Denman Island, British Columbia
Flash Slave Trigger for Digital Camera |
Disclaimer: This is not my invention! I got the circuit from http://home.hiwaay.net/~cnaumann/smartpc/flash.html. However, the need for such a device is great, and the photo accessory industry has dropped the ball, so the Internet is the solution. The information is published here as insurance against that site closing down.
Digital cameras are wonderful, but their deficiencies can be maddening. I use a Nikon Coolpix 880. Among its other idiosyncracies, its flash has a maximum range of about six feet and is too close to the lens, producing "red-eye" even when red-eye reduction mode is selected.
An accessory flash unit would be the obvious answer, except for the criminal lack of an accessory flash ("PC") connector. The only way to use an accessory flash is to use a slave trigger.
And that brings up a problem of many digital cameras: they use a double flash. The first "pre-flash" is used to calculate colour balance information. The second flash is the "real" one that illuminates the exposure. Unfortunately, a slave trigger will fire the accessory flash on the pre-flash, leaving the exposure dark.
The need, therefore, (one which has so far been neglected by industry) is for a modification to a standard slave trigger, allowing it to fire on the second flash.
Research in the 'Net showed one manufacturer of digital-camera-compatible flash units. This manufacturer also sells a digital-compatible trigger unit, but the cost (US$80) is unreasonable.
I also found several schematics for delay-based slave trigger modifications. The theory is that the slave unit is triggered by the colour-balance pre-flash, and an adjustable timing circuit delays firing the flash until some number of milliseconds have elapsed, hopefully coinciding with the open shutter. While this would work, adjusting the time delay sounded too fiddly to justify building these devices.
The device I settled on (from http://home.hiwaay.net/~cnaumann/smartpc/flash.html) is an analog counter. Rather than firing the flash after a timed delay, it actually counts pulses, firing on the second one. It is powered from the flash unit, and uses a minimum number of cheap components.
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Pinout for 2N2222 transistors: |
The circuit fits comfortably inside a film canister. Rather than perform surgery on the slave trigger, I used a hotshoe to PC cord adapter (because the slave trigger had no PC connector), and spliced the device into the PC cord. This allows the trigger to be used unmodified for conventional photography, and with the adapter for digital photography.
(From http://hiwaay.net/~cnaumann/smartpc/flash.html.)
Resistors R1 and C1 have a time constant much greater than the time between the two flashes. R3 limits the current into the base of Q1, and ensure that Q1 is turned on long enough to discharge C3 through R4. The time constant of C3 and R4 is long enough to allow the flash to fire. Diode D1 and capacitor C1 form a simple power supply, allowing the unit to be powered from leakage current from the flash. Resistor R5 supplies a bias current to the slave adapter so that it can see the flashes. The pre flash turns on Q1, and Q1 stays on until C2 is drained through R3. The hot shoe adapter employs an SRC in its output, the current supply by R5 must be less than the sustain current of the SCR, but the current supplied by both R2, R3 and R5 should be greater than the sustain voltage. I believe that I may have R3 a bit large. With Q1 on, Q2 cannot turn on. After the pre flash, C2 recharges almost fully before the main flash, however, C1 does not recharge significantly. When the main flash is picked up by the slave unit,.Q1 does not turn on, and therefore Q2 is allowed to turn on and the external flash fires. The resistor and capacitor value are not highly critical, and with some tinkering, this circuit should be adaptable to a large number of flashes and hot shoe adapters. Another good modification would be to replace Q2 with an SCR, and possibly increase C1 so that the unit can suppress red-eye reduction flashes too.
Figure 1: components.
L-R: slave trigger, Hotshoe adapter, digital adapter
Figure 2: The assembled stack
Figure 3: Proof that it works!
Kids, don't try this at home. Flashing directly into the camera is probably unhealthy for the CCD in the digital camera. I was intending to take the photo in Figure 2, but the flash, though switched off with the ready light off, still had enough charge to flash. It does offer dramatic proof, though!
| Comparison: photos below are unretouched. | |
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Figure 4: taken with on-camera flash only. Colours are washed-out; illumination fades noticeably with distance. |
Figure 5: taken with slave flash bounced off ceiling. Colours are brighter; illumination is more even. |
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The slave trigger does not fire on long-range (subject 10 feet from flash and camera) shots. It could be because the Coolpix 880's flash is just so winpy. Or it could be that the adapter is stealing power from the flash that the slave trigger wants to steal. Some simple tests would indicate if the latter is the case, but I have not performed them yet.
Regardless of the cause, it is probably something I will have to live with. My planned solution is to make a bracket that will hold the flash and camera side by side on the tripod. The bracket will position the slave trigger so that it is out front, facing the camera's flash, where it can easily be trigered.
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Copyright © 2006 Keith Walker
Last modified: 19-Apr-2006
