## Saturday, July 21, 2018

### Pulsing and ringing a coil (or earbud)

The question is what would it sound like. My best guess is it would be good since the circuit was presented in magazines and data books but the debate is on going. Let's look at at couple of factors. My first question is why use the unijunction? If we drove the speaker directly from multivibrator how would that work?
The data I present is for an earbud but would apply to any coil. Remember the coil has resistance, inductance and capacitance and therefore has self resonance.
If you pulse with a single spike the coil it will 'ring'. It will oscillate at its resonant frequency and the signal strength will fall off as show above.
If you pulse it with a square wave it will ring as above.
The coil is being pulsed at a low frequency and rings at its resonant frequency.
The pulse frequency has been raised so the ringing forms the sine wave and re-enforces it before it drops off. So you should be able to see the coil will 'shape' the wave and the self resonance of the coil will be the controlling factor. Now the question is 'what is the resonant frequency?'.
This is from a headphone spec sheet. It show the phase angle and impedance. Remember the phase angle is 0 at resonance. so we see resonance at 100 Hertz. If we pulse at a rate less than 100 hertz we have ringing above 100 hertz the next pulse arrives before the coil produces a full cycle and thus no ringing.

This is the response of the unijunction transistor. The 47K resistor and 22nfd cap trigger the unijunction. In the circuit above the supply is DC. In the circuit being discussed the supply is a square wave.  It could be said the multivibrator is unnecessary and redundant. Connecting the battery directly to the top of the 47K resistor will produce a tone. Using the square wave supply will produce a tone control by controlling the charge rate of the 22nfd cap. Does the cap charges in one cycle by the multibrator or 10 cycles? That is the question!