Need help understanding sequential photo-flash trigger circuit

David, yes it appears that this circuit ignores subsequent triggers for three seconds. C1 and R3 establish this time constant. It is a non-retriggerable one-shot.

I ran a SPICE simulation to be sure. It takes a -0.4V or greater trigger.

Thank you so much for the reply and answer to my question!

(Sorry for the delay in responding, I haven't had internet access for two days ... and still don't have regular access, unfortunately.)

This SPICE simulation has me intrigued (never knew that existed and am also realizing I probably incorrectly tagged this thread as being about PCB DESIGN - sorry 'bout that.)

There are about three other questions that come to mind:

1) Would it be possible to modify one of these circuits to where there is no set delay between triggering each subsequent flash, but instead each is triggered in succession based upon subsequent trigger signals from the camera?

Actually, if that is possible then the other questions are not relevant, so I'll leave it at that and keep my fingers crossed.

It would be much easier to design a circuit to do what you want rather than the modify this existing circuit.

I would use a 4007 CMOS decade counter as the heart of this circuit. I'll post a schematic. Do you want the same -1 volt trigger capability as the old analog circuit?

I would try and convert this to modern day digital techniques and using PIC16F88, 16F84A dependant on how many time you want this to trigger At least that way you are to all intensive purposes able to control your rate of flash and also time delay you could also control continued flashing sequence

In other words dependant on how many frames you want to capture, This again you could control within your software also using the interrupt to start the flash sequence off

This could be controlled on Port A for example via Led detector or LDR coupled to transistor pulling the port pin of your choice from High to Low or visa versa depending on PNP or NPN substitution initiating an interrupt routine such that port B attached to SCR’s would then control camera flash using right shift or left shift bit movement through PORTB calling a delay sequence increment in us , ms , or seconds via keypad increment or decrement attached to PORTA after each bit shift

Wouldn’t this be far better and simpler in terms of how much control you would have over this I can think of a very neat nice design at the moment which could be quite attractive to camera enthusiasts?

Hope this helps you

Mark Harrington

www.harrington.force9.co.uk

David, that would be fantastic .... thank you so much!

Mark, thank you also for helping me on this. What you've described though sounds way out of my depth, and I really like the idea of keeping it simple. Also, the original circuit was not ideal in that I would be having to sync the rate of flashes with the camera's frame rate through trial and error. Making the flashes operate based upon the trigger from the camera is the way I would actually prefer to have the unit function.

The same trigger voltage should work fine. I know that it cannot be more than 6 volts, as the camera shutter can be damaged by flashes that operate with a trigger voltage higher than 6 volts. The flash units in question use a trigger voltage of around 3 to 4 volts. I can measure the voltage if needed. (I'll measure them just to see what they are and post back)

Once again, thank you for helping me with this! It is truly appreciated.

Attached is a sequential photoflash circuit using a 4017 decade counter.

The trigger is a negative going of 1 volt or more.

The SCR's are 400 volt sensitive gate types.

Here is the corrected schematic.

Wow, that is amazing David! Thank you so much for putting that together for me!

I do not understand some aspects of it at the moment. However, I'm going to run my questions by someone I know that has electronics experience to keep them to a minimum here.

Thanks again ... you are very, very kind!

(the voltage on the flashes measured 4.46v)

David,

It seems that I've been confusing the flash trigger voltages with the camera's sync pulse.

I did not realize there was a voltage coming from the camera at all. It seemed to me that all that was happening was the camera was completing the connection, like a switch, for the voltage that the flash carried.

I've measured the voltage on the camera's sync terminal and it is -0.5 volts. This was with a hand-held meter, so I'm not certain if there might be a transient pulse higher than -0.5 volts which occurs at the beginning of the pulse and is too quick to see with this meter. It was tested using the bulb setting on the camera (shutter stays open until the shutter button is released), and the voltage went to -0.5 volts and stayed there for as long as the shutter was held open.

My apologies for confusing the voltage you were referring to when asking if -1 volt would work. I'm guessing that it may not work as it is right now?

Hi David,

I'm wondering if the capacitor near the camera sync connection is there in order to produce a "de-bounced" signal?

David L, can you obtain an oscilloscope to take a look at the camera sync pulse?

Unfortunately there isn't one available to me. I'm going to try building it as you have designed it and hope for the best.

So long as there isn't anything like a greater than 6 volt charge put across the camera's sync contacts. I'm guessing that it would be isolated due to the capacitor that is there.

Still very thankful that you posted that in the first place ... very nice of you to do that!

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