The Thomas White Resonant LPG 292c PCB utilizes a 3 pole on-off-on switch to control the mode of the module. You can manually select from lopass, gate or both.
The fonitronik expansion board replaces this switch by a logic, it is actually nothing but a gate controlled 3-pole on-off-on switch.
The layout of the board is optimized for use with Thomas White’s great PCB for the Resonant LPG292c. However it can be used as a gate/manually controlled switch for other projects or even as a stand alone.
Since this project utilizes a CD4053 it is limited in the amplitude of signals that can be switched. It covers a range of about 15Vp-p, that is from -7.5V to +7.5V. Take this in consideration when building a stand alone or using the board with other projects. For the LPG 292c you will be fine.