I am trying to trigger a special strobe flash and another device by a momentary contact switch that is part of a sealed intelligent controller device. That is the really simple part, as when the controller issues the command and closes the contact, the flash fires. Now the interesting section.

The flash has a trigger voltage of 4.4 volts DC. For several seconds prior to the momentary contact switch closure (approximately 0.1 sec), the controller issues a series of rapid momentary switch closures at a rate of about 30 per second which does not cause the flash to fire. The flash will only fire once the momentary closure is of a long enough duration, probably because the flash has the circuitry to ignore the short duration switch closures. I think this is set up in this fashion to keep this special flash at peak charge right up until the moment of firing.

I would like to be able to know just when the pulse becomes long enough to actually fire this first flash, so I can use this as an output to trigger another device that doesn't ignore the short pulses.

I am hoping for a simple circuit that can ignore the short duration switch closures, and only complete this secondary circuit when the switch closure becomes much longer.

If anyone has some ideas it would be most appreciated.

Michael

 

Hi jmb,

I'm not sure from the description (what is the mysterious "other device"?) but unless there is a reason not to, I'd think the easiest solution would be to skip trying to detect the pulse duration change, and just use a photodiode to detect when the flash itself fires, and output the start signal from that.

If that won't work, then can you use microcontrollers? What about 555s?


Torben

 

Torben: Thanks for the initial feedback. The other device is sometimes another flash, sometimes a counter, or could be tied to a relay, but it doesn't matter, as every other device I try is being set off by the faster initial pulses.

I don't want a photodiode to detect the first flash, I am trying to sync the two events via hardwire. I have used the 555 chips before in some simple timing circuits, and don't know just how that might be used to detect when the pulse goes to a longer duration. Can you describe it in more detail for me?

Simpler is definately better, and I would also like to try and find a way without using the 555, if that is at all possible.

Again, thanks all in advance to all for your input and possible suggestions!

Michael

 

I hope this makes sense.
I believe that the initial short pulses can be ignored by placing a low pass filter between the pulse source and the secondary device.

 

OK, I figured that you might have considered the photodiode idea and dropped it for some reason. I'll take a shot at a circuit for it.

Mine does use a 555 but is still fairly simple. This works (at least in the simulator) for input pulse widths up to at least 15ms and down to at least 1ns. However, the width of the firing pulse must be at least 2ms. It doesn't matter how long it is as long as it's at least 2ms.

Anyway, I'm sure that there are other solutions, and I am seeing if I can come up with a simpler one tonight. If I do I'll post it, but likely one of the local gurus will beat me to it.


Good luck!

Torben

Attached Images

flash_trigger_1.png (18.5 KB, 28 views)

 

hi torben
i need a good simulator to test my circiuts. i see you're using one. is there any which you would recomend . preferably freeware
thanks.

 

Torben: Thanks for the help! I will gather the parts and try it, as well as shirazmacuff's suggestion of a simple low pass filter. I will post a go-nogo as soon as I can test either one (I have to go borrow the controller again once I am ready for the test).

Michael

 

Hi Shiraz,

I'm using LTSpice http://www.linear.com ; it is free and comes with a large built-in library of models and there is an active Yahoo! group for it with a good forum and a large library of extra models and example circuits.


Torben

 

thanks so much i'll try it out