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I've made many RC circuits and RLC circuits ( series and parallel resonance). I want to make sure that these circuits work and make resonance, but I don't have CRO device. Is there any way to detect the resonance using Multimeter or something like that ?

I tried to hear the resonance. Human ear can hear 20 to 20K Hz, so I used 100uF coil and 1 uF capacitor to make Resonance of 15KHz frequency. ( 12 volt / 50 Hz AC power source ). when I connect the 8-ohm speaker, I didn't hear sound. actually, it was a very low sound. It looks like the sound that I hear when I connect the speaker to the power source directly.

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    \$\begingroup\$ Do you have a signal generator? How are you driving the LC? The AC voltmeter function of your DMM 'may' go up to a few hundred kHz. \$\endgroup\$ – George Herold Aug 27 '14 at 1:52
  • \$\begingroup\$ Hi @GeorgeHerold , I don't have a signal generator. I made this circuit: postimg.org/image/yzovo6yrv is it possible to hear any sound from a circuit like this ? or should I make 2 circuit with antennas, one is a transmitter and the other is reciever ? \$\endgroup\$ – Michael George Aug 27 '14 at 2:31
  • \$\begingroup\$ Hi @user282... that's not going to work. You need a voltage source at the resonant frequency. (15 kHz is also a bit high, you might find ~1kHz easier to hear.) That 100 ohms in series will also "spoil" the Q of your circuit. The PC sound card idea from Andy is good. You could also make your own oscillator (A few R's and C's and an inverter 74HC04) \$\endgroup\$ – George Herold Aug 27 '14 at 13:28
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You have a PC (because you are posting here). Presumably it contains a sound card - use it for measuring the resonance using free software that can both analyse a waveform and produce a stimulus.

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A relative low cost device to check resonant circuits is what used to be called a grid dip meter because it measured the grid current in a vacuum tube oscillator circuit. When the coil of the oscillator is placed close to a resonant circuit, and the oscillator is tuned to the resonant frequency of that circuit, the resonant circuit will draw power from the oscillator and cause a lowering (dip) of the grid current. Grid dip meters were constructed so that the inductor was external to the meter and could be easily coupled to a another circuit and also could be interchanged to allow a wide tuning range. Newer versions used a transistor oscillator instead of a vacuum tube but the principle remains the same. These meters typically covered a range from about 100 kHz up to 250 MHz and were calibrated with an accuracy of a few percent. They were made by companies such as Eico, Heathkit, Miller and Kenwood. You should be able to obtain one at a very reasonable price on Ebay.

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What basically happens with resonance is that you insert a tiny bit of energy in a circuit at given times (resonance frequency) and because of the properties of the circuit this tiny amount of energy has nowhere to go other than swinging up an down from capacitor to inductor.

What happens if you keep supplying this tiny amount of energy is that the total amount of energy in inductor and capacitor builds up and as a result the voltage and current in the circuit increase until an equilibrium is reached. At that point the load resistor (or your oscilloscope, or multimeter, or ... ) drains just as much energy from the circuit as is being fed.

This means: For a parallel circuit you want a alternating voltage source with a high impedance (or even better a current source), so you feed a tiny amount of energy into the circuit and at the same time the source doesn't load the resonant circuit too much so it will build up energy. The same goes for your meter, you want a device with a high impedance to check for resonance, otherwise the meter will drain too much energy from the circuit to measure the resonance.

The same goes for a series circuit but with low series impedances and a low impedance voltage source. The 100 ohm resistor and the speaker will dampen the circuit too much to notice too much of the resonance. The easiest way to prove resonance in this circuit is to use a voltage generator which you can vary the frequency of. That way, despite the load resistors, you should be able to hear the volume change with changing frequency. Do remember though that your speaker will have a certain volume/frequency response too (probably including resonance frequencies of its own).

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If your circuit generates enough power, you could connect it through a coupling capacitor to a full-wave bridge rectifier with a parallel filter capacitor. If your circuit is oscillating, you should be able to read DC across the filter capacitor with your multimeter.

You may want to use Schottky diodes for that bridge rectifier, maybe not, if your circuit generates enough voltage and you're only trying to detect oscillation.

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