A recent previous post detailed the first and second passes at converting Norman 200B strobe packs to low voltage so as to be compatible with modern cameras. Those were not quite up to snuff because the first pass stole too much voltage to be usable and the second allowed 20mA of current to flow through the camera or trigger it was connected to.
After doing some reading about the subject, it would appear that at least for some older cameras, the trigger circuitry used an SCR, in which case the 2nd implementation above would fail because the constant 20mA would keep the SCR latched on. It might also be way too much current for the trigger circuitry to handle. The numbers I recall were 1.5mA max trigger current that decayed to something under 750uA.
So, back to the drawing board… Here’s what I came up with:
U2 is a negative regulator (required because of the positive ground of the Norman 200B) to bring the voltage at the SYNC contacts down to 5V or less. The resistor network consisting of R2, R3, and R4 control the current into the base of Q1. Capacitor C3 in parallel with R3 causes the transistor to turn on for a short period when the SYNC connection is shorted to +12V; R3 discharges C3 when SYNC is left floating. R1 limits the current through the LED in the optoisolator.
The transistor circuit was assembled on a solderless breadboard and a plain red LED was used instead of the optoisolator. When the SYNC connection was shorted to the positive supply, the LED flashed briefly and then went off. The maximum current is 1.56mA when C3 is completely discharged. When C3 equilibrates (if SYNC is left shorted to the positive supply for a long time), the current goes down to under 20uA. Calculations are in the comments here.
To make things simple, I designed a small printed circuit board (0.75 inches square) and submitted it to BatchPCB.com. All the components are in stock and available in single quantities from Digikey.com. Here is the layout:
Here is the parts list:
Digi-Key PN Description 445-5142-1-ND C1 0.33uF (0603 SMT) 445-1316-1-ND C2 0.1uF (0603 SMT) 445-1321-1-ND C3 1uF (0603 SMT) 568-1738-1-ND Q1 PMBT2222A (SOT-23) RMCF0603JT180RCT-ND R1 180 Ohms (0603 SMT) RMCF0603JT1K00CT-ND R2 1K Ohms (0603 SMT) RMCF0603JT360KCT-ND R3 360K Ohms (0603 SMT) RMCF0603JT2K20CT-ND R4 2.2K Ohms (0603 SMT) 160-1379-5-ND U1 MOC3023S (SOIC-6) 497-1219-1-ND U2 79L05 (SOT-89)
The boards and parts have been ordered. Now to wait for their arrival…
UPDATE: 3/8/2013: U1 had the wrong Digikey part number (it was a PDIP and not an SOIC as I’d intended). I found out when the package arrived… The corrected part number is above.
The boards went off to fab on 3/6/2013. Hopefully they will not take the 4 weeks worst case given by BatchPCB.com.
UPDATE: 3/22/2013: Boards arrived yesterday but as yet unbuilt… And another mistake in the BOM discovered: previously a 18kOhm resistor for R1 was specified. It has now been changed to a 180Ohm resistor.
Doh! And instead of an SOIC outline for the optoisolator, a smaller one was used…