Building an Electrical Pick-up Device

Now we’re going to make the pickup for the double bass, including the preamplifier circuit, which we use to offer a high-input impedance to the pickup, and a low output impedance for our audio amplifier that will follow it in the signal chain. And it’s all built around this. It’s what’s called a piezoelectric buzzer. This is the sort of device, sort of transducer that you get in a smoke alarm to make the beeping sound when it detects smoke. The white is a disk of a ceramic material, that’s lead zirconium titanate, sandwiched between two metallic electrodes. There’s one very thin one on the top of the material and this thicker one on the base and then wires soldered to those electrodes.

This material shows piezoelectric behaviour which means that when we deform it, we get charges induced on the surfaces and a voltage across the element which really we can think of as being a capacitor. So this is what we’re going to make our pickup out of. And we can see how it responds to vibration if we look on the computer screen. So on the screen here we’re showing output from the oscilloscope which is just being run through the computer. And if I knock the desk underneath our piezo sensor, you can see the vibrations being picked up. And we’ve got an amplitude of maybe a couple of hundred, maybe 400 millivolts from that sensor.

We can see that this transducer is very sensitive to vibration. It will also be sensitive to vibration if the wires move. Because they can deform the surface of our sensor. So what we want to do is stop that movement. And I’m going to do that by setting the whole thing in some epoxy resin. So first, I’m going to put some gloves on to protect my hands. And I’m going to mix up the resin in this old dish that nobody wants. So the resin I am using is just a two part epoxy resin. I lay out two equal length strips of the two components. And now I’m going to mix them together. This resin takes about 20 minutes to harden.

I can make the hardening a little more rapid but also make the resin more fluid if I heat it up. So I’m just going to warm it with a hairdryer. [SOUND OF HAIRDRYER] So the resin’s a lot easier to work with now. I’m going to dip my transducer in it. And in particular, I want to fix those wires in position so that they don’t vibrate and move the surface of the sensor. Remembering to pick it up out of the dish. Otherwise, it will be stuck there forever more. You could blob on where the wires come. Now we wait 20 minutes for that to harden.

So we’ve made our piezoelectric transducer, which is going to provide electric signals, generate electric signals from the mechanical vibrations of our double bass. Now we’re going to make an amplifier circuit that is going to process those signals. We’re going to build it on this prototype board. And on the board, I’ve arranged for a +9 volt power rail, a ground rail, and a -9 volt power rail shown by the red, the green, and the blue respectively. And the first thing I want to do is have an arrangement on here which shows me that there is power applied to the board.

And I’m going to do that with a light emitting diode which has a turn on voltage of about 1.8 volts and a current limiting resistor, this 1.5 kiloohm resistor, which I’m going to connect in series with the light emitting diode from the positive supply down to ground. There we are and the light is on. Now I’m going to build my amplifier circuit. And we’re going to build that around a op-amp chip. So here we have it. And I’m going to plug that into the board across the central divide so that these pins don’t short out with the ones on that side. And I’ve got to provide power to this op-amp circuit. It needs 9v into pin number seven.

And it needs -9v into pin number four. So I’m going to put those wires in now. So how do I know which leg is number seven? Well, there’s a little depression on the top here, that I can see, and I can count around one, two, three, four, five, six, seven, eight. So that’s my nine volts into pin seven. And I’m going to put minus nine volts into pin number four. And you’ll notice I’m using the colour coding so that I can keep track of what’s happening on the circuit just by looking at it. And I’ve cut my wires to be the right length here so they’re nice and neat around the op-amp. Here’s our circuit fully assembled.

This is the 741 type op-amp and the power rails that you saw me putting in earlier, the +9 and -9v to pin seven and pin four. Now we’ve built a non inverting amplifier. Which means we need a feedback loop which goes from pin six to pin two, incorporating one resistance and then another resistance from pin two down to ground. When I built the circuit I put the resistor in first from pin six to another point on the board, and then made some bridging wires to bring me though to pin two. And that means I can cut them all nice and short and keep the loop neat.

And then to check that the circuit was going to work, I turned the power off. And I measured the resistance between pin six and two. And I got 33 kiloohms, which is the resistance of that resistor there. Then, I put in the resistor from pin two down to ground. And again I use the multimeter to check that I had the resistance I expected down to ground. And that’s all before turning the power on. So as this is a non-inverting amplifier, we’re going to bring the input of our signal into pin number three. So I’m going to bring a yellow jumper wire in to pin number three.

And for this circuit on the breadboard, the other end of that wire is going to a jack socket. And I’m going to plug our sensor via a cable into the jack socket over on the far end of the circuit board. Now our sensor, I said at the beginning was really a capacitor, some ceramic insulating material between two electrodes, and if we’re using the capacitor as a source, then we need to be a little bit more careful with our amplifier circuit. I’m just going to remove that yellow cable for the moment to show you what I mean. At pin three here, I’ve put in one extra component down to ground.

And that’s a one megaohm resistor and normally you don’t see this in the circuit for a non-inverting amplifier. But because the inputs to the amplifier don’t draw any current, in order for our sensor to provide any signal we need to create a current path. And that’s going to have to be a current path through a large resistance down to ground. And that’s the function of the one megaohm resistor. Without it our circuit won’t work properly. So now all we have to do is plug in our sensor to the jack socket. Here’s our sensor, the piezoelectric disk, coated with the epoxy resin which is now almost dry.

I’ve cut back the orange and green wires that we saw earlier cut them very close to the disk and soldered on instead the conductors from this twin core shielded audio cable. I’ve then put some heat shrink tubing on over the joints. And I’m going to shrink that down with the soldering iron next, so that I have a nice tight wire and secure wire connection onto my sensor. At the other end of this cable I’ve got a miniature jack plug. And that is going to just plug in to there on the sensor. Now if I was building this system onto my double bass properly, I would solder all of this onto a circuit board.

It would all be enclosed in the box. And then that would just be a socket in the wall of the box. And I would probably cover this also with some tape or some other material just to keep it safe. [SOUND - BANGING TABLE] You can see that we’re picking up vibration from our transducer going through our amplifier circuit. So now we have our pickup. Let’s go and put it on the double bass.