Replacing the logic in a broken macro ring flash with ATtiny85

Hi

I have a cheap LED ring flash for macro which has stopped working, yet the two half-rings of LEDs are fine.

Rather than bin it, since it's not E-TTL or anything fancy my intention is to pull out the busted board, and hook up an ATtiny85 and drive the flash with a logic level MOSFET (they're switched via the drain side). According to this site the main pin to monitor is the central one marked X in the diagram - high means trigger. The hotshoe connections in the existing flash include GND to the outside of the hotshoe which is black, a blue wire to X for the trigger sense, and a red wire to the pin marked CLK in that diagram.

I can't presume the colour code of red is +6V both because that doesn't make sense, and because it's a cheap flash so the wire colour could just be what was on hand at the time. There is no voltage on that wire when I put batteries in, but then that could equally be due to the board being history rather than design.

Has anyone done something similar? If so what should be on that CLK (or are 2 wires for trigger and GND fine)? I'd have thought the CLK would only make sense if the flash accepted communications from the camera, and this one certainly couldn't.

Secondly I expect an opto-isolator would be a sensible thing between the ATTiny and the trigger...or is that a thing of the past with modern cameras?

For testing I was going to run the LED rings flat out, but with a MOSFET and PWM output it's possible I could have settings for power levels like it originally had - is there anywhere you can recommend to show the mathematical relationship between LED luminescence and PWM level (ie is it inverse squared, or directly linear, or something else?). When I get to that part I'll test through the camera, but again if it's been done somewhere already I'm happy to read and learn from others' efforts before tackling this.

Thanks in advance, Geoff

My understanding from the wiki page is that CLK is an alternate trigger but only after a specific command is sent and only on some cameras. I can't imagine why your simple flash would have a connection to it. We're I in your shoes I would leave it unconnected.

Secondly I expect an opto-isolator would be a sensible thing between the ATTiny and the trigger...or is that a thing of the past with modern cameras?

How much is your camera worth? How good are you at coding? Designing a circuit? Building a circuit? You may want to include the opto-isolator to protect the camera.

strykeroz: Secondly I expect an opto-isolator would be a sensible thing between the ATTiny and the trigger...or is that a thing of the past with modern cameras?

It depends on the voltage. I've talked to somebody that burned out his camera's hot-shoe on a recent camera (Olympus E-PL1) when he put an older flash that dumped a lot of voltage through the hot-shoe.

In any case, I would imagine since the grounds of the Arudino and camera aren't connected, you would want an opto-isolator in any case.

MichaelMeissner:

strykeroz:
Secondly I expect an opto-isolator would be a sensible thing between the ATTiny and the trigger…or is that a thing of the past with modern cameras?

It depends on the voltage. I’ve talked to somebody that burned out his camera’s hot-shoe on a recent camera (Olympus E-PL1) when he put an older flash that dumped a lot of voltage through the hot-shoe.

In any case, I would imagine since the grounds of the Arudino and camera aren’t connected, you would want an opto-isolator in any case.

The grounds are connected - the outside of the hotshoe is the GND connection. In the Canon case the voltage is documented at between 3.3 and 5V depending on model; which probably explains why my old Metz Mecablitz isn’t compatible with the newer models as your friend found out on the Oly. Despite the lower voltages I’ll take the prudent path and isolate, especially following Mike’s suggestion above that should something go wrong on the flash that’s less benign than the previous failure (perhaps due to a goof on my part) I could be trashing my dSLR for the sake of this project.

Thanks again. Very much looking forward to this one once the workbench is cleared from the present project.
Geoff