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Forty

Thu Aug 18 2011, 04:50PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

after building two sets of photo gates that can slide along a slotted barrel and be fixed in place with set screws, I decided I never want to do that again (it would also be near impossible to adjust while within a portable coilgun.)

So i looked in to some simple delay circuitry so that i could fix and photo gate a good distance away from the coil, and adjust the delay between the trigger signal and the coil firing.
After some searching, I found these two designs:

This one uses a 555 timer and a JK flip flop. A 1M trimpot varies the length of the one-shot pulse from the 555,
and the flip-flop triggers on the falling edge of the wave, causing it's outputs to switch.

This one uses two 555 timers and has an output that triggers instantly, and a second output that triggers after a delay,
again, tuned by a 1M trimpot.

The inputs for both of these circuits are just the output from a led + phototransistor pair, and the output from the delay circuits would simply be used to signal the gate of the firing SCR. Both circuits are designed to reset themselves shortly after the triggering event.

i'll try to run some numbers today and see if i can't simplify the ranges on the component values (because I don't have any 1M trimmers) down so that the delay time always lies within a reasonable range for coilgun use. I'll also look in to replacing the dual jk flip flop ic because only half of it is used, and i don't have any of them anyways (only have d-type flip flops)

surprisingly, no questions from me so far. I just wanted to share a triggering concept I haven't seen used before in coilguns.

Benj

Thu Aug 18 2011, 06:11PM

Registered Member #4050 Joined: Sun Aug 07 2011, 04:06PM
Location: France
Posts: 17

Hi, with some help I "designed" this optical triggering circuit, more or less similar to the one you found

Do you think it would work?

Forty

Thu Aug 18 2011, 07:03PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

Can't see the circuit without login. could you copy it from there and post it here?

Benj

Thu Aug 18 2011, 08:28PM

Registered Member #4050 Joined: Sun Aug 07 2011, 04:06PM
Location: France
Posts: 17

Here it is.

Forty

Thu Aug 18 2011, 09:22PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

hmm i like that it uses smaller trimpot values, but does each 555 really need one?

Benj

Thu Aug 18 2011, 09:45PM

Registered Member #4050 Joined: Sun Aug 07 2011, 04:06PM
Location: France
Posts: 17

Well, the values can still be changed if needed ( as it depends on how fast the projectile is ), since the duration T of the pulse is given by:

T=1.1*R*C

The first NE555 allows you to delay the pulse. Yet, if the projectile is fast enough and the optical bridge close enough to the coil, there is no need to .

I was thinking about using the first NE555 to power the SCR, and the second one to power the other SCR in the V-Switch circuit (have a look here

).
Have you ever tried to build one? V-Switch+optical triggering seems to be a good way to improve coilgun's efficiency.

are the pull-up resistors ok? (location, value...).

Turkey9

Fri Aug 19 2011, 03:02AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Personally, I would use a microcontroller for the delays. Much simpler when it comes to the actual circuitry and delays are super simple.

PSCG

Fri Aug 19 2011, 11:15AM

Registered Member #3792 Joined: Sun Mar 27 2011, 06:07PM
Location:
Posts: 136

@Benj: Is it possible to use only one NE 555 to delay the pulse? And if is, can i replace the optical sensor with a coil to sense the projectile movement (as explained here)?

Benj

Fri Aug 19 2011, 12:03PM

Registered Member #4050 Joined: Sun Aug 07 2011, 04:06PM
Location: France
Posts: 17

Hi pscg! Using an only 555? well, how would you delay the pulse then?
My circuit works like this:
-The projectile makes the comparator's output switch from +12V to GND, thus triggering the first 555 ( because a NE555-based monostable is triggered when a falling edge comes on the trig pin).
-You can choose how long the first monostable is on by using the potentiometer.
-After T=1.1R*C,there is a falling edge on the 555 far right.
-This 555 turns on, powering the scr.

So, if you use only one 555, you won't delay anything ( The projectile passes the sensor ( optical or inductive)===>the scr is triggered right away ).

I think an inductive sensor would also work. You should have a look on the website i talked about in my last post: the guy used a simple inductive sensor to trigger a scr. I don't know if it is as effective & accurate as an optical gate.

just one thing: I'm not an expert ( far from it ), i'm a real begginer in optical triggering and "serious" coilguns.

Forty

Fri Aug 19 2011, 03:38PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

Hmmm. well i imagine the 555 triggers on the falling edge after the voltage crosses one of its internal comparator voltages (2/3 or 1/3) so perhaps a capacitor could be placed on the external comparator so that it is charged to 12v, and then discharges through a trimpot once the comparators output switches.
So the delay would be set before the 555 triggers, thus allowing a single timer to be used.
sound accurate?

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