If you need to buzz a telephone and keep a LED lit to remind the operator to call back, this is a simple solution!

This board has a relay and supporting components and implements a latch circuit for alerting operators to call the Dispatcher as part of a model railroad telephone system, typical applications include:

• Lighting an indicator (a limiting resistor is provided for LEDs, but a 12V light bulb can be substituted) after a station is initially called (buzzed): the indicator will remain lit until the associated phone is picked up. This allows for an unmanned location to be called or can be used at a manned location where the operator may be busy with another task and might forget to call back.
• Lighting an indicator at the Dispatcher’s desk indicating that a call request has been made but not answered

 Features:

• 12 VDC operation
• Option of on-board power from a wall wart via 2.1mm plug or screw terminals for use with 12V layout auxiliary power bus.
• Latch is held in series with a normally closed contact from the associated phone or with our “Off-Hook Detector”
• Current limiting resistor (normally 1K ohm) for LED indicator on board
• Jumper to select 12V or ground for LED/Lamp supply
• Extra +12V and GND connections for easy daisy chaining and connections to phones
• Compact (1.2” x 2”) footprint allows the relay latch to be placed inside standard phones, close by our Cup Hook Phone or placed at a central location close to the Dispatcher

Instructions

Schematics, CAD Files, Bill of Materials