91.5Hz Tone Generator

Some notes on the generator

  1.  Perhaps a disclaimer note should be inserted by EMDRC with suitable words to the effect that its application is at the user’s risk.
  2. The schematic provided gives a way to generate a low frequency tone.
  3. The output could be, say, inserted after the microphone preamplifier via a high value resistor, perhaps >20k. The generator output would be adjusted to just access the repeater.
  4. The schematic shows an AF transformer for isolation purposes. A direct output without the transformer may be more convenient. The 220n capacitor on the transformer primary was found by experiment to clean up the output waveform a bit more. The stepdown transformer turns ratio is not critical.
  5. For accuracy, the frequency or waveform period needs to be measured. The 10 turn tuning potentiometer makes adjustments easier than a skeleton preset!
  6. Not tried, but two 741 ICs could be used instead of the LM358.


Regards Ed.


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