This is a excerpt from Sam's Strobe FAQ

Modified by Carl McMillan. The parts source is updated.

Optoisolated Adapter for Older Flash Units to Low Voltage Cameras

(From: Brian L. Zimmerman (

Many electronic flash units can have a very high voltage between the trigger contacts that are shorted to trigger the flash. For example, the trigger voltage of the "Digi-Slave" DSF-1s flash unit being sold in 2001 for use with digital cameras (See for example, Slave Flash Products) measures 218 V fully charged. This would be dangerous to use on many modern electronic cameras such as Canon digital cameras which specify no more than 6 V.

This design adapts such a high voltage flash for use on a low voltage camera by using an optocoupler to electronically isolate the camera's contacts from the high voltage. It can then be triggered by the camera using the 6 volt supply from the flash unit's batteries, but also works at a lower voltage. The use of a triac optocoupler has the added advantages of using fewer parts than other optocoupler designs and can use the power from the flash's trigger circuit to fire the SCR switch instead of a separate power source.

The adapter circuit can be inserted into the lines inside the flash unit going between the flash trigger circuit and the flash unit's contacts as shown below. Since there are only 4 small parts in this circuit, there is a good chance that you can build it right into the available space inside the flash unit's case.

(WARNING: High voltage precautions apply here - be sure to safely discharge all capacitors!)

For a GIF picture of the drawing from a CAD program. (better drawing).

System connections


              o--------------------- | ----------------------- (+)
   Flash hot-shoe contacts           |                Flash trigger circuit
              o--------------------- | ----------------------- (-)
                               Cut and insert
                           adapter circuit below

Flash adapter schematic


                  +---------------------+      R2
 (-) o-----------(1)---+         +-----(6)----/\/\----+----o Flash Trigger (+)
                  |    |  OPTO1  |      |     5.6K    |A
 To Hot-Shoe      |  __|__   ____|____  |           __|__
 (and camera)     |  _\_/_-> _\_/_/_\_ (5)NC        _\_/_   SCR1
                  |    |         |      |           / |     400V,6A
 (+) o---+   +---(2)---+         |      |          |G | K   (RS#276-1020)
         |   | NC(3)             +-----(4)---------+  |
         /   |    +---------------------+             |
     R1  \   |          Optotriac                     +----o Flash Trigger (-)
    330  /   |           MOC3010
         \   |         (RS#276-134)
         |   |
         o   o
        (+) (-)
   To 6V Battery (Flash unit battery pack)

Parts list

  1. SCR1: SCR, 400V, 6A, (RS#276-1020, $1.29, TIC106D, etc.). See note
  2. OPTO1: Triac optocoupler (MOC3010, RS#276-134, $1.99)
  3. R1: 330 ohm Resistor.
  4. R2: 5.6k ohm Resistor.

RS# indicates Radio Shack part numbers. Total cost (October, 2001) is $3.75.

Note: Radio Shack has discontinued the SCR, 400V 6A RS#276-1020. 

These parts can still be bought at Electronic part stores.
The part that is currently available for SCR, 400V, 6A is SCR, 400V 8A, which is a heavy duty part. One source is: Mouser Part#: 511-TYN408  Mfgr. Part#: TYN408  STMicroelectronics
September 28, 2003 the price is $0.80.

The MOC3010 Optocoupler can also be bought at

Mouser Part#: 512-MOC3010M  Mfgr. Part#: MOC3010M 
Fairchild Optoelectronic Devices
 DIP-6 NON-ZERO TRIAC Current price September 28, 2003 is $0.87.



The camera shutter shorts the hot-shoe contacts, causing current to flow from the 6V battery through the IR-emitting diode at pins 1 & 2 of the OptoTriac. The current is transferred via light pulse which switches on the triac at pins 4 and 6 of the OptoTriac which is powered by the voltage from the flash trigger circuit. Current flows into the gate of the SCR, switching it on and causing discharge of the flash trigger through the anode and cathode of the SCR. The 330 ohm resistor (R1) limits the current through the hot-shoe to about 18 mA, and the 5.6k ohm resistor (R2) limits the current through the triac to about 40 mA. You may need to use a different value for R2 for your particular flash and SCR, since this is based on a 218 V trigger voltage. The triac can handle a larger current (1.2 A peak), but SCR's typically only use a small gate current for triggering.


The circuit with component values shown seems to work reliably (at least so far) for this particular combination of Canon digital camera and slave flash. Others may be quite different. Some info can be found at Kevin Bjorke's: Non-Canon Strobe Page with a List of Trigger Voltages. Just knowing the trigger voltage isn't really enough information as it doesn't imply anything about the available current. Adding an input buffer using a transistor or CMOS gate would eliminate this as a concern.

SCRs and triacs should be driven hard when they are controlling high current sources to make sure they turn on quickly and minimize time where they are passing significant current with a significant voltage drop. The optotriac's output is current limited so this isn't much of an issue. However, the SCR discharges the trigger capacitor through the trigger transformer and this could amount to several A switched in a few microseconds. A gate current 10 to 20 times the minimum spec in the datasheet is recommended so long as this doesn't exceed the maximum rating in the datasheet. In any case, the worst that will happen is that the SCR will fail or become unreliable after running with marginal gate drive - no great loss considering its cost.

Page last edited: April 26, 2015