Experiment 8 (Chapter 8) -- Fun With Electrets

It's a strange name, but Charles explains in Chapter 8 that an electret is a mix of "ELECTrostatically" and "magNET" -- it's a small sensor that reacts to sound saves. Experiment 8 is super simple to wire up, and as you can see in the following photo, it's just a few wires, a single resistor, and 9V of power. I could have used a 9V battery, but my variable power adapter goes from 3V to 12V, so I'm able to just switch it to 9V and go.

Two wires on right (red + black) are for Voltage readings

Wired in with a 4.7k resistor, I discovered that the electret isn't really all that sensitive. The chapter tells you its best not to tap on the small microphone, but that's about the only way I could get it to register any fluctuation in voltage (AC) on the multimeter. You'll see this in the video.

Charles was also correct about identifying the GND and +V terminals on the bottom of the electret. After flipping it over, the three little "fingers" were easily visible. I used a Sharpie marker to label them on the outer edge of the component, and I didn't have to solder on any leads as my electret came with two leads already added on.

Three "fingers" (tiny tracings) on right indicate GND lead

Once everything was wired up, I discovered the round shape of the electret made it hard to insert a separate wire to take voltage readings. A simple adjustment of moving the LED to a separate row on the breadboard allowed me to insert a wire so I could take an AC voltage reading between the positive lead on the electret and GND. Again, the electret doesn't seem all that sensitive, and my talking in the video didn't even register on the multimeter. I ordered a spare and got the same results with that one. Once I'm done with the electret in any upcoming experiments, I'll probably try and break one open to see what's inside. If I do, I'll post pictures in a follow-up.

Before moving on to Experiment 9, be sure to read over the last section of Chapter 8 that talks about a split power supply. Charles' solution is a good one, although you need to understand why he's recommending a very low value of paired resistors if using this method. If you don't understand why splitting the voltage at the 4.5VDC will make it more difficult to obtain accurate voltage readings, read it again... it hit home for me on the second read once I thought about what was actually going to be involved in taking readings of the output voltages.

And here's the Experiment 8 video: