I have an LED Flash which I want to trigger on and off just before and after my camera captures an image for a Machine Vision application. Any ideas on what circuit set-up I can use for doing this? The Flash uses a 7.4V, 2200mAh 16.3Wh Li-ion battery. This is a link for the Flash I intend on using: https://www.amazon.in/Digitek-LED-D204-Professional-Battery-Charger/dp/B01A5ZS98O/ref=pd_cp_147_2?_encoding=UTF8&psc=1&refRID=Y64HXVVGE3093911NP92

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    \$\begingroup\$ That's a floodlight rather than a flash. So it doesn't get triggered so much as switched on and off. Do you want to be able to use it as normal as well? What switches does it have (the listing doesn't give much detail) \$\endgroup\$ – Chris H Apr 19 '18 at 11:00
  • \$\begingroup\$ How's this : diyphotography.net/how-to-build-a-flash-with-an-optical-slave ? Practically you want to build a camera blitz. \$\endgroup\$ – Overmind Apr 19 '18 at 11:11
  • \$\begingroup\$ @ChrisH Yeah,sorry, more a floodlight than a Flash. It has one on-off button and a knob for light intensity. I'm not sure about the internal circuitry \$\endgroup\$ – Rakshith Madhvan Apr 20 '18 at 7:12

I like to use solid state relays for this sort of thing. They provide isolation (often for your safety but in this case to avoid frying your kit). They're faster and use less current than a mechanical relay, and simple to integrate.

An Arduino can apparently deliver 40 mA on a GPIO pin. This arbitrarily selected DC SSR wants ≤30 mA and drops around 1.5V so will only need a simple resistor to drive it off a GPIO pin. It can switch 10 A if the two internal MOSFETs are connected in parallel, and the through-hole version can be assembled on stripboard or dead bug style (glued down on its back with its legs sticking up. The current capability is more than enough, because we don't know how much that lamp draws.

As it has a dimmer as well, there's probably an LED driver chip in there, but I assume from my experience of hacking LED lights that the on/off switch is connected to the power supply line (in contrast to bike lights, where it's often an input to the LED driver).

When it comes to making the connections inside the lamp, that depends on exactly what you want to do. Your options are:

  • Replace the on-off switch with the SSR
  • Wire the SSR in parallel with the existing on-off switch (this allows you to turn the light on without the Arduino)
  • Wire the SSR in series with the switch (this allows you to force the lamp off).

The biggest problem I see is if my assumption above is wrong, and you have an on-off switch that connects to a logic pin on an LED driver chip. The only way to find this out is to open up the lamp and trace the circuit. You can usually replace such a switch with an opto-isolator, but the details will depend on the existing circuit.

  • \$\begingroup\$ Thanks a lot for the answer. I was wondering if I could use a simpler mechanism like replacing the battery with cables from a 12V DC supply, converted to the required 7.4V using a Buck converter and supplying the rated current of 2.2A. Would that have any significant effects on the life of the LEDs or any other negative effects? \$\endgroup\$ – Rakshith Madhvan Apr 23 '18 at 8:03
  • \$\begingroup\$ If you supply the correct voltage and enough current, the LED and its driver won't know the difference. But you'll still need to switch it somehow, probabkly using one of the approaches I suggest. Note that the output voltage of a battery varies with charge state; the home made power supply for my old DSLR is more reliable when set to the voltage of a full battery than that of a nearly empty one \$\endgroup\$ – Chris H Apr 23 '18 at 8:07

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