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3-Input Stereo Audio Switcher

Need more analog audio inputs for your stereo amplifier or home-theatre set-up? This 3-Input Stereo Audio Switcher will do the job. It works with an infrared remote control or you can just press one of the front-panel buttons to select a program source.

By John Clarke & Greg Swain

WHILE WE WERE developing the Input Selector board for the Ultra-LD Mk.3 Stereo Amplifier, we realised that we also had the basis for a self-contained project. All we had to do was move the PIC microcontroller to the main relay board, re-jig the circuit somewhat and house it in a metal case for shielding. That would give us a remote-controlled 3-Input Stereo Audio Switcher that would be ideal for use in any audio set-up.

Click for larger image
The rear panel carries the four RCA socket pairs, with the output pair at left.

In practice, it wasn’t quite that easy because we also had to redesign the switch board to include the infrared receiver and an Acknowledge LED. And we had to re-jig the firmware in the micro to suit the reallocated I/O ports and to eliminate the remote volume control feature used in the Ultra-LD preamp.

As shown in the photos, the unit is housed in a metal diecast case which we spray-painted black. The switch board mounts on the front panel, while four pairs of stereo RCA sockets on the main PCB (three for the inputs and one for the outputs) protrude though holes in the rear panel. Power comes from a 9-12V plugpack and the unit typically draws less than 600mW.

Virtually any universal remote control can be used with the unit and there are three different “modes” (or devices) to choose from – TV, SAT1 & SAT2. The default mode is TV but SAT1 can be selected by pressing (and holding) button S1 during power-up. Similarly, SAT2 is selected by pressing button S2 at power-up, while pressing S3 at power up reverts to TV mode.

Of course, having selected a mode you must also program the remote with the correct code. We’ll have more to say about that later on.

In operation, the unit lets you select between any one of three stereo analog inputs by pressing the “1”, “2” or “3” buttons on the remote. Alternatively, you can press the buttons on the front-panel switch board.

An integral blue LED in each switch button lights to indicate the selected input. This occurs both when a button is pressed and when the remote control is used. The blue switch LEDs also serve as power indicators, while the orange Acknowledge (ACK) LED on the front panel flashes when ever a valid remote control signal is received.

By changing a couple of linking options, you can also build the unit so that it responds to buttons 4, 5 & 6 on the remote, or to buttons 7, 8 & 9 (ie, instead of 1, 2 & 3). You might want to do this if buttons 1, 2 & 3 have been allocated to another piece of equipment (eg, to the selector board in the Ultra-LD Mk.3 Stereo Amplifier) or if you want to build two such units and control them using the same remote.

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