I'm experimenting with circuits to trigger my camera shutter and camera flash. My plan is to build a 2-stage trigger for high speed photography that triggers the camera a fraction of a second before an event like a balloon popping, and then a second trigger for the flash using sound, vibration, breaking an "electric eye" light beam, etc.

Both cameras and flashes use a 2-wire trigger that applies a DC voltage. When the 2 wires are connected with a near-short, it triggers the flash to fire/camera to take a picture.

Different cameras and flashes use widely varying trigger voltages. Older flashes sometimes use trigger voltages as high as 300 volts, although those are uncommon in modern flashes. It looks like the phototransistor is rated at 250VDC max, so it would probably be destroyed with a flash who's trigger voltage exceeds that level, but that's fine. Better to sacrifice the optocoupler than burn out my controller or my camera.

So far I'm using analog electronics for my trigger circuits, but I plan on using CMOS logic circuits (run at 5VDs) built around a micro controller. The microcontroller will allow me to add things like variable delay, enabling/disabling the whole setup, sequencing the triggering, quenching LED room lights just before opening the shutter, etc, etc.

The circuits I've built so far use a low-power PNP switching transistor like a C1740 to do the final triggering of the flash or camera. (I just connect the flash/camera trigger directly to the collector an emitter of the PNP transistor.) That works fine, but I wanted to provide some protection to both my trigger circuit and to my cameras/flashes. I've been using a 5-6 volt supply for my test analog circuits in anticipation of using regulated 5 volt supplies for my digital circuits.

I use Nikon cameras and flashes, which use very low voltage triggering (According to my meter, my hotshoe flash puts out about 3.6 volts. I haven't measured the trigger voltage from the camera, but I think it's also <= 5VDC.)

I bought some PS2501A-1-A Optocouplers, and am having trouble understanding the data sheet. Here's a link:

Datasheet for PS2501A-1-A Optocoupler

My questions:

Is the "diode" mentioned in the data sheet the LED on the trigger side of the photo coupler?

What is the minimum current needed at 5VDC to trigger the optotransistor "wide open"? It looks to me from the data sheet as if the LED needs 30 mA, but that it can tolerate .5A forward current. Is that right?

If I drive the LED at 30 mA and 5 volts, can I push 250V through the phototransistor? (I don't know how much forward current one of the older high trigger voltage flashes needs to trigger. It's my understanding that it is a transient voltage though. I believe that for these older flashes, the flash feeds it's flash tube voltage through the trigger line to a step-up transformer that generates something like 10kV, which ionizes the xenon gas in the flash tube, triggering the flash. If that's the case the current would be quite high for a tiny fraction of a second.)

If my trigger circuit can't trigger these old flashes that would be ok, although not ideal. At least the optocoupler would prevent damage to the other components in the system.


1 Answer 1


Yes, the "diode" is the LED.

There is no such thing as "wide open"- the current at the LED is reflected (within limits) at the transistor by the ratio "CTR" = Current Transfer Ratio.

If you put 5 mA through the LED you get somewhere between 2.5mA and 20mA through the transistor (until it saturates)- that's what the minimum/maximum CTR figures of 50% to 400% at If = 5mA and Vce = 5V mean. Vce =5V means that it's far from saturation. So if you use a resistor in series that limits the current to (say) 1mA (eg. 5K on a 5V supply) you'll have it saturate with 5mA into the LED. Note the since they vary over an 8:1 range, the manufacturer has ranked some of them and marked them in different "bins"

N : 50 to 400 (%)

H : 80 to 160 (%)

W : 130 to 260 (%)

Q : 100 to 200 (%)

L : 200 to 400 (%)

Naturally, the N version will tend to be the cheapest since the CTR can vary over the widest range, and includes the worst-performing units (50-80% CTR).

The transistor in the NEC part is rated at 70V and you should not put more than that across the Emitter-collector. There are higher voltage rated solutions.. for example the Sharp PC851XNNIP0F is rated at 350V.

You should make sure you have plenty of CTR- it degrades with temperature and with time (as the internal LED fades). Putting extremely high currents (like 30mA) through the LED will hasten the deterioration.

A proposed model of optocoupler aging is life \$\propto \frac{1}{Ie^{\frac {-E}{kTj}}}\$

Where k is Boltzman's contant 8.62\$\times 10^{-5} eV/K\$

Tj is junction temperature

E is the activation energy of approximately 0.15eV

so if you increase the current you not only get a decrease due to the current itself but an exponential decrease due to self heating. If I plug some plausible numbers into that equation, I get more than a 1000:1 reduction in life at 20mA vs. 5mA, with the same ambient temperature.

Note that if a relatively high current is only seen with a very short duty cycle (perhaps fitting your application) then the life is hardly impacted. It's the current and temperature integrated over time that causes the deterioration.

Bottom line is that you want to keep the current as low as is reasonable to make the thing work (and since operation is guaranteed at 5mA, that's not a bad place to start). You should also make sure it will work at (say) 3mA so even if it ages a bit it will still continue to work. In some cases you may have to buy more expensive optos with higher CTR than the cheapest ones if you need long life. Anecdotally, there is significant difference between different manufacturers' products. Personally, I tend to stick with the best-known Japanese makers.

  • \$\begingroup\$ Thanks for your answer, but I still need help understanding the data sheet. Am I right that the max forward current through the LED is 30mA? Are you saying that I shouldn't put the max of 30mA through the LED because it will make it age faster? \$\endgroup\$
    – Duncan C
    Commented Jul 8, 2014 at 15:36
  • \$\begingroup\$ I see now where it says the max voltage through the transistor is 70V. I could have sworn when I ordered it that it was rated at up to 250V, but apparently not. Sigh... \$\endgroup\$
    – Duncan C
    Commented Jul 8, 2014 at 15:37
  • \$\begingroup\$ @DuncanC Yes, 30mA is the Absolute Maximum rating when the ambient temperature is 25°C (and no higher). You should never get even close to that, if the current is sustained. 5mA or 10mA is more sensible. \$\endgroup\$ Commented Jul 8, 2014 at 15:40
  • \$\begingroup\$ Ok, I'm trying to learn how to interpret the data sheets on components. The current rating on page 4 of the data sheet is maximum current for the diode, 30 mA. How do I figure out a reasonable operating current for the diode? You say 5-10 mA, but how would I figure that out for myself? Also, how do I figure out which "bin" of component I have? Is that encoded in the part number somewhere? And is CTR equivalent to gain? \$\endgroup\$
    – Duncan C
    Commented Jul 8, 2014 at 23:35
  • \$\begingroup\$ Spehro, would you mind clarifying? I'm trying to understand how to interpret data sheets for myself. \$\endgroup\$
    – Duncan C
    Commented Jul 10, 2014 at 20:20

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