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?
Best Answer
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.