FAQ
TL;DR: The legacy trigger is a non‑retriggerable one‑shot that ignores new pulses for about 3 seconds and needs ≥0.4 V negative input; “It is a non‑retriggerable one‑shot.” [Elektroda, DAVID CUTHBERT, post #21661616]
Why it matters: This FAQ helps photographers and hobbyists build or update a sequential photo‑flash trigger that syncs reliably with camera pulses without trial‑and‑error timing.
Quick Facts
- Original analog design blocks retriggers for ~3 s via C1–R3; needs a −0.4 V or greater negative trigger. [Elektroda, DAVID CUTHBERT, post #21661616]
- Recommended redesign uses a 4017 decade counter; trigger is a negative‑going ≥1 V; outputs drive 400 V sensitive‑gate SCRs. [Elektroda, DAVID CUTHBERT, post #21661621]
- Camera sync safety: keep trigger voltage ≤6 V; typical flash trigger is approx. 3–4 V. [Elektroda, David L, post #21661620]
- One user measured camera sync at −0.5 V in Bulb mode; transient amplitude unknown. [Elektroda, David L, post #21661624]
Does the original circuit retrigger if camera pulses occur quickly?
No. It’s a non‑retriggerable one‑shot that ignores any new triggers for about three seconds, set by C1 with R3. Rapid camera pulses during this window will not advance the sequence, so missed flashes are expected if you fire faster than the set delay. “It is a non‑retriggerable one‑shot.” [Elektroda, DAVID CUTHBERT, post #21661616]
How can I trigger flashes one‑by‑one on each camera pulse without a fixed delay?
Design a fresh sequencer around a CD4017 decade counter rather than modifying the analog one‑shot. The 4017 advances one output per valid trigger, so each camera sync pulse fires the next flash. “It would be much easier to design a circuit to do what you want.” [Elektroda, DAVID CUTHBERT, post #21661618]
What input level does the 4017 sequencer expect from the camera?
The posted design expects a negative‑going pulse of 1 V or more to clock the counter. If your camera delivers less amplitude, the counter may not advance reliably, so level‑conditioning could be required before testing the rest of the chain. [Elektroda, DAVID CUTHBERT, post #21661621]
What SCRs should I use to fire the flash tubes?
Use sensitive‑gate SCRs rated around 400 V as shown by the designer. These devices let the low‑level logic output safely trigger the high‑voltage flash circuits when properly isolated. Confirm your flash’s trigger polarity and wiring before final assembly. [Elektroda, DAVID CUTHBERT, post #21661621]
Is my camera’s sync line just a switch, or does it output voltage?
A user measured about −0.5 V at the camera’s sync terminal in Bulb mode, indicating the camera presents a voltage, not only a dry contact. Without a scope, any short transient above that level remains unknown. Treat it as an active signal source. [Elektroda, David L, post #21661624]
Is −0.5 V enough to clock the 4017 design as posted?
Possibly not. The shared schematic calls for a negative‑going pulse of at least 1 V in magnitude. With only −0.5 V available, the counter may fail to advance, so you should verify the actual pulse or condition it accordingly. [Elektroda, DAVID CUTHBERT, post #21661621]
How should I verify the camera sync pulse shape and level?
The designer recommended checking the sync pulse on an oscilloscope. That reveals the true amplitude and any fast transients that a handheld meter misses, ensuring your interface threshold and polarity are correct before wiring flashes. [Elektroda, DAVID CUTHBERT, post #21661626]
What’s the safe trigger voltage for the camera shutter contacts?
Keep the trigger presented to the camera at or below 6 V to avoid shutter damage. In the thread, flashes used approximately 3–4 V triggers, which stays inside that limit. Insert isolation and verify wiring to protect the body. [Elektroda, David L, post #21661620]
What does the input capacitor near the sync jack do—debounce?
A participant asked if that capacitor debounces the camera sync input. In practice it often shapes or AC‑couples the trigger, but the thread leaves it as an open question about that exact schematic placement. [Elektroda, David L, post #21661625]
Can I build a microcontroller version instead of using a 4017?
Yes. One contributor proposed a PIC16F88/16F84A solution using an external detector to trigger an interrupt, then shifting bits on PORTB to fire SCRs with programmable delays. It offers flexible rates and frame counts via firmware. [Elektroda, Mark Harrington, post #21661619]
How do I hook up and test the CD4017 sequencer to my flashes?
- Feed the camera’s negative‑going sync pulse (≥1 V) into the 4017 clock.
- Connect each Q output through a 400 V sensitive‑gate SCR to a flash trigger.
- Fire the camera; confirm flashes advance one‑by‑one per pulse.
[Elektroda, DAVID CUTHBERT, post #21661621]
Why did the analog version miss shots in burst mode?
Its timing network (C1 with R3) holds the output active for about three seconds and blocks retriggering during that window. In burst shooting, any pulses within that hold‑off are ignored, so only the first frame fires the flash. [Elektroda, DAVID CUTHBERT, post #21661616]
