Now the challenge was to turn the breadboarded circuit into something that could fit into the housing of a vintage lamp. This is what I started with:
|Breadboarded dynamo circuit with standlight.|
|Schematic of flashing dynamo standlight circuit (click to enlarge)|
I used EagleCAD to do this, which is a very capable and free piece of schematic capture and PCB layout software. Most of the components I had to make in my own custom library, which I'll eventually post someday. Most standard packages (SOT23, 0805, SOIC, etc) are already available. I placed the parts as logically as I could and used the autorouter, which did a pretty darn good job of routing everything in a circle about 1.6" in diameter. My design rules included larger minimum trace widths and more generous spacing around components than the default rules. After much revision, I wound up with a two-sided PCB design:
|PCB layout in EagleCAD|
|BatchPCB boards. Not bad for $3 each!|
|Populated PCB. Huge 20F supercapacitor dominates!|
To my amazement, the whole thing worked on the first try. The only problem was that I didn't include an input capacitor for the 555 timer which I had on the breadboard. Without it, the 555's output is very erratic while the supercapacitor is charging. I soldered a 10µF leaded capacitor across pin 1 (GND) and pin 8 (Vcc) of the 555 and it worked exactly as it did on the breadboard. Revision 1.1 of the PCB will have to correct that!
The 6.8V Zener, which regulates the input voltage, gets darned hot as it shunts all the dynamo current during the off cycle in flashing mode. This could cause a potential failure of the Zener. I need to test it extensively to see if it fails. If it does, then I'll have to figure out what to do with the dynamo voltage when the LEDs are disconnected during the off period of the flash. No ideas yet.
Another potential pitfall is that the LEDs are connected in series, so if the rear light becomes disconnected, the series circuit will be broken and the front light won't work. The advantage of being in series is that both LEDs get the same current, simplifying the standlight circuit. For now, I think it's a good compromise.