You could use an LM567 (NE567) tone decoder chip tuned to 750 Hz to
drive the LED. 567's were very popular in touch tone decoders.
Isn't the receive offset in the HW-101 1 kHz, not 750 Hz?
Dale Saukerson wrote:
> Way back about 1978 or so....I built a couple of different circuits
> from either QST or 73.
> One was a simple audio filter tuned to 750 Hz. A LED would indicated
> a received tone equal to 750 hz.
> The other was an audio frequency generator set to 750 Hz. You would
> audibly beat that against a received tone. If the tones matched, you
> had only one tone to listen to. If they did not match, you would get
> both sum and diff and an audible warbling.
> I know I used them with my HW-101. Don't remember if I used them with
> my Corsair or '745.
> Any of you remember something like this? (I no longer have my
> archived magazines).
> They were probably OP-Amp circuits.
> Dale Saukerson
> Ex, soon again, N0AOZ
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