Wireless flash sync for photography - Page 1
Forum section: electronics / beginners

author: John0922
author_rank: Contributor
timestamp: March 12, 2020, 05:22:53 am
content: Hi- I’m trying to build a wireless sync to fire my studio flash when I trip my camera’s shutter. I’m stuck on step one, which is firing the flash from a short 5 volt signal.The power pack for the flash has a sync output, which consists of two leads. When these are momentarily shorted, the flash fires. Apparently these leads when shorted discharge a small capacitor into a larger one, which ignites the flash tube.Across the leads measures 14 volts. I’ve tried using an optoisolator, putting the 14 volts across the collector / emitter, but the flash doesn’t fire when the led is turned on. I’ve tested this on the bench with 14 volts from my power supply, and it works fine there, but the real flash doesn’t fire.I also tried a MOSFET, which again works great on the bench, but won’t fire the flash.I know I could use a mechanical relay, which is guaranteed to work, since shorting the leads by hand works, but I worry the relay will be too slow, since the flash must fire when the camera shutter is open.Any help would be greatly appreciated.Thanks, John

author: magic
author_rank: Super Contributor
timestamp: March 12, 2020, 07:38:20 am
content: Quote from: John0922 on March 12, 2020, 05:22:53 amAny more details on that?

author: Zero999
author_rank: Super Contributor
timestamp: March 12, 2020, 09:06:18 am
content: A MOSFET or BJT should work. What MOSFET did you try and what was the gate voltage?

author: John0922
author_rank: Contributor
timestamp: March 12, 2020, 06:19:33 pm
content: Here’s a schematic of what I tried so far. On the bench, the MOSFET creates a perfect short, but it won’t fire the flash.I’m starting to think I need a thyristor. A lot of these circuits use one, and there’s even one in the camera.I don’t know why a thyristor is chosen instead of a MOSFET. The sync output is a discharging capacitor— maybe that has something to do with it?Thanks again for any help you can provide.

author: magic
author_rank: Super Contributor
timestamp: March 12, 2020, 06:41:25 pm
content: You must connect circuit ground with sync negative for the FET to work. Not sure if you did.

author: StillTrying
author_rank: Super Contributor
timestamp: March 12, 2020, 06:43:37 pm
content: The 6V's ground has to be connected to the mosfet's source.

author: John0922
author_rank: Contributor
timestamp: March 12, 2020, 06:45:12 pm
content: Yes I did it. I forgot to draw it. Thanks.

author: magic
author_rank: Super Contributor
timestamp: March 12, 2020, 07:00:21 pm
content: So post everything you know about the flash internals.

author: StillTrying
author_rank: Super Contributor
timestamp: March 12, 2020, 07:05:35 pm
content: Are you sure the mosfet's drain is connected to the +14V side of the Sync, and the source to the 0V side.I've electrically flashed many HV and LV flashes and not had a problem, even using a transistor.

author: John0922
author_rank: Contributor
timestamp: March 12, 2020, 07:17:36 pm
content: Thyristor worked!! I have no idea why.Thanks, John

author: StillTrying
author_rank: Super Contributor
timestamp: March 12, 2020, 07:28:49 pm
content: Thyristor worked!It is a reliable method, maybe because once it starts conducting a thyristor sticks on ?

author: John0922
author_rank: Contributor
timestamp: March 12, 2020, 07:39:14 pm
content: Quote from: StillTrying on March 12, 2020, 07:28:49 pmYes I was thinking along those lines too. I was reluctant to hook the scope up to it, because I have no idea whether ground is floating inside the power pack.

author: ArthurDent
author_rank: Super Contributor
timestamp: March 12, 2020, 08:57:50 pm
content: The small studio flash units I have can be triggered by an SLR camera’s built-in flash and unless you are using large format, the camera flash should trigger most modern studio flash units that have built-in sensors. There are also inexpensive transmitter/receivers that can be used that cost about $20 up that can be used as well.If you are going to build your own trigger, check your camera specs because some SLR hot shoe trigger circuits are only rated for 5 volts and depending on what you do you could damage the trigger circuit in the camera. I’m assuming the equipment you are using may be older because both my Pentax and Lumix DSLRs have built-in wireless so the camera can trigger the on-camera flashes remotely. I would go with one of the commercially built wireless systems just to be safe.

author: Brumby
author_rank: Unknown
timestamp: March 13, 2020, 02:37:06 am
content: Optical slave flash units have been around for decades - even in kit form. The ones I've seen use an SCR to fire the flash, so I would have suggested learning from the past rather than trying to re-invent the wheel.Silicon chip did a "smart" design in 2003 which counted the pre-flashes used for red-eye reduction, only firing when the main flash was triggered - but simpler designs have been around a lot longer.

author: John0922
author_rank: Contributor
timestamp: March 13, 2020, 04:21:48 am
content: Thanks Arthur. I’m using a Nikon D800 and a Norman 2400 watt second studio flash. Since the flash sync puts out 14 volts and the camera handles it fine when there’s a direct wired connection, I’m assuming I can get away with 5 volts or so for the transmitter.I’m aware of the inexpensive wireless units, but wanted to do this myself as a learning exercise. Hopefully I won’t screw up any of my equipment.

author: flolic
author_rank: Frequent Contributor
timestamp: March 13, 2020, 07:24:58 am
content: All Nikon and Canon DSLR's hot shoes are high voltage tolerant. For example, Canon 6D uses SFT1423 500V/2A rated mosfet for flash triggering.

author: StillTrying
author_rank: Super Contributor
timestamp: March 14, 2020, 12:49:08 pm
content: Make a sound and/or light beam triggered flash now.