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I was wondering if it would be possible to increase the frequency of a signal by a fixed amount. Basically, I have a negative wire soldered to a speaker and a positive wire that very quickly breaks/unbreaks the circuit. As expected, this causes the speaker to pulse each time the positive wire breaks and unbreaks. However, the frequency is very low and I was wondering if it would be possible to have a circuit that "adds some frequency" to the current frequency so that I would get a continuous beeping instead of some pulsing with noticeable space in between. The circuit would have to be pretty simple since I'm making it myself. Is this feasible or should I just be happy with what I have?

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  • \$\begingroup\$ To clarify: you're a beginner to electronics, and you're trying to make a speaker play a simple sound? \$\endgroup\$ – immibis May 19 '15 at 4:47
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A mixer might be what you're looking for.

A mixer multiplies two signals together. \$x_{out}(t) = x_1(t) \times x_2(t)\$ if \$x_1\$ and \$x_2\$ are two input signals. From the multiplicative property of the Fourier transform, this means that in frequency space, the output contains sum and difference frequencies of the input.

Say \$x_1(t)\$ is the signa produced by you moving your wire around. And \$x_2(t)\$ is a sine wave at some frequency \$f_0\$. Then if you put these two signals into a mixer, the output will be your \$x_1\$ signal shifted up in frequency by \$f_0\$. There is also a shifted version of the negative-frequency part of your input signal, but that might not be important or it might be what you actually want, depending what you're trying to do.

You can also see that if \$x_1(t)\$ is a square wave that's sometimes 1 V and sometimes 0 V, and you multiply it by a sine wave, you get out a pulsed sine wave, or a series of beeps, if \$f_0\$ is an audio frequency.

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  • \$\begingroup\$ Thank! This seems perfect. The end noise from the speaker will still be reflective of how fast the wire is disconnecting/connecting but also makes it a continous noise and not just spaced out beeps. \$\endgroup\$ – rareyankee May 20 '15 at 2:21
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Assume your signal is a square wave and that its frequency is \$f_0\$. This square wave is actually the sum of sines of frequencies \$f_0\$, \$3f_0\$, \$5f_0\$, and so on. You can design a band-pass filter around one of these higher-frequency harmonics to produce a continuous sine wave of a higher frequency.

These sines are known as the signal's harmonics, and they decrease in amplitude as they increase in frequency; you may need to amplify the signal after filtering. You could do all this with two or three op-amps. Search for Texas Instruments application report SLOA093 for easy recipes for filter design.

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  • \$\begingroup\$ This might be what OP wants, but it would not increase the frequency of the input by "a fixed amount" as requested. \$\endgroup\$ – The Photon May 19 '15 at 1:52
  • \$\begingroup\$ @ThePhoton, OP can't choose any arbitrary frequency, that is correct. Unless s/he can control the frequency of the square wave. However, he also wants the speaker to play a "continuous tone", which can be achieved by what I proposed. Using a mixer, he will get the sum of many tones, not a single one. \$\endgroup\$ – MBaz May 19 '15 at 13:21
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I don't think anyone's actually read the question properly.

"As expected, this causes the speaker to pulse each time the positive wire breaks and unbreaks. However, the frequency is very low and I was wondering if it would be possible to have a circuit that "adds some frequency" to the current frequency so that I would get a continuous beeping instead of some pulsing with noticeable space in between."

"This causes the speaker to pulse each time the postive wire breaks and unbreaks". "Add frequency", it sounds like he actually wants to generate an audible tone from the speaker, which means constructing an oscillator.

Search for "Square wave oscillator" on the net, which will produce a square wave or a "Wien bridge oscillator" which will produce a pure tone - a sinewave only.

You can look for circuits using a 555 timer chip, "audio circuits using 555 timer" You should find some that will drive a speaker directly.

If your request is actually different to this, and you have a signal which is too low in frequency, and you want to shift the frequency, then a mixer circuit might be a good way to do it, but it's a little more complicated because you'll end up with effectively two signals (side bands) and you will need a filter to remove one of them.

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