Digitizing an Analog Photoflash - Page 1
Forum section: electronics / beginners

author: DrDnar
author_rank: Newbie
timestamp: February 13, 2023, 06:27:06 am
content: I have a pair of old Sunpak 611 flashes which I'd like to convert to digital control. They have a wired external light metering apparatus that is based on analog circuitry whose principles I can only vaguely guess at, and which also interfaces with a camera's flash trigger circuit.So my question:How does the circuit pictured below control the flash pulse duration?And how can I get a 3.3 V microcontroller to speak the same "protocol" (without exploding it)?Some Short Background on FlashesFlashes vary their brightness by controlling how long the flash tube fires for; the light coming out of the tube is the same brightness for whatever duration is chosen. In an example below, the flash appears to fire for 2 ms, but if the metering circuitry thinks less light is required, it might only fire for 0.5 ms.Old flashes often put a very high voltage across the camera's trigger output, because traditionally the trigger was a physical switch that could handle the high voltage. Modern cameras use logic-level outputs, but some professional-grade dSLRs have solid-state circuitry that can still handle high voltages. These flashes produce up to 320 V across the external trigger.These flash units have a flash head unit that has batteries and a trulygiganticcapacitor that's over an inch in diameter and several inches tall. A 4-wire metering/trigger unit plugs into the flash head. The metering unit has a photoresistor that senses ambient light. A dial on the top lets you enter your film speed and aperture; it works by sliding a shaded piece of transparent plastic in front of the photoresistor. (That is, a physical light-blocker is used instead of directly futzing with the metering circuit.)Please Enjoy Looking at This Old Analog CircuitryI took pictures of the control and light metering board and then hooked up an oscilloscope.Here's a picture of some very hand-drawn circuit traces. The bottom side (bottom half) has been mirrored already for your reverse-engineering convenience.How do inline attachments work? This should be sunpak611auto.jpg (below).And here's my crudely-drawn schematic:This should be sunpak611auto-.png (below).I Didn't Fry My OscilloscopeThis is an oscilloscope trace showing the flash firing in a not-very-brightly-lit room. (That is, it fired fairly brightly, but not maximum brightness.) Trace 1 shows the wire labeled "A" on the schematic (yellow in the photo). Trace 2 shows the wire labeled "D" on the schematic (red). Trace 3(note the different vertical scale)shows the wire labeled "C" in the schematic (white). They are all referenced to the wire "B" (black) which according to my meter is a short to the negative battery terminal. (The flashes run on 4 C cells.)This should be 005-amb.png (below).QuestionsBased purely on looking at the scope trace, my guess is that the A line is shorted to the D line to trigger the flash to start firing, and the D line is shorted to ground to stop it.Is this correct?I have no idea what the "C" line does, other than giving me something to trigger the oscilloscope on.As for microcontroller control, I'm looking atan optocouplerto allow the microcontroller to generate the start and stop pulses with a precision of at least 10 µs. (Though if you have an idea for higher precision, do suggest it.) I'm assuming here that the flash unit is looking for voltage pulses and isn't trying to dump the whole flash current through the metering unit, which seems very unlikely but I'm still a bit concerned.The "D" wire also appears to reflect the current voltage on the main capacitor, so I'm also considering using a 100:1 1 MΩ voltage divider to monitor the line witha zener diodeas a voltage clamp just to be safe; this would allow the microcontroller to monitor how charged up the capacitor is.Bonus points: generate a 3.3 V DC 25 mA power supply from the 70-320 V supply voltage so I don't have to run an additional wire from the flash head.

author: DavidKo
author_rank: Frequent Contributor
timestamp: February 15, 2023, 07:52:03 am
content: My experience with Pentax is, that old TTL flash does not work with camera that does not have TTL sensor or new flash with lens which does not have automatic aperture. The final exposure is wrong. I expect that in your case is enough to have synchronization with shutter (you can set your exposure manually).You can buy hot shoe adapter for given camera/flash (wireless if you want to be sure not to damage the camera).

author: Kleinstein
author_rank: Super Contributor
timestamp: February 15, 2023, 08:53:11 am
content: The old flash circuit is still having a high voltage at the trigger input. So it is not safe to use with a new camera (e.g. past ~1990).So the first point would be to make the trigger work with low voltage (e.g. 3-5 V) instead of some 150-300V and thus to make it possible to use the flash with a modern camera without damaging it.This change would not need a µC or much digital parts, more like a high voltage driver or SCR.A modern flash (e.g. TTL) with active controled flash needs quite some extra effort for turning off the high flash current. That would be more like build new from scratch, maybe keep the case.The old circuit looks like it is more adjusting the votlage in the storage capacitor so that the strength of the flash is adjusted from the measured light conditions. This part could be chanced to a digital system with a little more freedom to adjust (the relation can be nonlinear).However this would than need to operate the camera in manual mode (set the aperture manually and if needed adjust the ISO settings). Not all cameras support this.

author: ledtester
author_rank: Super Contributor
timestamp: February 15, 2023, 08:13:44 pm
content: QuotePerhaps the description of this auto thyristor flash will help:http://web.archive.org/web/20090216011934/http://repairfaq.cis.upenn.edu/sam/strbfaq.htm#strbvi2It uses two SCRs -- one controls the current through the flash bulb and the other seems to quench the current in the first one which will turn it off.The gate of the second SCR is triggered by a simple RC network formed by a CDS cell and capacitor which in a way integrates the amount of light received by the CDS cell.Perhaps all you need to do is to have the MCU trigger the second SCR.

author: nightfire
author_rank: Frequent Contributor
timestamp: February 15, 2023, 09:48:03 pm
content: I did some homework on those so called "automatic" flashes, as german manufacturer Metz also had some time back then lots of them available for small money.And, doing some event photography in nightclubs, they were surprisingly reliable with my Nikon D7000 in those times.Anyway, they basically work in this principle: Flash capacitors are charged, flash fires by getting the contact pins shortened, and the photocell measures the amount of reflected light and cuts off when a certain threshold is reached.This usually uses an angle comparable of a 50mm lens on a full frame (a.k.a. 35mm or "Kleinbild" format). Reflection is measured like with a 18% grey card, giving zone V in the classic zone system of Ansel Adams.As far as I remember, the capacitor gets charged and cuts off the lightbulb firing.Note: Older flashes can have nasty voltages at their middle contacts, so the logic to replace it should have max. 6V to accommodate modern digital specs. Nikon on contrary specified being 250V safe on most of the DSLR. With other manufacturers this may depend if talking about the hot shoe (where it can be lower) or some auxilary jack or classic P/C contact, those are mostly fit for 250V.