Building the new MOSFET switching board for the 1053 Emulator - almost done

MOSFET SWITCHING BOARD CONTROLS FOUR FEEDBACK SIGNALS TO THE 1130

The IO Selectric typewriter that is the basis of the 1053 console printer has microswitches that control four feedback signals - End of Line, End of Forms, CB Response and CR-LF-Tab Interlock. Three of these signals deliver +12V or are open circuits, while the CB Response signal delivers +48V or is open. 

My switching board design uses P channel MOSFETs to act as the microswitches, controlled by a 0 or 5V logic output from the Arduino inside my emulator. An NPN transistor controls the gate of each MOSFET, either leaving it at the supply voltage (12 or 48) or dropping it enough to switch on the MOSFET. 

For the three signals that are 12V level, I used an MPS2222A transistor but that won't work for the 48V signal. Every NPN transistor I had in my shop had a maximum voltage between emitter and collector of 40V or under. In operation, with the MOSFET turned off, the transistor will see the full 48 or 12V. 

I chose a BD139 transistor, which has an 80V maximum Vce rating, but had to order it. It will arrive tomorrow afternoon when I can add it to the board and wire this into the emulator.