Often you need to
connect output from more than one source (preamplifier) such as tape
recorder/player and CD (compact disc) player to audio power amplifier.
This needs disconnecting/connecting wires when you want to change the
source, which is quite cumbersome and irritating. Here is a circuit that
helps you choose between two stereo sources by simple touch of your
hand. This circuit is so compact that it can be fixed within the audio
power amplifier cabinet and can use the same power supply source. The
circuit uses just two CMOS ICs and a few other
componenets. The ICs used are MC14551/CD4551 (quad 2-channel analogue
multiplexer) and CD4011 (quad 2-input NAND gate).
When touch-plate S1 is touched (its two plates are to be bridged
using a fingertip), gate N1 output (IC1, pin 3) goes high while the
output of gate N2 at pin 4 goes low. This causes selection of CD outputs
being connected to the power amplifier input, which is indicated by
lighting of LED1. When touch-plate S2 is touched, the outputs of gates
N1 and N2 toggle. That is, IC2 pin 3 is pulled ‘low’ while its pin 4
goes ‘high’. This results in selection of tape recorder outputs being
connected to the input of power amplifier. This is indicated by lighting
of LED2. Pin 9 is the control pin of IC2.
In the circuit, the state of multiplexer switches is shown with pin 9
‘high’ (CD source selected). When pin 9 is pulled ‘low’, all the
switches within the multiplexer change over to the alternate position to
select tape player as source. Note. Although one can connect pin 7 (VEE)
of IC2 to ground, but for operation with preamplifier signals going
above and below ground level, one must connect it to a negative voltage
(say, –1V to –1.5V) to avoid distortion.