Beej, on Mar 13 2010, 01:38 AM, said:
This is an awesome little train, congratulations to anyone that has it in their collection. I'd be interested to see how far you got with working out the circuit Mark, a whistle activated train would definitely be something different these days.

If you raised the frequency of the tuned circuit maybe you could use a dog whistle instead of an audible one.

I've reverse-engineered the 118 unit and drawn up the
circuit diagram (when moderated).
As I suspected, there is a high frequency tuned circuit on the front end, to detect the whistle frequency, followed by a flip-flop to switch the state, followed by a power transistor to connect the battery power to the motor.
The RC tuned circuit has an input high-pass corner frequency of about 12.25kHz (a bit lower, subject to microphone impedance) and a feedback low-cut corner frequency of about 3030Hz. The bistable flip-flop relies on capacitors discharging together, the 4uF one balancing the 10uF ones for each transistor. The output stage has protective chokes and a large capacitor.
Now that I have a circuit I could probably make an electronic version of the whistle, or replicate the circuit with modern components. In the 60s they used germanium diodes (0.2V drop rather than 0.7-1V) and the gain of transistors was a lot less at low frequencies, so some extra high-pass filtering might be necessary. You can't just replace a 60s transistor with a 21st century one! It was cutting-edge toy technology back then.
I see from the Peeron photos of the 138 unit that the bidirectional unit contains a 14-pin chip and 4 transistors in an H-bridge. Still 5 electrolytic capacitors, so I suspect a similar technique is used for the bistable operation. No surprise about the H-bridge for bidirectional drive but I'd like to know which IC it is. I guess there are 2 flip-flops in there, one for on-off and the other for forwards-backwards. Probably either two tuned circuits or two different capacitor sizes too, to separate the long and short whistle bursts. Bricklink shows an adjustable whistle with a yellow bottom, so could it have used two frequencies?
Steve, is the lid loose on your unit, so that you could see the chip markings? It could be a gate chip (7400, 7402), a pair of flip-flops (7473, 7476), op-amps (less likely as most need more voltage than 4.5V) or a transistor array. Take care with the wires to the unit's bottom terminals if you open the unit. I had to re-solder mine once! Of course I would reverse-engineer the circuit if I had a unit handy!
Mark
Edited by Mark Bellis, 09 April 2010 - 04:11 PM.