I have two mini slave flashes, that have no stand-by modes by themselves. So, even when charged and ready to flash, current keep flowing from the two AAA batteries each, and, if I forget to turn them off, their batteries run out of charge.

So I assembled this circuit a year ago maybe, with an arduino Micro (Leonardo) that has a raw pin to accept a wider voltage and I use one NP-F550 lipo battery (8.4V) to power the arduino and the flashes. In each flash, in the space where the batteries would be inserted, I soldered a small voltage regulator breakout board to make the 8.4V to 3.0V. And I used a TIP120 to turn off the flashes with an arduino, after some time it was on. So I created a standy-by code in the arduino for the flashes. And only one battery to replace instead of four.

What happens one time or another, it's not frequent, is that, when the flashes are fully charged and I turn off the unit (I put a on-off lever switch also on the battery wire) and later I turn it on again, as they are then fully charged, sometimes they both trigger without being told too.

These flashes are slave ones, so they need another flash to trigger them. But in these conditions they trigger just because I turned on the switch. And it happens one time or another. I can't replicate this when I want to.

It never happens when leaving stand-by mode (with the arduino "energized). It only happens when I cut power from the battery with the flashes fully energized and ready to be triggered. And one time or another. It never happens also when I turn off the switch with the flashes not fully energized (there's a little led in each unit that tells you if they're charged or not)

Is I made this circuit a while ago, I remember that each flash when charging drew close to 0,8A. So both together don't draw more current than 1.6A, for maybe 5 seconds (time to fully charge). So I didn't use any heatsink on the TIP120.

So, why does this happen? Is there any way to prevent this sudden trigger to happen? Should I use a different circuit to turn on and off them?

Thanks for reading!

I’ll add a link I’ve just found for a hack someone did on this flash this year. Shows the disassembly of the unit. The flashes I used are two of these Vivitar DF120 units

One thing to mention is that, the OFF position of the flash sliding lateral switch is also a “firing test” position. So it has a OFF/ Test position, and three slave modes to choose from. That fires on the first, second or third flashes, from diverse camera types.


Second Edit:

I managed to measure the current of this flash (DF-120):

At 3V, it charges in close to 10 seconds by drawing 0.84A. At 12 seconds current drops to 0.3A. At 25 seconds current drops to 0.13A. At 30 seconds, after firing and re-charging it stabilizes at 0.12A and stays forever at 0.12A, until next firing. So it's stand-by current is 0.12A, after 30 seconds of have fired.

When they are fully charged, and arduino enters stand-by, as soon it leaves stand-by the flashes are already at 100% charge. They don't slow re-charge. The green led, goes from dead (stand-by) to full green instantaneously.

The same effect happens when I turn the battery switch off, that powers both the arduino and the flashes. If I then turn again the switch on, they are instantaneously at 100% power ready to flash. So, as I never turn off their own switch (that is a Off/test switch), they hold their charge.

And today I tried and tried and never managed to make them fire when switching on and off the battery....

enter image description here

  • \$\begingroup\$ Are these optical slaves? Do the flashes have any switch? It is possible that the slave circuit requires power to prevent misfiring. When you turn on voltage, it may take a while for voltage to rise in the slave circuit (e.g. bypass caps). [Be extremely careful if you open it up to check. The main flash capacitor stores lethal voltages.] \$\endgroup\$
    – Indraneel
    Oct 7, 2018 at 15:59
  • 1
    \$\begingroup\$ Yes, they are optical. No physical connections. There is a sliding 4 position switch. Off, slave mode 1, 2 and three. I always leave it on the slave mode 1, which works with the main camera flash. I have a big discharger (25w 1k resistor) to discharge it when I needed to open it). One thing to notice is when I switch the slave flash off by changing it from the slave mode 1 to the off position (on each flash’s switch) the flash also triggers... \$\endgroup\$
    – Rodrigo
    Oct 7, 2018 at 17:30
  • 1
    \$\begingroup\$ If it triggers with the switch, then it way even be a protection "feature" of the circuit. What is the brand and model? Photo of the flash unit? \$\endgroup\$
    – Indraneel
    Oct 7, 2018 at 19:58
  • \$\begingroup\$ I would guess that the units are not designed to wake up reliably with a slower supply rise without false triggering. The slower supply rise is probably due to your 3V regulator. You could test this by adding an extra switch to the 3V supply connection (or use the original power switch) and see if this causes false triggering on power up when fully charged. I would also consider using 2 transistors to switch them. The second unit flashes because it slaves of the first. You could use an over specified regulator with an enable pin that switches fast to control them perhaps. \$\endgroup\$
    – KalleMP
    Oct 8, 2018 at 5:24
  • \$\begingroup\$ From the video, it seems the off switch will always test flash and discharge the capacitor if ready light was on. I cannot see how the flash could fire when turned on, since it would already be discharged. I'm guessing the off switch is not a soft switch. Also, it seems this is a super duper intelligent flash, so in standby mode it is always drawing some current (all flashes do, but they have bigger batteries), maybe 10 mA. That would be 100 hours or less on fresh batteries. \$\endgroup\$
    – Indraneel
    Oct 8, 2018 at 7:23


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