I have obtained a military vibrating-reed frequency meter that I would like to turn into an educational display unit, with a knob that adjusts the frequency from 380 to 420 Hertz, giving the user the ability to hear the frequency on a loudspeaker as well as see it in the frequency meter.

However, my experience is only with digital circuits like 74 logic, some BJT audio circuits, etc... have never generated AC myself before. How should I generate a 26 volt 400Hz signal?

I was thinking something like... Microntroller to generate a variable sine wave, which I would feed to an audio amplifier. One channel of the audio amplifier would go to a speaker and the other would go to a step-up transformer and then to the meter. Would that work?

  • \$\begingroup\$ Do you know what the 26V means on the meter? Is it the maximum safe input, minimum detectable input, or typical input? I wouldn't go the microcontroller route myself. Probably easier to start with a VCO, divide to generate a square wave, and lowpass filter to convert back to a pure sine wave if needed (you may be able to find a VCO chip with built-in output divider that can generate the square wave directly). \$\endgroup\$ – Justin Oct 8 '19 at 17:50
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    \$\begingroup\$ I think the first thing I would try is see if it accepts a 26V 400Hz square wave, if it does then you could just use a high side mosfet to switch it on and off and a 555 timer. How many terminals are on the back? \$\endgroup\$ – Voltage Spike Oct 8 '19 at 17:59
  • \$\begingroup\$ 2 terminals on the back. Could I stress the mechanism by applying a square wave? It physically excites a bar with the reeds attached. A square wave will subject it to much higher accelerations than a sine wave. \$\endgroup\$ – Akdinv Oct 8 '19 at 18:01
  • \$\begingroup\$ Odds are it has a rectifier in it, if you could crack open the back and post a pic that might be helpful. Put a DMM in diode mode on it and see what it comes up with \$\endgroup\$ – Voltage Spike Oct 8 '19 at 18:04
  • \$\begingroup\$ Windows app Audacity.exe can generate your voltage, which you can use a transformer to possibly boost the voltage if < 12V. You can choose Slow frequency sweep into a new record then play it. audacityteam.org \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 8 '19 at 18:53

The frequency meter is simply an electromagnet that excites the magnetic reeds into mechanical resonance at double the marked frequency for each (since the magnetic field effectively rectifies the input current).

Your proposed arrangement should work. Another approach would be to use a micro with either a pot or a rotary encoder to control a DDS such as an AD9850/9851, which would provide a nice sine wave.

One attraction of that path is that you could use an inexpensive China module for the DDS and an Arduino for the micro, and chances are someone has done much of the programming for you.

It appears that kind of frequency meter device likes to see the nominal voltage +/-20%.

There may or may not be a series resistor inside to adapt it to the standard (one of several) 26VAC/400Hz aircraft voltage level.

It might work okay with a square wave since the first harmonic will be at more than 1kHz, but the sound would be ugly and the amplitude would have to be adjusted appropriately. I would not give it too much DC content. That’s taken care of with a transformer, of course, but if you are considering a direct coupled circuit it may not be.

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