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Old 27th Mar 2019, 1:32 am   #21
Argus25
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Default Re: Low frequency indicator?

...but the original request was for the circuit to produce a DC voltage. The 555 circuit only works because the integration is carried out by the meter movement. The circuit config I suggested produces the DC voltage, proportional to frequency that was requested, and it can all easily be done with "one IC" a quad op amp.
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Old 27th Mar 2019, 2:48 am   #22
Argus25
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Default Re: Low frequency indicator?

Actually, now I think about it, the perfect IC for this job is the National Semiconductor LM3900. A vintage quad OP amp which is a Norton current differencing op amp.

I'm pretty sure that in the original NS Application notes, for the LM3900, they included a great circuit for a frequency to voltage converter. I can't attach it as I'm on a limited mobile device (limited by the operator no doubt), but if you look it up on Google, these op amps are very convenient for applications in the frequency spectrum you require and very easy to use and would make a terrific solution.
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Old 27th Mar 2019, 7:25 am   #23
Terry_VK5TM
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Default Re: Low frequency indicator?

My mistake, I had it in my mind a meter was being used for some reason, sorry.
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