Nuts and Volts - September/October 2018

maximum power applied to it will be about 1.26W with a 4.5V supply. Note: The RMS voltage of a 0V based/50% duty cycle square wave is

Vrms = Vpk ÷ √ 2

Refer to https://masteringelectronicsdesign.com/how-to-derive-the-rms-value-of-pulse-and-square-waveforms for more details.

The PN2222 can drive a larger speaker if you need more volume. I used the PN2222 instead of a newer transistor (such as the 2N3904) because the PN2222 can handle more collector current: 1A vs. 200 mA. R5 is present because there is very little volume until you reach about 2.5V driving the base resistor.

If you use a more powerful speaker, you may want to decrease the value of R5, or eliminate it completely by replacing it with a jumper. If you need more sound output, a larger speaker will work, but I would not use one less than 8Ω.

I tried a speaker I have with taps for 40Ω, 20Ω, and 10Ω. The speaker (about 30 years old with no power rating on it) is 5” x 7” and was quite loud using any of the three taps, with the 10Ω tap being the loudest. I recommend that you use a diode across the speaker input since most speakers present an inductive load.

You can also use a higher voltage supply for just the speaker. You’ll still need a lower voltage for the PIC; it will operate anywhere between 2.5V and 5V. You’ll need to be careful that you don’t cause the PN2222 to dissipate too much power. Its spec is about 0.6W. Of course, you can add an external power transistor if you need lots of volume. If you look at the board layout in Figure 2, you can see that the speaker connections are quite close to both the battery pins as well as Q3. This was done deliberately to keep the high current path of the speaker away from the rest of the circuit.

Note that the current for the speaker does not go through Q1. This was done because I didn’t want the high current to be switched by Q1. My first design had the current go through it, which created a problem. Due to