# Driving a voltage boost circuit with a piezo driver circuit

For a university project, I'm creating my own small speaker. This speaker includes a speaker driver circuit consisting of a transistor and 3 resistors (see figure below).

simulate this circuit – Schematic created using CircuitLab The power source is limited to 1.5V, but I would like to boost this voltage using a voltage boost circuit.

The voltage boost circuit actually is an oscillating circuit including an inductor, diode, switch (oscillating) and a capacitor (see figure below). I think the "timed switch" can be attached to the "feedback" pin from the piezo.

simulate this circuit

Since the piezo driver circuit is a kind of oscillating circuit, I thought it would be nice to combine both circuits. So the piezo driver circuit would be used as the oscillating circuit of the voltage boost circuit.

Both circuits work separately, but not combined. When replacing the light bulb with the piezo, the piezo doesn't vibrate since the capacitor keeps the voltage constant, but the piezo needs to be turned on and off very quickly in order to vibrate.

Somehow, the capacitor needs to power the piezo driver circuit again, but I cannot yet figure out how. Below is my attempt.

simulate this circuit

Can any of you maybe guide me in the right direction?

I hope you understand what I'm trying to do. Otherwise I'll be happy to try to explain it in a different way.

• My mistake! I mean inductor indeed! – Daan Aug 21 '18 at 11:49
• the piezo doesn't vibrate since the power from the capacitors is constant Unclear what you mean, capacitors don't provide power directly. Here the capacitor keeps the voltage constant. You should first try to make your piezo driver work on the higher voltage using a lab supply. The voltage at the output of the "boost circuit" will depend on the load you're connecting (bulb vs driver) since you have no voltage feedback. What is the boost circuit's output voltage when bulb or driver is connected? – Bimpelrekkie Aug 21 '18 at 11:53
• Yes, the capacitor keeps the voltage constant. I'll edit it, thanks. The piezo driver also works on a higher voltage. The circuit is directly copied from a Sonitron 1.5-30V piezo speaker. In a simulation, when the "timed switch" had a frequency of 3000 Hz (resonant frequency of the piezo), the output voltage of the boost circuit was about 9V, with a 6V (30mW) light bulb. – Daan Aug 21 '18 at 12:26
• 30 mW doesn't sound right for a light bulb, are you sure it is not just 6 V, 50 mA making it 300 mW? What if light bulb and driver are both loading the boost circuit? Use an oscilloscope to see how much ripple there is across boost circuit output. Be clear what is in simulation and what is on your bench. Especially boost converters work much better in a simulator compared to on a bench. In the simulator there is often no series resistance and on the bench there is. – Bimpelrekkie Aug 21 '18 at 12:31
• So the piezo driver circuit would be used as the oscillating circuit of the voltage boost circuit. Then maybe make a variant of the "Joule thief" circuit, see: talkingelectronics.com/projects/LEDTorchCircuits/… and then section 2 transistor circuits. Replace the LED by your piezo. Not sure if that will work but I do know such a circuit can generate 20 V AC if the LED is not connected. Then leave the "feedback" pin of the piezo unconnected. – Bimpelrekkie Aug 21 '18 at 12:35