Building a Microphone and Amplifier

Watch-O-Scope works by "listening" to the watch being tested, measuring time between and within ticks to determine rate, beat error, and amplitude. To do this, it needs a sensitive microphone that will detect the sounds of the watch, while ignoring ambient sounds. It also needs an amplifier to raise the sound enough for Watch-O-Scope to hear it.

In the past, when a ready-to-use microphone stand with amplifier was not available, the pair of articles below were used as a guide on how to construct the necessary microphone and amplifier hardware yourself. If you are interested in doing so, this page describes how.


Building a Microphone

Building the microphone stand, shown on the right in the picture below, requires some basic woodworking skills and a bit of wiring.

In addition to a microphone, you will also need an amplifier to amplify the weak signal from the microphone so that it can be detected by your computer's sound card.

Materials Needed

The microphone stand is constructed out of wood. Almost any smooth hardwood will do, such as poplar, birch, or maple. The example shown here was constructed out of birch plywood.

The following parts are needed to construct the microphone stand:

All parts except the last three can be obtained from a good hardware store, although you may need to visit a hobby shop for the brass rod or tubing.

The piezo element can be purchased on-line from a variety of sellers on eBay. These elements can also often be salvaged from musical greeting cards, where they are used as tiny speakers.

The audio cable and plug can be purchased separately from an electronic supply company, but it may be easier to just purchase a pre-made audio cable and cut one end off. A stereo cable may be used, in which case the extra wire that connects to the ring between the tip and shield of the plug is left unconnected.

Preparing the Components

The main components of the microphone stand are three blocks of wood as detailed in the plan below (click image for a high resolution PDF version):

Start by cutting the three pieces to size. You will need to cut one 2" × 2-1/2" (50mm × 64mm) piece of 1/2" (12mm) thick wood, and two 2" × 1-1/2" (50mm × 38mm) pieces of 3/4" (19mm) thick wood. Grain direction doesn't really matter.

Drill 1/16" (1.5mm) pilot holes in all three blocks for the holes marked a through f. When drilling holes a and b, clamp blocks A and B together to ensure the holes line up. Likewise, when drilling holes c and e, and holes d and f clamp blocks B and C together. Also drill 1/16" (1.5mm) pilot holes, 3/8" (10mm) deep, for the dowel holes in the top of block C.

Next, drill a 1/8" (3mm) hole for the cable, 1/2" (13mm) deep, into the top of block A, centered in the width of the block, and 3/16" (5mm) back from the front face.

Using a 1-1/8" (28mm) Forstner bit, drill the hole for the microphone element in block A, centered 3/4" (19mm) from the top. Drill this hole 3/16" (5mm) deep, which will expose part of the cable hole you previously drilled.

Use a 5/32" (4mm) drill to enlarge holes a in block A to clear a #6 (3.5mm) wood screw. Using either a countersink bit, or a 5/16" (8mm) drill bit, countersink these holes on the back side of block A so the screw heads will sit flush. Also enlarge holes c in block B (but not holes b) to 5/32" (4mm).

Enlarge holes b in block B to 1/8" (3mm) so the wood screws will screw in easily, but still hold securely.

In block C, enlarge holes e to to 11/64" (4.5mm) so that the brass rods or tubes can slide through them fairly loosely.

Using a 3/8" (10mm) brad-point or Forstner bit, enlarge hole d in the front face of block B, to a depth of 1/8" (3mm). Similarly, enlarge hole f in the rear face of block C, but to a depth of 1/2" (13mm). This will give the spring space to compress into.

With a 1/2" (13mm) brad-point or Forstner bit, enlarge hold d in the rear face of block B, to a depth of 3/8" (10mm). This is the space that will be occupied by the head of the brass bolt.

Complete holes d and f with a 1/4" (6mm) bit. For the cleanest cuts, use a brad-point or Forstner bit again, but an ordinary twist drill will also work. If necessary, clean up the holes with a small round file. Alternatively, hole d can be drilled to only 13/64" (5mm) and then threaded with a 1/4-20 (M6 × 1) tap so that the bolt screws into the block, instead of needing to be glued in.

Drill the dowel holes in the top of block C to their full 1/4" (6mm) diameter.

Prepare two 1-3/4" (45mm) pieces of 5/32" (4mm) brass rod or tubing. If using rod, this is most easily cut with a hacksaw, whereas tubing is best cut with a hobby-sized tubing cutter (a miniature version of the device plumbers use to cut copper pipe). File or sand the ends to a slight bevel.

Assembling the Microphone Stand

Solder the audio plug to the shielded cable (or use a cable that already has a plug installed). Thread the cable through the hole in the top of block A. If the cable is a loose fit, you may want to add a layer of heat-shrinkable tubing as was done here, so that it fits the hole more exactly to prevent stress on the microphone.

Solder the shielded cable to the microphone element after unsoldering any leads that it may have come with. The cable's outer shield is soldered to the brass disk, and the inner conductor is soldered to the microphone circle. If you are not comfortable soldering directly to the element, cut the existing leads very short (about 1/2" or 13mm) and solder those the the shielded cable, being careful to avoid short circuits.

Apply double-sided foam tape to the back of the microphone, covering the entire surface except where the cable is soldered on. Before peeling off the backing, test fit the microphone to ensure it can seat firmly in its recess. If necessary, add another layer of tape, or carve away some wood to make room for the soldered connection.

Once everything fits properly, peel off the backing, carefully slide the microphone into position, and gently press it down so the tape can take hold. Be careful that the microphone does not touch the surrounding wood, otherwise it will pick up far more ambient noise.

Next, insert the bolt into the back of block B, and apply a bit of glue to the bolt head so that it cannot turn. Epoxy or thick (gel) cyanoacrylate is ideal for this. Use the wing nut to hold the bolt in place until the glue cures.

Attach block B to block A using the brass #6 × 1" (3.5 × 25mm) wood screws. If the screws are difficult to screw in, enlarge the holes slightly, as trying to force a brass screw will twist the head off. It helps to hold the two pieces together with a clamp or vice to keep them aligned while installing the screws.

Install the two brass rods or tubes into their holes, pressing them in until they are stopped by block A. If you need to tap them in, do not hit them with a hammer directly, or you will bend them or widen the ends. Instead, hold a small block of wood over the end of the rod/tube, and tap the block gently with a small hammer.

Slide the spring over the bolt and ensure it seats all the way into the counterbored hole. Then slide block C over the bolt and rods, pressing it all the way up against block B. If it won't go all the way because the spring is fully compressed, count how many coils of the spring remain visible, and cut this many off from one end.

Once the spring is properly adjusted, install the wing nut and washer, and then press the dowels into their holes. If the fit is loose, use a bit of wood glue to secure them.

Install the vinyl or rubber caps over the dowels, trimming the dowels slightly if they are too long for the caps to cover them completely.

Clean the completed stand with a soft brush, vacuum, and a tack rag to remove any traces of sawdust. Then, to isolate the microphone from the desk or workbench it is being used on, apply soft rubber or vinyl feet to each corner of the bottom, left side, right side, and back.

Finally, apply a thin bead of glue where the cable exits the top of the stand so that pulling on the cable does not strain the solder joints on the back of the microphone element.

Congratulations, your microphone stand is complete! Visit the amplifier page for plans and instructions on building an amplifier to interface this microphone to your PC.


Building a Microphone Amplifier

The microphone used in the microphone stand above produces a very weak signal, generally on the order of 10µV, whereas a sound card is expecting a signal of about 500mV to 1V at the Line In jack, or 50-100mV at the Microphone jack. The amplifier boosts the signal level up about 6000-fold, and also helps to reject background noise.

Building the amplifier, shown on the left in the picture above, requires some electronic construction experience.

The Circuit

The amplifier consists of three stages, each using one operational amplifier (op-amp) of a TL074 quad low-noise op-amp IC. Each stage consists of a passive high pass filter, followed by an amplifying active low-pass filter. An additional high pass filter follows the last stage, and together, the high pass filters greatly attenuate 50Hz or 60Hz AC (mains) hum. The active low pass filters, with a cut-off frequency of about 11kHz, reduce high frequency background noise. The schematic and circuit board layout are shown below (click image for a high resolution PDF version):

The fourth op-amp in the TL074 provides a stable "virtual ground" at about 40% of the supply voltage. This allows the entire circuit to operate from a single-polarity 9V power source (a 9V battery).

A piezo-electric transducer like the one used as a microphone in this project has a very high DC impedance and acts in a circuit like a very low-valued capacitor, on the order of 20 to 30nF. Normally, when designing a piezo amplifier for audio applications, one wants a very high input impedance in order to obtain sufficient bass response. For a watch microphone however, low frequencies should be rejected. By using a fairly low input impedance (33kΩ), the piezo transducer itself becomes part of the first high pass filter stage.

Parts and Materials

The amplifier can be constructed on a printed circuit board, or any other circuit fabrication technique you are comfortable with (an earlier prototype, for example, was constructed on stripboard). The remainder of this article assumes the use of a printed circuit board. To aid in rejection of 50Hz or 60Hz AC hum, the amplifier should be enclosed in a metal case, electrically connected to the circuit's ground. Shielded audio cable must be used for all connections to the amplifier.

The following parts are needed to construct the amplifier:

All capacitors should be rated 15V or higher. C1 through C7 should have 10% tolerance or lower. All resistors should be 1/8W or larger with 5% tolerance or lower.

The circuit board was designed with specific jacks and a switch in mind, the technical details of which are shown here:

If using different parts, you may need to modify the circuit board layout to suit. Alternatively, you can use panel mount parts, connected to the circuit board by short leads.

Construction

If using a printed circuit board, prepare, etch, and drill the circuit board as shown in the plan above. One way of doing this is described in the article, Making Excellent Printed Circuit Boards. A properly scaled version of the artwork can be found in the PDF version of the plan.

Begin by installing a socket for Z1, with the notched end pointing away from the jacks and switch edge of the board. Then install all the fixed resistors, followed by the capacitors. Note that the locations for capacitors C2, C4, and C6 have three holes each. This is to allow use of two different sizes of capacitors (0.1" or 0.2" lead spacing). If using the smaller size, be sure to install the two leads into the upper pair of holes (the two holes that are not connected on the copper side of the board).

Finally install the jacks, switch, and trimmer potentiometer R7. You may need to enlarge the holes for the jacks and switch slightly.

Install the battery clip next. There are two holes for each lead, with the ones closer to the edge of the board being for strain relief. Enlarge these holes to 1/16" (1.5mm) and relieve the copper around the holes with a larger drill bit. Feed the leads up from the bottom of the board, loop them over in a large loop, and then solder them into the other pair of holes. After that, pull the leads back to leave just a small loop on top of the board.

Enclosure

The amplifier should be enclosed in an aluminum case to shield against AC hum. A steel case should not be used, simply because it could be, or become, magnetized, which could affect the functioning of a mechanical watch. The case used here was 112mm × 64mm × 24mm, and the drawing below shows the size and location of the three holes that needed to be drilled into the panel for the switch and two jacks:

Any similar enclosure will do, so long as it has enough space to house the circuit board and a 9V battery.

Final Assembly

Double check all your work, ensuring that the all the components are installed in the correct locations, and that the polarity of C9 and C11 is correct. Also check the board carefully for solder bridges (a magnifying glass is helpful for this). Install Z1 into its socket, with the notch at one end of the IC aligning with the corresponding notch in the socket.

Install the board into the case. If you used the jacks shown, the nuts that fasten the jacks to the case are sufficient to hold the board in place. Set potentiometer R7 to its midpoint, connect a 9V battery, and assemble the case.

For the battery, the amplifier uses a standard 9V alkaline or NiMH battery, which will provide about 100 hours of continuous operation. To save money and the environment, it is advisable to use a rechargeable NiMH 9V battery (which come in 8.4V or 9.6V versions, either of which will work). The low-self-discharge variety (often sold as "precharged") is preferrable, as it will retain its charge longer during periods of non-use.


Using the Amplifier and Microphone

After finishing construction of both the microphone and amplifier, you are ready to use it.

The amplifier requires a standard 9V alkaline or NiMH battery, which will provide about 100 hours of continuous operation. To save money and the environment, it is advisable to use a rechargeable NiMH 9V battery (which come in 8.4V or 9.6V versions, either of which will work). The low-self-discharge variety (often sold as "precharged") is preferrable, as it will retain its charge longer during periods of non-use.

Plug the microphone stand's cable into the amplifier's "MIC" jack, and place the amplifier and microphone stand in a convenient location on your workbench. The microphone cable is relatively short in order to avoid picking up AC hum from lamps, appliances, and building wiring.

Plug the audio cable into the jack labelled "OUT" on the amplifier, and plug the other end of this cable into your computer's Line In jack. If your computer does not have a Line In jack (many laptops and mini-desktops do not), use the Microphone jack instead.

If you have a netbook or tablet that has a single jack used for both microphone input and headphone output (one that accepts a four-conductor smartphone-style plug), you will need an external adapter that splits out the microphone input and headphone output to separate jacks, such as the StarTech 3.5mm 4 Position to 2x Position Adapter from Amazon.

Notice that the microphone stand has soft rubber sound isolating feet on four sides. This enables testing the watch in four positions without removing it from the stand.

Finally, turn on the amplifier's power switch. It should now be ready for use with Watch-O-Scope (you may need to set your computer's audio to select the correct audio connection if there's more than one).

If you're unable to see a watch signal, check the following: