# Replacing a 8 ohm Speaker with a Piezo. Will it work?

Like Replacing a piezoelectric speaker with an electromagnetic one, but the other way round.

simulate this circuit – Schematic created using CircuitLab

A DC squarewave will turn on and off ASW (Analog SWitch) for a buzzing sound from a constant voltage. The 4K7 pulldown is for discharging the cap. when ASW is open.

I want to use a piezo for the simplicity they provide. Are the schematics equivalent, sound-wise?

• No the schematics aren't equivalent. I can tell because they are blatantly different. One will clearly receive the square wave in series with a 1Mohm resistor, the other will not and even if it did, it'd be in series with a measly 8 ohm resistor. – Funkyguy Aug 27 '15 at 1:38
• @Funkyguy The cap+spkr one will still get a square wave, b/c caps pass AC components. But I will clarify what I'm looking for. – Alexander M Aug 27 '15 at 2:03
• Not if it is a DC square wave, unless it is a high enough frequency which you didn't specify,, it will still block much of the signal rendering the speaker useless – Funkyguy Aug 27 '15 at 2:06
• @Funkyguy Yes it will even if it's a DC square wave. They're merely AC square waves with the lowest point at GND. Besides, it's for an audio project. – Alexander M Aug 27 '15 at 2:11
• Why do you want to replace your speaker with a piezo? I get their cheaper, but their quality is blech – Funkyguy Aug 27 '15 at 2:14

## 1 Answer

If you do a source transformation on the voltage source and resistor in parallel for the $8\Omega$ speaker, you get a current source which is what we care about when driving a speaker. We know that the current through the cap can be written as $I_c=sCV$; this means that as the time rate of change of voltage increases, current will increase, and vice versa. The result is that we've protected our speaker from passing too much current and eventually (potentially) being destroyed. Aside from this, impedance of the speaker typically comes into play when we want to maximize the power transfer between an amplifier and the load (speaker). In this case, seems like we don't care too much. A piezoelectric speaker works by virtue of the fact that an applied voltage results in a strain on the material. To my knowledge it is safe to apply the rated amount of DC to one of these, and the high resistance makes it safe to apply this voltage directly without creating enough heat to burn up the device. This circuit should work as expected IMO.

• I'll add that piezo's are capacitive loads, not inductive like traditional speakers. This article has some noteworthy things to say about driving them. Most notably, that their impedance is very dynamic! – rdtsc Sep 11 '15 at 12:19