I just bought Jeff' F-stop timer application. Great work, Jeff! I just have one minor change request:
Can you provide a square area on the screen that lights up in red when the enlarger should be ON?
The reason is I want to use a phototransistor taped to the screen to control a relay to switch my enlarger ON and OFF.
I would suggest the top left area of the screen, since this is unused in the current UI.

An alternative solution is to put out a continuous tone (e.g. 1000 Hz beep) during exposure. This would be a more robust way of switching on and off my relay.