The Cutting Edge

Construction of an audioswitch.

Background:
When you have a lot of stereo equipment, and few inputs in the amplifire, you need some sort of switch than can select more inputs at one channel, i.e. an audioswitch.
Because I felt like building something cooler than to connect everything to a switch connector I started to look at digital multiplexers. (To take several inputs and sending them to a single output is called demultiplexing.)
It appeared I already had one in my collection, so I started to connect... Wow, it worked! So I fixed a counter and decoder that could step between the channels. The trickiest part was to make sure the button didn't cause many impulses per push. This is because the contact make tiny bounces when it changes state, and you get several (extremely short) pulses. It is enogh for the fast electronics to register as multiple button presses. Hmm, how do I solve it? Well, a low-pass filter that only allows pulses of low frequencies... then the first push is the only one registered.
I put a couple of LED's (light emitting diodes) in to indicate which channel that's selected to be active.

[ schematics ]

Construction:
Power supply
Because I planned to run the circuits by battery I use CMOS-circuits because they have logic levels based on the power supply voltage, and will work until the battery is really empty.

The multiplexer needs double power supply and ground. First I tried a virtual-ground generator.

[ battery with virtual-ground generator ]

(V_CC=positive power supply, V_SS=ground, V_EE=negative power supply)
It worked, but the LED's didn't light up properly.
An old fashioned voltage divider then? It should probaby draw more current. A little laboration and I managed to drive it properly without sucking current, so I chose that solution.

[ battery with voltage divider ]

Note the uneven voltage distribution 10k/1k. It probably depends on that the positive voltage is loaded more.
A 9V battery is sufficient to get 2*4.5V (+ and -). The measured total current cunsumtion measuerd on the finished product is about 4.3mA. That mean a normal alcaline 9V battery of 550mAh should suffice for 127 hours!
I also put a power switch on the back of the box so you can switch on and off the power supply.

Circuits
The project is based on a 4052 analogue double 4-channel multiplexer/demultiplexer. It works both ways, so that you could use the audioswitch to switch an output signal too. (I haven't tried this myself.) The counter is a 4040 12-step binary counter, which is enough for quite a few channels (4096 to be exact). I only use 4 channels, the rest of the bits are unused. The decoding for the 4052 is made with the AND-circuit 4081 which contains 4 units, and inverting schmittriggers ((1)4584), used as NOT-circuits. Two schmittriggers are used in the pulse forming together with the filter.

In Sweden you can order the things from ELFA. They have a superb online store with a lot of pictures.
My order looked about this. (I have forgotten if I had some of it previosly at home... - check it!)

Prices are in SEK (and from the days of the construction).

Pos ArticleNr Description              #    price/unit   Total  English comment
-------------------------------------------------------------------------------
  1 60-104-33 Motst 330ohm 5% 1/4W     10       .90       9.00  Resistors
  2 42-258-27 Phono chassihylsdon      10      7.05      70.50  Phono connectors
  3 42-042-02 Batterikontakt            1      2.05       2.05  Battery contact
  4 48-135-80 IC-sockel 16-pol löd      3      5.80      17.40  IC sockets
  5 48-135-64 IC-sockel 14-pol löd      2      5.10      10.20  IC sockets
  6 50-107-15 Plastlåda svart 112x      1     45.50      45.50  Plastic box
  7 73-780-60 14584 hex Schmitt-tr      1      7.85       7.85
  8 73-696-30 4081 4x 2-in AND          1      4.65       4.65
  9 73-678-24 4040PC MM5640BN logi      1      7.50       7.50
 10 73-679-49 4052 logikkrets           1      6.05       6.05
 11 35-084-21 Ministrömst slutande      1      6.35       6.35  Push button
 12 75-029-09 3mm LEDhållare 10st/      1     14.95      14.95  LED sockets
 13 73-242-62 TLE2426CLP jordgener      1     12.35      12.35  Ground generator (see text!)
 14 55-288-15 Kopplingstråd 6x10 m      1     71.50      71.50  Cables
 15 55-035-94 Buntband 99x2,5 natu      1     15.65      15.65  Cable ties
 16 50-569-40 Gummifot 10 mm A 921      4      1.30       5.20  Rubber feet (for the box)

Total sum:                                              306.70
Rebate:                                                   9.20
Price excl. VAT:                                        297.50

As seen you can leave out some parts too...
I seem to have changed to 220ohm resistors after the order. (Had some at home.)

Assembling

The circuit assembly are in sockets on a veroboard with 2.54mm hole distance (1/10"). With that method you won't have to etch anything.

The lines must be cut at the right places to make it work. (se figure)

Color codes:
Red	Line with + power supply.
Black	Line with ground level.
Blue	Isolated connecting wire.
Green	Marking to cut the veroboard line.
Light blue dots are soldering points for external connections.

[ board layout (as seen from above) ]

Tip:
- Cut the lines with a razor blade or knife.
  I prefer razor blade, because you can get support with the fingers against the board for a more precise cut. (Don't cut the wrong lines!)
- First fix the veroboard, then the soldering of sockets, and last the soldering of components.
- Use a vice or something to hold the board.
- Drill holes for the attachment. I mounted the board in the box with a little help of some threaded distance bolts and screws.
- Try to make as many of the short connections as possible by soldering across neighbouring lines. Reduced amount of cables gives a much cleaner (and prettier) construction.
- Solder cables to the RCA connectors before you attach them to the box! Or else the box might melt from the heat, plus that you won't be able to reach the soldering points. Use cable ties for the cable bundle.
- Think about when and where the connector nuts are used, so you won't have to unsolder to get them in place.
- Solder IC-sockets on the board and you won't have to worry as much about ESD protection or soldering heat damaging.
- The construction gets very compact, so make sure to keep the wiring short and think over where everything will go. The critical part is to fit the battery.
- Check all soldering and cutting with a magnifying glass before you begin short circuiting something.
- Check it again.
- Try it out before mounting. (If you aren't a lucky bastard.)
Mounting in a plastic box makes drilling holes very easy.

[ front view of the box ]

Drill holes for the LED's, switches and the connectors on the back side.

[ layout inside the box ]

Remember, all images links to bigger versions!
Good luck!