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Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
I put my detectors in front of the coils, rather than behind the coil to be fired. Because of this, a delay of maximum 5 ms is needed. I was wondering if this would work: 1) a LM339 comparator detects projectile, out put goes high 2) the output triggers a monostable 555 circuit, prolonging the pulse 3) The out put of the 555 connects to the IGBT that is to trigger the SCR. In parallel with the IGBT, a capacitor will be placed. The capacitance determines delay time. Once the 555 output goes high, the capacitor will show a voltage drop, and delaying the triggering until the capacitor is charged to the minimum voltage the IGBT needs. The gate threshold for my IGBT is 3.75V. My only worry for this is that a unreasonably large capacitor will be needed because of the low voltage.
Is this a reasonable method, or is there some other more sensible way?
Registered Member #511
Joined: Sat Feb 10 2007, 11:36AM
Location: Somerset UK
Posts: 55
Hi rp181
I would use an NE556 IC, this is simply two 555s on one chip. Configure both sides as monostables the first with an adjustable pulse length and the second with a fixed short pulse length. Then connect the output of one to the input of the other via an edge trigger. You will need to set up your photogate circuit so the output goes low when it detects the projectile because the trigger pin on a 555 is active low. Also include an edge trigger here so that the length of time the gate is blocked will not effect the time delay.
It should work like this, in the rest state the input to the first 555 is high and both outputs are low. When the projectile blocks the photogate its output goes low but the 555 input sees a short low pulse caused by the negative going edge. This will trigger the first 555 and its output will go high, at the end of the preset delay the output will go back to low, this negative going edge will trigger the second 555 to produce a short positive pulse at its output. You can use this pulse to drive your SCR through some sort of amplifier.
There are probably more elegant ways to do this using discrete components but I like the 555.
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
Sounds like a good idea, couple of questions. 1) whats an edge trigger? 2) how do i invert the output of the LM339? Ive done it before, but is was on accident. 3) wouldn't the second 555 trigger in the rest state because the input is low?
Registered Member #511
Joined: Sat Feb 10 2007, 11:36AM
Location: Somerset UK
Posts: 55
Hi rp181
1) whats an edge trigger?
I suggest you Google that one, you will learn a lot more. As an interface for the input to a 555 it can be done with 1 capacitor and a pull up resistor.
2) how do i invert the output of the LM339? Ive done it before, but is was on accident.
If you are using the LM339 to compare two voltages, just swap the two inputs.
3) wouldn't the second 555 trigger in the rest state because the input is low?
Good point, thats why you need the edge trigger because it only responds to a change of state.
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Delay timing circuit
