A quad bidirectional and bipolar CV controlled SPDT switch. When the CV input is low, the Common I/O will be connected to the Normally Closed I/O. When the CV input is high, the Common I/O will be connected to the Normally Open I/O. Handles the full range of eurorack voltage levels safely. Handles AC or DC signals just fine, though it may produce a click when used with some audio signals. Has optional current limiting which can be included to protect your other modules from being patched output to output accidentally, or omitted to better preserve gain levels. There are four identical circuits in each module.

The logic level for the CV control is about 100 millivolts by default, set by the 1k and 100k resistor divider closest to the TL074. You can change the 1k to 10k to change this to 1 volt or to 27k to change this to about 2.5 volts. 

When building, start with the lowest components first, resistors, capacitors, ICs and diodes. The resistors labeled 1k* are the optional current limiting resistors. If you want to include current limiting DO solder in these resistors and DO NOT close the solder gaps. If you would rather omit current limiting to preserve gain levels, exclude these resistors and close the solder gaps. When all of the SMD components are soldered, place the board connecting pin headers in their homes in the upper and lower boards and solder them with both boards in place to ensure correct alignment. Place the I/O jacks in their homes and install the faceplate while soldering, again to ensure correct alignment.