Introduction

There are a number of ways to build the engine starter switches with automatic jump back.  Martin van Liempt is a colleague cockpit builder who lives not far from me which makes contact very convenient.  He probably has built the nicest 737NG cockpit in the Netherlands, it's quite amazing.

He has devised a simple mechanism that slips over the switch axis, so there is no need to take the switch apart. 

The solenoid can be obtained from Conrad, part number ITS-LZ 1949.

Operation

If you turn the switch to the left (GND) it moves the plunger into the solenoid.  A small spring moves the switch back in the OFF Position.  As soon as the switch contact is closed, the software energizes the solenoid via a Phidgets Relay board and the solenoid stays energized until the starter valve closes.  If you move the switch to the CONT or FLT positions, the lever that is fixed to the switch axis will move freely through the plate that is connected to the solenoid.

The beauty of this is that when there is no power or sufficient bleed air, the software will not energize the solenoid and the switch will jump back to the OFF position.

Software

My panels program provides two digital outputs for the solenoids.  It checks the value of FSUIPC offsets "fstarter1" and "fstarter2" to be 1 and then makes the output high.