AUDIT Audio Test Instrument

PRODUCT INDEX

• Introduction
• Features
• Option Switch
• Option Indicators
• Power Indicator
• RUN/STOP Switch
• Output Jacks
• Enclosure
• Overview
• Front Panel
• Rails
• Panel Finishing
• Bottom Panel
• Back Panel
• Top and Side Panels
• Final Assembly
• Alternatives
• Wiring
• Motherboard
• Front Panel
• Operation
• Troublehooting
• Parts List
• Revision Summary

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Introduction

The following describes how to build the AUDIT (AUDIo Test) instrument that generates audio test signals, including white and pink noise, square wave tones and audio test signals. This instrument is based on the AGEN module. Because the AGEN module is documented elsewhere, this document only describes how to build, wire, operate and troubleshoot the instrument.

The intent of this project is to show how an AGEN module can be connected to a few external components to make a simple audio test instrument. Because the instrument was built primarily for internal use, it is not as polished as one might expect from a typical project. For example, some of the labels are not placed optimally and the panels are not highly finished. But, the project does show how simple circuitry and construction techniques can readily produce a useful instrument using the AGEN module.

The AUDIT test instrument is enclosed in a custom-fabricated case that is built with surplus circuit board that is cut to size with metal shears and fastened together with aluminum angle stock, screws and glue. Because the construction of this enclosure may be of interest, it is also included in the project description.

The parts list in this document includes only the additional components and material needed to build the AUDIT intrument.

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Features

For the following, refer to Schematic 1 titled "AUDIT AUDIO TEST INSTRUMENT." The schematic shows the components, circuitry and pinouts for the AUDIT Module and also shows how the module is wired to produce the AUDIT audio test instrument. You can also download this schematic.

As shown in the function table, the AGEN module has four inputs that are coded as a single hexadecimal digit ranging from $0 to $F. Thus, the hexadecimal digit "$A" corresponds to a binary-coded input of "L H L H" in the function table. This indicates that, if the processor inputs for port bits 5, 4, 1 and 0 are wired low, high, low and high (respectively), then a 7 kHz square wave appears on the "HIGH" output and a 700 Hertz square wave appears on the "LOW" output.

The output functions available at the jacks and their drive capabilities are described in the AGEN documentation.

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Enclosure

Photo 7 shows the inside of the finished enclosure and the connections between the proto board and front panel. Note the AGEN module located on the left side of the board.

To add some polish to the project, a custom overlay was designed for the front panel using Front Panel Designer®, free panel layout software available from Front Panel Express, LLC. This software produces simple full-scale front panel layouts that can be used to lay out panel cut and drill points. The panel layout can also be printed on heavy photo paper and glued to the front panel to produce a professional-looking front panel overlay. An engraved panel can also be easily ordered using the software. The software provides a cost estimate so that various panel materials and features can be adjusted to reduce costs.

The front panel template is available on the documentation disk in the /projects/audit/hardware directory to make front panel fabrication easy. Problems with labels and component positioning that are visible on the photos have been corrected in the layout file so that your results should be better than ours.

Before cutting the front panel blank, we suggest that you print out the template so that the cut size of the panel exactly matches the template. The front panel uses a 5" wide by 4" high piece of 1/16" thick copperclad circuit board stock. Before cutting, the panel was scribed with a square using the panel edges to determine the edge start points. The panel was cut out of a larger panel using a metal shear with 4 inch jaws (the larger the better).

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Once the front panel is cut, the template can be glued or taped to the panel blank to determine the drill points (use the print option that turns on the centering marks). The best way to mark the drill points is to use an optical punch. If you routinely do layouts, this can save a lot of frustration: punch points are always positioned correctly and punched vertically. The punch we use was purchased from The Flexbar Machine Corporation and is model 14517. The cost is about $50, but is well worth it if you regularly do any layout work.

Once the drill points are marked, the panel holes can be drilled or punched. If you use the template, these will exactly match the panel overlay. To produce a finished panel overlay, print another template (without the hole centering marks) on heavy duty printer paper. We used the heaviest stock we could find with a glossy finish. For some reason, the semi-gloss results were disappointing, but this was probably more a matter of technique than materials.

Be very careful when cutting and handling the front panel. For easier handling and to facilitate the next step (spray coating), leave an inch margin around the overlay edge markings. The overlay should be handled only by grasping these margins (or by the edges). In Photo 2 you can see some of the background printing has rubbed off at the left side because it was touched before final preparation.

Before applying the overlay to the front panel, it must be coated with a clear acrylic spray to give a glossy finish and protect against abrasion and water damage (ink jet inks are not waterproof).

Almost any high-quality clear acrylic spray coat can be used. We have used several different brands without noticing much difference. The essential "trick" to getting a good coating is to keep the overlay completely flat during spraying and drying. To ensure that the overlay remains completely flat during spraying, we suggest taping it to a scrap piece of flat plywood or pressboard using painter's tape applied across the unused margin of the rough-cut panel overlay. Alternatively, you can fasten tape to the board with the sticky side out and stick the cut-to-size overlay to that. Photo 11 shows such a tape-up for a different audio generator that uses this technique.

Spray the overlay in an sheltered space that that is relatively free of airborn dust (outside is not recommended). If possible, spraying should be performed in a reasonably warm environment (e.g., during the day). Of course, the spray area should be well-ventilated.

Spraying technique is not particularly critical, but we recommend thoroughly shaking the can before spraying. Apply several light coats first to ensure full coverage across the overlay surface. After the light coats, spray heavily. The spray distance is not too critical: a foot away or somewhat closer produce good results. The spray should be close enough to heavily coat the overlay. Keep the can moving: don't stay in one spot. The coats may visibly build up in some spots but, if the surface is flat, any buildups should flow together smoothly. If you haven't prepared an overlay before, we recommend making a small test overlay. Once sprayed, the overlay should be left undisturbed overnight or until the coating tests completely dry (test in the areas that will be cut out).

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The preparation of the overlay is not as difficult or involved as the above instructions might make it seem.

Once you have a sprayed and thoroughly dried overlay, there is one final step before applying it to the front panel: cutting out the holes marked on the overlay. The best way to cut out the holes is to use a set of hollow punches. These are heavy metal punches that are typically used to cut out round washers and gaskets from leather or rubber sheets. We used an inexpensive 9-piece set, #3838, purchased from Harbor Freight. These cost under $10.

To cut the holes, tape the overlay flat on a piece of pressboard or plywood with the hole markings facing up (see Photo 11). For each hole, select the appropriate punch -- a little oversize is better than undersize because nuts and flanges will usually cover the excess cutout.

Punch the holes with a large heavy hammer with a big head (you don't want to miss and hit the panel). A small hand sledge hammer or shot-filled hammer is ideal. Most holes will take considerable force and perhaps several hits at different angles. Be sure to cut completely through: don't be tempted to pull out a "hanging" hole piece.

Again, it isn't all that difficult but a few test punches on the overlay edges will help to get a feel for the punch technique. One last caution: do not punch the overlay before spraying! If you do this, the acrylic spray will soak into the backing paper and discolor the overlay. Also, the overlay will tend to stick to the spray board (not good). There is no substitute for a clean cut hole. Cutting the hole with a exacto knife or punching out small holes with an awl are a waste of time. Above all, don't even think about drilling, even with a Forstner bit.

After punching, cut the excess edges off the overlay (don't touch, use disposable gloves). Now you should have a finished overlay that is ready to glue to the drilled front panel. There are probably many good ways to glue the overlay to the panel blank. Contact cement, rubber cement or spray adhesive should work, but we haven't tried these.

We use our old friend E-6000®. For panel work we use a thinner consistency E-6000® labeled Quick Hold®, but normal consistency is also satisfactory. The cement should be applied in a zigzag pattern in two directions at right angles to each other. The glue should also be applied near (but not at) the edges. Glue should be applied in an ring around the bigger holes but not close enough to them to bleed out into the hole space. E-6000® is very forgiving and if there is a little excess at the edge or in the holes, it can be easily trimmed after drying. It is often not necessary to spread glue out to the hole edges because indicator flanges and panel mounting nuts will hold the overlay near the hole.

After drying, excess glue can be removed by gently rubbing it with your fingers or a soft cloth. Wiping the glue while it is wet is generally not advisable, but usually successful.

To press the overlay flat against the panel, a rubber or plastic roller works well. As long as there isn't any excess glue, you can place a heavy weight on the panel while it dries. Put weight on the panel with the overlay side down -- using the plywood spray/punch sheet provides a flat, clean glue-up surface. If the panel sticks slightly to the board, it can usually be lifted off without damage to the overlay (E-6000® is very forgiving for small amounts of excess glue -- it usually rubs right off).

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Photo 2 shows the front panel overlay glued on a piece of circuit board. Some of the panel mounting hole outlines have being punched out with a gasket punch to reveal pilot holes drilled according to a layout overlay. Other holes have been drilled, reamed or punched to their final sizes.

One final note: the glue instructions say that it can be used like contact cement by coating both surfaces to be glued and letting them dry to a semi tacky consistency. This may be a better way to glue the overlay to the panel but we have not tried it.

For the following description, refer to Photo 7.

The enclosure is held together with rails made of 1/2" by 1/2" aluminum angle stock. Photo 7 shows two of these panels joining the front, bottom and back panels. The angle stock is fastened to the panels, with glue or 4-40 screws, as appropriate. In most cases, the rails are attached with screws, but the front panel rails are glued to the panel to eliminate drilling screw holes through the front panel.

For screw-fastened rails, two holes are drilled in the rails and tapped to eliminate the need for a fastening nut. Users who do not want to tap holescan use nuts. A "poor man's PEM nut" can be made by gluing nuts or small tapped standoffs to the rails after the rail is screwed down. Screw holes are spaced consistently with a drilled template rail used as a drill guide. The four drill holes on the template should be drilled so that each hole pair is spaced differently for each rail side. Otherwise, longer screws might interfere with each other.

To fasten side panels to the enclosure, 1/2 inch threaded standoffs are glued in the corners of each rail end (see photo). This provides a secure threaded attachment for the sides. When gluing the standoffs to the inside corners, do not glue the standoffs flush with the rail ends. The standoffs should be glued so that they are pulled back from the rail end by about 1/16" -- this ensures that the side panels fit flush against the rail ends and do not rest on the standoffs which could otherwise vary in their protrusion from the end rails.

The bottom panel is built with a 4" by 5" piece of copperclad. The prototyping board for the AGEN module and other components is mounted to the bottom panel by first assembling the proto board with four threaded standoffs mounted in on its corners and then glueing the finished assembly to the bottom by applying a dab of glue to the standoff ends. You can increase the strength of the standoff to panel joint by fastening a washer on the standoff ends before applying the glue. This mounting method is used in a number of our projects because it eliminates having to drill accurate mounting holes in a panel. Photo 10 shows this mounting method.

The rail for the front panel is first glued to the finished bottom panel. In retrospect, fastening the rail to the bottom panel with screws and a tapped rail might provide a more secure fastening (but, the glue is extremely secure after a few weeks).

The front panel is glued to the bottom panel as shown in Photo 3 and Photo 4. As shown, the glue-up is facilitated with a corner clamp commonly used for picture frames. The bottom panel is not glued flush with the front panel: it is glued so that the bottom is 1/8" up from the bottom edge of the front panel. This is shown Photo 5. This is particularly important if you mount the front rail to the bottom panel with screws.

The rail for the back panel was also glued to the bottom panel but mounting with screws may be somewhat faster (i.e., glue-up time is eliminated). The bottom panel is, like the front panel, raised up 1/8" from the edge of the back panel. In Photo 5, note that, during the glue-up, the bottom panel is spaced up from the back panel with an 1/8" thick sheet of green plastic.

The back panel is a 4" by 5" piece of copperclad. A 1/2" hole is punched in the back to allow the 5 Volt power plug to pass through the panel and plug into the power jack on the proto board assembly. This pass-through is shown at the top right of Photo 10.

The back panel is fastened to the bottom panel with screws and tapped holes, as described above. The top rail is glued to the back panel with the top panel screwed in place, as shown in Photo 6. The back panel's top rail mounts about 1/8" from the edge so that the top panel will rest below the front and back panels. This also ensures that the screw heads for the top panel are not visible from the front panel.

In retrospect, it would probably be better to fasten the back panel's top rail with screws rather than glue. Although the glue is secure enough, screws are very secure and allow small adjustments. Also, with several right angle joints secured only with glue, the enclosure flexes and appears a bit flimsy. This is ok because everything becomes secure after the enclosure is buttoned up and allowed to dry for several days. However, it is better to use screws whenever possible.

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The enclosure described above was an experiment to see if a custom enclosure could be built with parts on-hand. But, it probably makes sense to use pre-fabricated railing and less-expensive panels. One rail material that looks interesting is supplied by MicroRAX. The rail runs about $0.25 an inch, and the connector plates are extra. This makes a reasonably low-cost enclosure but is not nearly as cheap as the home-brew system described here.

Another custom chassis alternative is to use an existing enclosure and cover its front panel with a custom overlay like the one described above. This is much faster and probably cheaper than custom-fabrication.

One good source of nice project cases is old Heathkit instruments. The enclosures (or even complete instruments) are often available for a few dollars at Ham swap meets. They come in various sizes. A nice small case size is the one that is used for Q Multiplier and SWR meters. The next larger size case is one typically used for electronic switches for old oscilloscopes. This function is now obsolete and the switches are typically available with all knobs for around $5.00.

We have found that front panel switch and control cutouts can often be re-used. Because the overlay covers the entire panel, unused holes are covered up. Photo 13 shows a Heathkit front panel overlay for another project. The LCD display cutout was nibbled out using the front panel template temporarily pasted to the panel. The cutout can be a bit rough as long as it is slightly larger than the cutout pattern. That way, the precision-cut overlay covers the rough metal edges and presents clean edges.

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Photo 7 and Photo 10 show the two ends of the motherboard with the AGEN module plugged in.

For the following, also refer to Schematic 1 to show the correspondence between the schematic and the instrument components described below.

Two 8-pin rows of socket strips are soldered to the left end of the board to receive the pins of the AGEN module. A power jack and input capacitor also mount near a corner of the left end to receive the power jack and act as a power distribution point.

On the other end of the board there is a 16-pin machined-pin socket for the DIP-format resistor array. PL3, a 2 by 5 shrouded IDC plug, is mounted near the resistor network because its pins wire straight across to the resistor network. The network provides current limiting for the Option indicators (HEX display) and the power LED on the front panel (two resistors in parallel).

Connections to the Option LEDs are via a color-coded IDC socket and cable assembly. The socket, SO3, connects between PL3 on the motherboard and the HEX display LEDs on the front panel. The indicator cable is stripped out on the LED ends and soldered to the individual LEDs. We used heat shrink on the wires to avoid shorts and dress up the wiring. Tie wraps secured over the heat shrink neatly bundles the LED wires an they come off the front panel.

A separate 2-pin Molex plug/socket pair (PL5 and SO5) mounts on the front right edge of the motherboard to connect the resistor array to the Power LED.

Similarly, a 2-pin Molex plug/socket pair (PL2 and SO2) mounts on the back right edge to connect the RUN/STOP switch to the reset line on the AGEN module.

PL4 is a 2 by 5 shrouded plug that wires the HEX-coded option switch to the motherboard, similar to the IDC plug and socket used for the PL3/SO3 combination. Because the HEX switch had pin connections for soldering to a circuit board, a socket was attached to the panel end of PL4 so that connections are made with a plug/socket pair at the panel.

Lastly, the three front-panel jacks connect to the motherboard via the PL1/SO1 plug and socket combination. Because I used some on-hand board-mount BNC jacks, it was easier to wire the jacks and PL1 socket assembly before attaching the jacks to the front panel, as shown in Photo 12. With normal panel mount BNC jacks it may be possible to wire the jacks in place.

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When the unit is plugged in, the Option (HEX display) LEDs will light up in a pattern that corresponds to the HEX-coded option switch. Turning the option switch will change the pattern. The functions corresponding to each pattern are listed in Table 1 on Schematic 1. For example, selecting the pattern with all LEDs on ($F), will produce siren tones at the High and Low outputs when the RUN/STOP switch is cycled.

To start a new set of tones, turn the Option knob to the HEX pattern for the desired tones and cycle the RUN/STOP switch. The forces the processor to reboot with newly-selected voltages on the pins used to select options. After reading the option pins, the processor produces the selected tones until the RUN/STOP switch is pressed again. The AGEN documentation describes the functions generated for each switch Option.

The following parts list is for the enclosure, motherboard and chassis components used for the AUDIT project. Refer to the AGEN documentation for a description and parts list for the AUDIT Module. As noted earlier, the AUDIT Module is an AGEN Target Module programmed with AUDIT firmware.

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Revision Summary