FAQ
TL;DR: Optical slave triggers may use ultra‑high values (e.g., 30 MΩ) because a photodiode can deliver only “a few nanoamps,” so “you need big resistors.” [Elektroda, Mark Nelson, post #21670863]
Why it matters: This FAQ helps DIY photographers and tinkerers diagnose, clone, or improve optical flash slaves without false triggers or damaged gear.
Quick Facts
- Observed parts: photodiode (e.g., BPW34‑like), unknown SCR, 0.047 µF 400 V cap, 4.7 kΩ, and two 30 MΩ resistors. [Elektroda, Elly Elisa, post #21670865]
- High resistance lets a timing capacitor reach a trigger threshold before firing the SCR. [Elektroda, Chuck Sydlo, post #21670864]
- Legacy hotshoes can present approx. 300 V; modern isolated syncs stay at ≤6 V. [Elektroda, Elly Elisa, post #21670866]
- Potting in clear resin admits light yet complicates repair and inspection. [Elektroda, Frank Bushnell, post #21670870]
- Resistor color bands can mislead; orange/brown/red and blue/grey may appear similar under resin. [Elektroda, Frank Bushnell, post #21670870]
Why would an optical slave use 30 MΩ resistors?
To bias or sense nanoamp‑level photodiode currents and let a small timing capacitor ramp until the SCR triggers. “They used high resistance in the trigger circuit to force the voltage to build in the capacitor.” This raises threshold, improving immunity to ambient light until the flash pulse arrives. [Elektroda, Chuck Sydlo, post #21670864]
Are those parts really 30 MΩ, or are the color bands misleading?
Bands under resin can shift hue; orange, brown, and red may look alike, and pale blue can read as grey. Visual misreads happen often in potted assemblies. Confirm values only by measuring a bare sample; photos alone can deceive. [Elektroda, Frank Bushnell, post #21670870]
How does a basic optical slave trigger actually work?
A photodiode senses the master flash. The circuit drives an SCR that shorts the strobe’s sync contacts, firing the tube. Many units act purely as an electronic switch, not as exposure control. Set the flash to manual when using an optical slave. [Elektroda, Elly Elisa, post #21670866]
What components were observed in the discussed unit?
A photodiode (BPW34‑like), an unreadable SCR, a 0.047 µF 400 V capacitor, a 4.7 kΩ resistor, and two 30 MΩ resistors in series. These appear in both the hotshoe and plug‑type versions shown. [Elektroda, Elly Elisa, post #21670865]
Would a 10 kΩ in place of 2×30 MΩ make it more sensitive?
Yes, but it would likely false‑trigger constantly. The poster saw even 10 kΩ create excessive sensitivity. Very high values help reject ambient light and noise during the brief flash impulse. [Elektroda, Elly Elisa, post #21670869]
Could the “M” marking mean 1,000 (old notation) instead of megaohms?
Historically, “M” sometimes meant 1,000 in the 1930s–40s, later replaced by “K” in the U.S. by the 1950s. However, modern four‑band resistors in photo gear are unlikely to use that convention. [Elektroda, Mark Nelson, post #21670863]
Does this circuit limit flash power (a smart TTL feature)?
No. The described unit is just an “eye” that shorts the sync to fire the flash. It is separate from the strobe’s exposure control. Users typically select manual power when using optical triggers. [Elektroda, Elly Elisa, post #21670868]
What voltages must an optical slave withstand at the sync?
Older strobes can present about 300 V on the hotshoe at discharge; newer isolated designs keep sync potentials at or below 6 V. Design for the higher case to avoid failures. [Elektroda, Elly Elisa, post #21670866]
How can I reduce false triggers in bright daylight?
Add an RC filter across the photodiode or bias network to slow response to steady light. Shade the sensor or use a dark cap to prevent photodiode saturation outdoors. This addresses the common daylight failure mode. [Elektroda, Elly Elisa, post #21670871]
What is an SCR in this context?
An SCR is the solid‑state switch that, once triggered by the sensor network, shorts the strobe’s sync contacts to fire the flash. It latches during the discharge and then resets. [Elektroda, Elly Elisa, post #21670866]
Why do some builders pot the board in clear resin?
Clear resin admits light to the photodiode and makes the device rugged. The downside is difficult diagnosis and rework because components are encased. [Elektroda, Frank Bushnell, post #21670870]
Could those tan, resistor‑shaped parts be inductors instead?
Unlikely in this case. The contributor checked inductor color codes and noted the blue third band didn’t fit. Photos of both units showed consistent resistor coding. [Elektroda, Elly Elisa, post #21670871]
Is there a simple way to prototype a robust optical slave?
Try this: 1) Photodiode feeding an RC to ground to filter ambient light. 2) Bias into an SCR gate with a high‑value network. 3) Wire SCR across the sync to short on trigger. Test indoors and then in sunlight. [Elektroda, Elly Elisa, post #21670871]
Where can I find reference schematics or explanations?
Search “flash slave circuit” to find many variations and explanations. Comparing multiple schematics helps identify the RC timing and SCR gate choices that fit your parts. [Elektroda, Frank Bushnell, post #21670870]
Why not just buy a cheap optical trigger instead of cloning one?
Building your own teaches how sensitivity, RC timing, and SCR selection affect reliability. It also lets you tailor the design to legacy high‑voltage syncs or modern low‑voltage gear. [Elektroda, Chuck Sydlo, post #21670864]
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