This one can be left unused or you can us it to ring a bell animate a bandmaster the choice is yours.
This button is only used when setting up j-Omega Electronics Board see text.
Multi function LED see text.
To your MIDI playing device.
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EM interference would probably occur when the voltage is cut to the solenoid. Ensure that there are reverse diode on the solenoid coil to catch the back EMF at the source.