# Why does the buzzer in this circuit have a voltage drop of 0.7V?

I set up a buzzer circuit in my Radioshack learning lab with a 100 kohm resistor to do some test measurements with voltage dividers (I didn't expect it to sound, I was just trying measurements.). I measured the resistance across the buzzer to be pretty high, 1700 kohms. With two components being resistors, I saw this circuit as a voltage divider with a 3V input voltage , and expected the voltage drop across the buzzer in to be 3 * (1,800,000)/(1,800,000 + 100,000) = 2.84 V.

However, when, I measured across the buzzer, I got a reading of 0.7V, 2V below what I expected. Why did the buzzer give a reading of 0.7V when I expected 2.84V? I also measured the current to be 22.8 microamps. I tried ohm's law: a voltage across a 1700 kohm buzzer with a current of 22.8 microamps would be 0.0000228*1,683,000 = 38.4V. That's way off my previous guess and measurement. Why did my ohm's law guess give a buzzer voltage that big instead of 0.7V?

simulate this circuit – Schematic created using CircuitLab

• Are you sure it is 1700kOhm? Like 1.7MOhm? I doubt it.. Nov 8 '16 at 19:06
• @EugeneSh. I'm pretty sure. I set my multimeter to 2000k, and it read 1643 in the screen (meaning about 1,700,000 ohms). Here's an image. Nov 8 '16 at 19:12
• do you have a schematic for this box? Nov 8 '16 at 19:16
• If the buzzer is electronic, the resistance you measure probably depends on the meter polarity - measure with the leads connected one way, then swap the leads and measure again - you will probably get different values. Nov 8 '16 at 19:18
• It just says that the "buzzer" is not a linear part as you are assuming Nov 8 '16 at 19:22

You don't say what kind of buzzer it is, but if I had to guess then I suspect it is a three-tab piezo coupled with a simple BJT circuit like this:

simulate this circuit – Schematic created using CircuitLab

This might account for your multimeter measurements and also account for the relatively low voltage measurement across it when active through a voltage source and added resistor. It's really hard to say for sure, though.

Have you considered the idea of opening things up and looking to see?

• I opened the box up, and pried out the buzzer with a flathead screwdriver. I don't know what to think of it, I've never worked with green circuit boards before, only did the labs that came with the learning kit. Do these pictures here and here match up with your diagram? The top part is the photoresistor of the kit, so I'm assuming the buzzer circuit is the entire bottom part. Nov 8 '16 at 19:45
• @DragonautX No, the circuit board just shows two leads, I think. So the circuit I mentioned would be inside that black cylinder of the piezo buzzer shown there. Don't crack it open. Better would be to just buy another one and destroy it so you can see how they handle these inside. (I've got some here, which is where I got the circuit.) But the fact that this actually IS a piezo pretty much convinces me this is why.
– jonk
Nov 8 '16 at 20:12
• @DragonautX Your Radio Shack circuit uses a convenient two-wire powered piezo buzzer. Those are really easy to use and apply and are widely sold. Inside, there is a circuit, though. The three-tab piezos are rarely sold to the public, since they need to be driven with a circuit and most people don't want to do that. But years ago in manufacturing, the three-tab piezo with a circuit was cheaper in quantity than the two-wire convenient to use piezos. So those were purchased. Today, I don't know the economics. Might be always cheaper now to buy the completed capsules than to get the raw piezo.
– jonk
Nov 8 '16 at 20:16
• Oh, okay. That's pretty cool. I didn't know an entire circuit like yours would be in that one cylinder. I guess the complexity of the circuit is why I'm getting weird measurements. I thought a buzzer would be some special component I didn't know about attached to one resistor. Can you recommend a two-wire piezo buzzer for me to buy so I could try to open it up? Also, can you show me a possible diagram for it? I'd like to look more into it, but I don't know where to start. Nov 8 '16 at 20:36
• @DragonautX Look for buzzers that specify a voltage and do not say "piezo element" on them. They should look "tall" and not "thin." Something like this: adafruit.com/products/…
– jonk
Nov 8 '16 at 21:23