Not only have digital circuits altered the way photographs are captured, but they have also altered the compatibility between modern cameras and legacy equipment. Classic cameras were mechanical; not only in their shutters, but also in the way they triggered the flash. A piece of metal on the camera hot shoe would physically connect the wires of the flash at the right instant. The metal didn’t care if the flash or studio strobe applied 50 V, 100 V, or higher.
Today’s cameras use semiconductors to electronically connect the wires on the hot shoe to trigger the flash. There are voltage limits to these semiconductors, beyond which they will degrade or fail.
To avoid damage to the camera, the safest approach is to use equipment specified as compatible from the camera manufacturer. Also acceptable are remote or slave flash adapters, which do not require a physical connection between the camera and external flash. Alternatively, there are commercial adapters (such as from Wein) called safe-syncs that are professionally designed and industry tested to connect digital cameras to higher-voltage flashes.
Many amateur photographers prefer a do-it-yourself approach. For them, there are schematics posted online, to which I am adding one more.
There is a substantial risk of injury to yourself, others, and equipment when connecting to high voltages and large charges (in the flash capacitors or coils). Do not proceed if you are unsure of what you are doing or do not agree to assume the liabilities.
In order to operate, a safe sync (low-voltage hot shoe to higher-voltage external flash) must:
This circuit was inspired by Jean-Paul Brodier. This adds a datasheet-compliant trigger signal, reversed polarity protection, optional test button, and optional battery operation.
Schematic for safe sync to operate from battery or flash.
U1 L601E3 or MAC97A8 triac. 400 V, 1 A. When U1 is enabled, the + and - wires of the flash are electrically connected, just as though the camera hot shoe had done so. A triac is a semiconductor switch, similar to a bipolar or MOSFET transistor, except that it won’t conduct in either direction unless turned on at the gate, and it won’t turn off until the current across it subsides. This is beneficial for a flash, since it allows the high voltage to fully dissipate, even if the camera doesn’t assert the trigger signal long enough.
R1 18 Ω resistor. This limits the current passing through the gate of U1; to keep the current below 1 amp and to extend the amount of time it remains triggered in order to meet the 2.5 µs minimum. Both of these limits are specified by the L601E3 datasheet. Furthermore, this limits the amount of current across the camera hot shoe during discharge.
C1 0.68 µF capacitor. This stores the energy that will be released into the triac gate when the camera signals to do so.
D1 1N4148 or 1N914 diode. This is a one-way path that allows capacitor C1 to charge, but won’t allow it to discharge except through the gate of the triac. (Pictures of the current flow are presented on the next page.) The type of diode selected has a relatively low voltage drop, in comparison to the 1N4004 diodes, to allow C1 to charge almost completely.
D2 5.6 V Zener diode. This protects the camera by limiting the maximum voltage across the camera shoe to no more than 5.6 V. A higher-voltage Zener diode runs the risk of damage to the camera. A lower-voltage Zener diode reduces the maximum charge voltage of capacitor C1, which may prevent triac U1 from triggering.
R4 and R5 Two 4.7 MΩ resistors. These limit the amount of current that can flow from the flash, reducing voltage along the way. If too much current flows, the Zener diode can be damaged, it wastes power, and the flash may either constantly trigger or never trigger. Multiple resistors are used in series instead of a single resistor, since there is a limit on how much voltage a single resistor can handle across it. If you are not going to connect up to a 400 V flash, you can reduce the resistance to speed cycle time (reduce the time it takes to charge the circuit from the flash).
D3 and D4 Two 1N4004 diodes. These diodes are one-way paths. If the selected power source is connected backwards (reverse polarity, + and - swapped), it won’t harm the camera with a negative voltage across the hot shoe, because the diode blocks it. (By the way, in that case, no current will flow and the circuit won’t do anything until the power source is correctly connected.) The 1N4004 type of diode was selected since it blocks up to 400 V.
SW3 SPDT (single pole, dual throw), 400 V, 1 A switch. Selects the power source. It is more convenient to run the safe sync from the flash, because then there is one less battery to worry about. However, in my testing, draining power from some older flashes may cause them to falsely trigger or fail to trigger. So, this gives you a choice.
R3 1 kilohm resistor. This limits the amount of current coming from the battery, so that triac U1 doesn’t falsely trigger during charging and so that the camera hot shoe doesn’t receive so much current when shorted. Think of it this way, when the battery is the power source and SW1 is pressed, current will flow from the battery, through R3, through D3, through SW3, through SW1, and back into the battery. If R3 wasn’t there to limit current, the battery would be nearly short circuited. Depending on the battery: the battery would drain quickly, D3 would die, SW3 and SW1 would melt, and/or the battery would be damaged. Now imagine all of that current going through the camera hot shoe instead.
B3 3V to 6V battery. A lower voltage may not be enough to trigger the triac (U1), although there’s no harm in trying. A voltage higher than about 5 V drains through D2, wasting greater amounts of power as voltage increases. Flip the power selector switch to Flash to disconnect the battery when not in use.
SW1 NO (normally open) pushbutton. Pressing the button connects the hot shoe wires together, just as though the camera had triggered the flash.
Now let’s see how it works by examining the flow of current...