I have a circuit that I want to be able to detect the frequency of the current going on it (part of the circuit is AC, the power supply is going to be at least 3v, probably 5v, and 6v DC at most). I don't need to know the exact numerical value of the frequency, just be able to tell when it changes to a different frequency. I have a very basic knowledge of electricity, so this may be a very overly simplistic interpretation of how it works, but could I connect the circuit directly to a small speaker, with a constant sound frequency indicating no change in current frequency (I imagine a constant hum), but with a change in the sound frequency (the hum becoming higher or lower pitched) indicating a change in the electric frequency?

  • \$\begingroup\$ Probably, for some range of frequencies for which the speaker is reasonably capable. But the details would depend on the speaker and the impedance and power level available in the circuit. Rather obviously you cannot just take a speaker out of a stereo and connect it to something like the AC mains, without a matching circuit to syphon off a suitably small fraction of the power, and without substantial effort to making the design safe. Also, there are generally better methods of monitoring - in what you propose, the signal might be lost behind something else louder. And aggravating. \$\endgroup\$ – Chris Stratton Jun 12 '18 at 20:46
  • \$\begingroup\$ Your ear is probably better at relative frequencies, so if you have a reference frequency you can tune you can get pretty close to the actual value (+-2% maybe?). \$\endgroup\$ – HKOB Jun 12 '18 at 20:53
  • \$\begingroup\$ Could I connect the tiny speakers from earbuds to the breadboard or a small speaker? The whole thing is also battery powered by the way and uses a TLS chip running at a nominal 5v (although I may replace it with another that is 6v) along with a capacitor to create pulses that are sent through an induction coil \$\endgroup\$ – cluemein Jun 12 '18 at 21:07
  • \$\begingroup\$ Or if I'm being even more simplistic, could a simple buzzer work? I remember messing around with a circuit a while back that had a buzzer change its sound based on how fast or slow a motor/generator turned. \$\endgroup\$ – cluemein Jun 12 '18 at 21:10
  • \$\begingroup\$ @cluemein Well, seems to be you want the desing of a circuit to hear that "hum" firstly you need the preamplifier to connect the small speaker. The preamplifier needs to be connected to the frequency source, if the source is the mains (120V -220V) you need other circuit to reduce the signal to 3.5-5v, so this way avoid to explode your breadboard. once you have all of this you will need a well trained ear to notice each frecuency variation. Speakers from earbuds won't help you. \$\endgroup\$ – Fernando Baltazar Jun 12 '18 at 21:44


simulate this circuit – Schematic created using CircuitLab

Figure 1. (a) Simple test speaker. (b) Attenuator for PC speaker input.

Figure 1a might work adequately enough for you. C1 removes the DC component. R1 limits the current to a safe but possibly inaudible signal.

Figure 1b would allow you to plug into a set of computer loudspeakers with built-in amplifier. Start with the volume turned down.

enter image description here

Figure 2. gStrings for Android.

Many of the guitar or piano tuner apps have frequency readouts on them. Hold the phone up to the speaker and you can read the frequency.

  • \$\begingroup\$ What size speaker are we talking about in a? I have seen 8 ohm speakers that are rated 2W and ones rated in the 100s of watts. \$\endgroup\$ – cluemein Jun 18 '18 at 20:30
  • \$\begingroup\$ There isn't enough information in your question to answer that. \$\endgroup\$ – Transistor Jun 18 '18 at 20:42
  • \$\begingroup\$ The power supply is likely to be a 9v battery or something comparable (a group of aa or aaa is another possibility), or a USB. I have a circuit that converts it to 3.3v or 5v depending on what position a switch is in. I can also get a circuit that is a class d amplifier board rated at 2.5v to 5.5v \$\endgroup\$ – cluemein Jun 18 '18 at 21:18
  • \$\begingroup\$ Work out the current through your speaker for a given series resistor. Then use \$ P = I^2 R_S \$ to work out the power in the speaker, S. \$\endgroup\$ – Transistor Jun 18 '18 at 22:09
  • \$\begingroup\$ One other thing is that I think the current will be at 100-300 kHz range in frequency. Will that effect whether I can hear the sound or not? Will the sound come out at that frequency or will it be different since sound is not electricity (I'm thinking it would be different, but want to check to make sure). \$\endgroup\$ – cluemein Jun 19 '18 at 20:17

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