Here's one suggestion for a self-powered meter circuit. It full-wave rectifies the incoming AC and uses a zener to derive a supply voltage Vsupply in the range 9V-12VDC. The AC waveform also clocks a CD4013 latch to give a 50% duty cycle square wave at half the incoming frequency. The latch Q output triggers a monostable (U1a, U1b) to give pulses set by R4 to have a width equal to the period of the minimum frequency of interest (assumed 360Hz). Logic U1c, U1d combines the latch and monostable signals to give output pulses of width proportional to the difference between the incoming frequency and the set minimum frequency. These pulses are fed to the meter and are averaged by the meter's inertia (a 100uF cap could be added across the meter if desired). R6 sets the current to 100uA at the maximum frequency of interest (assumed 440Hz).
Simulation shows that with a stable Vsupply the meter drift is ~ +- 1% over the temperature range 10C-30C, but with a 5% variation in Vsupply the drift is ~ 6%. The zener could be replaced by a more accurate reference (LT1021-10, LM317?) to reduce drift.
Simulation shows that with a stable Vsupply the meter drift is ~ +- 1% over the temperature range 10C-30C, but with a 5% variation in Vsupply the drift is ~ 6%. The zener could be replaced by a more accurate reference (LT1021-10, LM317?) to reduce drift.
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