
Joined: Mon Mar 17 2008, 12:47PM
Location: Finland
Posts: 43
OK...but I would appreciate if someone could "continue the picture above" and draw the amplifier and main SCR, because I dont understand exactly what are you meaning.Thanks again!
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Question from triggering 3-stage coilgun
Thanks!
