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Copyright 2018 by Wayne Stegall
February 6, 2018.  See Document History at end for details.

Input/Output Bypass Board


It is perhaps common that some optional components may be desired to switch in and out of an audio system.  Because of this some such components have a bypass switch in addition to the power switch.  This works well, but it is possible to switch in the component with the power off.  Instead a relay board at the the input/output terminals could bypass the component by default and only switch it in if the power is on.  The circuit of figure 1 below will accomplish this task.


Figure 1:  Schematic of I/O relay board.


In the given circuit DUT represents the audio component which is switched in by relays X1-X4 which can be 4 SPDT or 2 DPDT relays.  The relays normally connect to bypass the component when not powered up and switch to the opposite contact to connect the component with power is applied.  Because the same raw power supply with power the component and relay board, the power switch with activate both component power and connection.

Design Choices

When I examined possible relays, coil current seemed inversely proportional to coil voltage.  Therefore the largest coil voltage that would work with the raw power supply would draw the least amount of power.  This choice I made to be 24V.  Drive to the coils would then be supplied by voltage regulators to prevent the nearness of the coils to the signal from injecting hum.  The TO92 regulators suggested should be replaced with those in the usual TO220 package if the relay coils require.  A LM317 datasheet suggested tantalum capacitors before regulators if they were at a distance from the raw power supply, thus giving rise to choices for C1 and C3.  C2 and C4.are not required, you may omit them or choose any value you like.  The circuit board is constructed with jacks on one side and electronic components on the other.  If you like, reverse biased diodes of the 1N4001 type could be paralled with C2 and C4 to protect the regulators.

Possible parts list.

Lin, Rin, Lout, Rout  
audio jacks
X1 X4

24V SPDT reed relay


24V DPDT reed relay (in X1/X2 and X3/X4 pairs)


78L12 12V voltage regulator (TO92 package)


79L12 12V voltage regulator (TO92 package)
C1, C3

2.2F tantalum capacitor
C2, C4

100F electrolytic capacitor

Final thoughts

I built and tested a similar circuit long ago.

Document History
February 6, 2018  Created.