This circuit is intended as an extension for (pre-) amplifiers, to increase the number of inputs. Ever since the introduction of MD-recorders, DVD players, etc., the owners of older amplifiers have had a chronic shortage of inputs. The application of this switch box also makes it possible to loop the audio outputs from the DVD player and video recorder to the audio system, without the need to turn on the TV. This is very handy when the audio installation is positioned some distance away from the video system and you only wish to listen to the sound from the DVD/MP3 player, for instance. The circuit makes use of two bistable relays, which have two changeover contacts each. This makes the circuit nice and compact and also avoids the need of exerting considerable force on the shaft of a rotary switch. The relays can now be operated with three small buttons (S1 to S3). The relays are 12-V models, which operate just fine on a 9- V battery.
In order to reduce the power consumption to virtually zero, a number of differentiator networks (C1/R2, C2/R5, C3.R8, and C4/R11) have been designed that generate the necessary pulses for the relay windings. Every relay has a SET and RESET winding. The third stereo input is directly connected, via the normally-closed contacts of relay Re2, to the output. The other two inputs are connected via Re1 to the normally-open contacts of Re2, and from there to the output.
To select the third input only Re2 has to be reset. That is the purpose of the small circuit around T3. When pressing S3, T3 is made to conduct for a sufficiently long period (several milliseconds) via C3/R8/R9, to ensure that the relay switches over. R7 is required to discharge C3 quickly when the pushbutton is released. The differentiator networks have the advantage that even when the pushbutton remains activated the current consumption is still very low (<25 μA). Only the initial pulse amounts to a little over 0.5 mA.
We can, therefore, expect the battery to last several years (beware of possible leaks). If the first or second input is desired then Re1 has to be set or reset via S2/T2 and S1/T1 respectively. Re2 has to be set when S1 or S2 is pressed. The fourth differentiator network takes care of this. When S1 or S2 is pressed, D1/D2 and the small circuit around T4 generate a set pulse for Re2, so that Re1 will be selected. A 1000-μF electrolytic capacitor in parallel with the battery acts as an ‘emergency power supply’ when the battery is nearly exhausted. Every pulse at 9 V amounts to nearly 10 mA. The relays are industry-standard types and pin-compatible with the common V23042-B2203-B101 from Siemens (called Schrack these days). R13 limits the ‘short-circuit current’ (through C5) when the battery is first connected. The connections for the three pushbuttons appear on the PCB in the locations where they were most convenient, in order to keep the PCB as small as possible. Whoever is tempted to use a slide switch has to keep in mind that in that case there is always one close contact and a small current flow as a consequence. To facilitate testing, the power supply (marked with ‘+’) is available next to each connection. If the three pushbuttons are mounted on a panel a single common ‘+’ will suffice of course. The PCB shown here is unfortunately not available ready-made.