I’m looking to build a few whistling accessories in the Post War style that respond to the classic whistle activation DC offset. In place of the usual whistle tender relay, I’d like to build a discrete circuit.
After some thought and doing a little research (in threads such as this one), my thought is that perhaps this is as simple as a suitably-tuned RC low-pass filter that leads to a DC relay, which in turn controls the power to the whistle. My question is: is it as simple as this?
If so, in looking at the ERR diagram in the thread above I wonder:
- What is the purpose of diode D1 if the filter is going to attenuate the AC anyway? Or is this to ensure only one DC polarity (whistle vs. bell) gets through?
- What is the value of the optoisolator (?!?) in place of a simple DC relay?