Could anybody please help me with this project.

I've got a LM324 OPAMP connected as a Comparator circuit with a LDR. The circuit works well as it is, but I would like to connect a Flash Gun to it.

I'm told I need to use the CP106D SCR but have not a clue how to connect it to this circuit.

I've attached the datasheets.

Any help will be appreciated/advice. Many Thanks - JDR04

 

Hey, why are you using old SCR as there are many semiconductor devices are available?

 

Make a suggestion.

 

Post a link to your flash gun specifications if you can, or at least give is a model number.

 

Make a suggestion.

I am not so old in electronics to suggest, but i am asking why is there any spefic need or use of using SCR's??

 

Hi Ron H, thanks for your reply. I've got the flash user manual so I'll try and get more details on the flash for you. In the meantime, its a METZ MECABLITZ 58 AF-1 C DIGITAL. The C is specifying that its for a a Canon EOS model camera.

Thanks again JDR04

 

I'm also new to electronics. I was told the SCR was best suited to handle the high voltages from the flashgun??? Thanks - JDR04

 

Ron H, had a look through the specs in the manual. There are no voltages or currents quoted. I could perhaps email support and ask them - how critical is it?

All I can tell you is it needs 4X 1.5V AA batteries. - JDR04

 

Ron H, had a look through the specs in the manual. There are no voltages or currents quoted. I could perhaps email support and ask them - how critical is it?

All I can tell you is it needs 4X 1.5V AA batteries. - JDR04

I couldn't find a manual online.
What port are you intending to connect the SCR to?
I find it hard to believe that they would not provide a low voltage, low current trigger input.

 

I cant find any other specs anywhere. The flash only has two ports, one is for an external power pack and the other is to download software updates.

I'm thinking the way to do it is the same way when you taske the flash "off camera" to avoid red eye etc. You have to use a hot shoe connection that either connects to the PC flash sync terminal on the camera or a hot shoe that connects to the cameras hot shoe?

I see a while back you posted a circuit that uses a SCR connected to 2N2222 trannies, could something like this be done?

I dont know if it would help, There are 5 pins at the bottom of the flash gun. Whatever is connected, will have to be connected via some of these pins, I think.

Cheers and thanks for your time - JDR04

 

I'm led to believe that only two of the pins are required for triggering a flash. In this instant, the other pins are for through the lens metering, zoom control in the flash etc etc. At this stage I'm nopt worried about metering and zoom control.

Can anybody provide clarity or any comments??? Thanks - JDR04

 

I'll put together a schematic that will use your existing circuit (with some modifications) to drive your SCR, but obviously, I won't guarantee that it will trigger your flash, or not be damaged, or not damage your flash unit.

 

OK, here it is. Note that the op amp inputs are swapped in order to make the SCR trigger when light increases. If you want it to trigger when light decreases, let us know. You can't do this by simply swapping the inputs.
Also note that the unused op amps are connected in a manner which will keep them from oscillating or upsetting the biasing circuitry.

 

the circuit looks fine. the only thing needed is reference to flash circuit (probably just connect negative terminals if the SCR is part of flash)

 

Thanks Ron H. I understand most of the circuit but not all. Could somebody explain the function of R6 please.(Is this to set some sort of amplification?)

Just two other points, I will need to power the circuit off a 9V battery so its portable and not 15V. I will also need to have it detect Darkness rather than Lightness (sorry) if that makes any sense. Can this be done by swapping U3 and R5 around??

Thanks a lot for your time and input, it's really appreciated. JDR04

 

Can this be done by swapping U3 and R5 around??

you can reverse circuit operation by swapping those two components

 

Thanks Ron H. I understand most of the circuit but not all. Could somebody explain the function of R6 please.(Is this to set some sort of amplification?)

Just two other points, I will need to power the circuit off a 9V battery so its portable and not 15V. I will also need to have it detect Darkness rather than Lightness (sorry) if that makes any sense. Can this be done by swapping U3 and R5 around??

Thanks a lot for your time and input, it's really appreciated. JDR04

R6 adds hysteresis. I habitually add it to comparator circuits, to prevent jitter and/or bounce and/or oscillation at the switch point. In this case, it is probably not necessary. The SCR will either switch or it won't, and once on, you have to interrupt the current to get it to turn off, i.e., the gate voltage no longer matters.
You are correct about swapping the LDR and the pot.

 

Concerning electronic diagram posted above,

CP106D has positive peak gate current about 0,2A, what is too much for the LM324 operational amplifier. So, I do not recommend the above electronic diagram.

But why are you required to use a thyristor? Is much easier is to use a MOSFET.

 

The CP106D has an IGT max of 200 μA, not 200 mA.

 

The idea is, to put SCR ala the moving, you need something more than 20mA as a support of the LM324 operational amplifier. That I wanted to point out or draw attention.

Later edit: Note that since the beginning of the datasheet is wrong: Max IGT of 200 μA

Look below: