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:

Those are the absolute maximum ratings.
Look at the Igt spec. It is 200uA.
If gate current exceeds 200mA, the device may be damaged.

 

I agree with this: If gate current exceeds 200mA, the device may be damaged.

I don't think that SCR works with 200uA gate current. I tested practically and it is not.

 

I agree with this: If gate current exceeds 200mA, the device may be damaged.

I don't think that SCR works with 200uA gate current. I tested practically and it is not.

Did you test that exact same part number (TICP106)?
BTW, my circuit is providing about 13mA of gate drive.

 

Did you test that exact same part number (TICP106)?
Yes.

BTW, my circuit is providing about 13mA of gate drive.
If you refer to the circuit previously posted, I noticed that but it's about the limit. Most likely it will work quite warm that LM324, therefore we recommend a solution MOS-FET. So, I think, is much better with MOS-FET.

 

Thanks panic mode.

 

It seems the part number is CP106D, somewhat difficult to see.

So, which way do I go now,SCR or MOSFET?

 

BTW, my circuit is providing about 13mA of gate drive.

The Bourns datasheet indicates 5 μA typical and 200 μA max. Why drive the gate so hard?

 

The Bourns datasheet indicates 5 μA typical and 200 μA max. Why drive the gate so hard?

Good question. I guess I picked a value before I saw the datasheet, and forgot to change it.

 

It seems the part number is CP106D, somewhat difficult to see.

So, which way do I go now,SCR or MOSFET?

Having no idea what the spec on the flash unit is, it's hard to say. What is the purpose of this project?

 

I'm wanting to use the circuit to fire a flash gun. The LDR will detect a shadow etc and then fire the flash.

Just a thought, would it be possible to use an optoisolater? The output from the LM324 turns on the optoisolater which then allows the flash to fire or would the voltages be too high? If this could work which one would you suggest?

Thanks again for your time - JDR04

 

Don't most of these flash units fire by sinking a trigger line to ground? I think I read that somewhere.

 

I'm told that only two of the pins are used to actually trigger the flash. The others are for through the lens metering, motor control on the zoom flash head etc etc. I think Ron H has a point in that we don't know enough about the flash so I'm going to check it out and try and give you guys better info to work with. Thanks again JDR04

 

Thanks for your patience guys. I've used a different flash for fear of damaging the expensive one.

The flash (Canon 300EZ) fires when two of the pins are shorted out. When fully charged, the voltage at these two pins is between 4.03Vdc and 4.06Vdc.

When the two pins are shorted and the flash fires, the voltage drops down to less than 200mV.

These voltages seem to agree with what I have read are safe voltages to use on the camera when using external or sync cable connected studio strobes/flash. Voltages shoul;d not be above 6V.

Does this help?? Maybe using a optoisolater will be sufficient???? Any comments - JDR04

 

Do you know how much current flows through the switch? Where did you get this information?

 

Dont know about the current. I took the measurement off the two pins that fire the flash when bridged. Put meter probes on them and fired the flash with its test button. I tried taking a current measurement but could not get anything??

What do you think? -JDR04

 

What do you want to happen after the flash has fired once? Do you want it to flash again if, after the illumination increases, once again decreases to the threshold? Or do you want to require a manual reset after the flash fires? What if, when you turn on power, the illumination is already less than the threshold?

 

Once the flash has fired, once, then it should charge up and be ready for the next activation.

If the illumination is less than the threshold on switch on, then the flash should fire. But..... it would be up to me to set the threshold before "plugging in" the flash. Perhaps a small led indicating the triggering point on the LM324 output? Many Thanks -JDR04

 

I thought you guys might be interested to know.

I hooked up a SFH618A-2 optocoupler to the LM324 output via a resistor. Connected the flash via the two triggering pins and the circuit seems to work so far. I've only triggered it about 20 or 30 times and there's no smoke yet......nor is anything warming up!!

I have noticed that when I adjust the LDR sensitivity via the 10K pot the triggering becomes jittery at a certain point. Is this what Ron H meant by adding a resistor to give hysteresis ie it would be avoided??

Comments and advice always welcome.Thanks a lot - JDR04

 

I thought you guys might be interested to know.

I hooked up a SFH618A-2 optocoupler to the LM324 output via a resistor. Connected the flash via the two triggering pins and the circuit seems to work so far. I've only triggered it about 20 or 30 times and there's no smoke yet......nor is anything warming up!!

I have noticed that when I adjust the LDR sensitivity via the 10K pot the triggering becomes jittery at a certain point. Is this what Ron H meant by adding a resistor to give hysteresis ie it would be avoided??

Comments and advice always welcome.Thanks a lot - JDR04

Yes, the hysteresis is there to avoid jitter around the trigger point. When the darkness threshold is reached, the output goes high. Some of this output voltage is fed back, raising the threshold. The illumination will have to increase by some amount in order to cause the output to low again, restoring the threshold to the original value.

 

Use a MOSFET, like IRF540, is more than enough. And if you use this IRF540 should not use an optocoupler because is not necessary.