Lighting the Epsilon 12

Information about the ε12 delay and offset detector can be found at AMB ε12. The Compact Tube Hybrid adds a lighting circuit that powers two LEDs, one for ON and one for OFF. Here is a partial schematic.

Lighting the ε12

The circuit is simple. The PNP devices are referenced to the V+ rail. When the relay is off the collector of the MPSA14 is pulled up to the rail. This turns off the first BJT pushing its collector to ground. This in turn supplies base current to the second BJT turning it on and lighting its LED. When the relay turns on the collector of the MPSA14 pulls to ground turning on the first BJT and lighting its LED. At the same time the first BJT pulls the base of the second BJT up to the V+ turning it (and its LED) off. The base resistors protect against too much base current and the LED resistors, obviously, set the LED currents. Either two single or one dual LED can be used.