Dwarf Signal Adaptor Board
The interface boards between the Raspberry Pi (3) and the dwarf signals in the parking lot are needed to translate the Raspberry 3v3dc GPIO output signal to the 12vdc used by the dwarf signals. Output pins are also included so the current stacked 5vdc LEDs can still be used.
Across the top in the photo below are the input headers from the Raspberry Pi, two ground pins and three control signals (green, yellow and red). The ICs in the middle are pvt312 photovoltaic, one for each signal color/aspect. Across the bottom are two 4-pin headers connecting a dwarf signal and/or a stacked LED signal in the parking lot. On the right is the 12vdc input voltage terminal block. The current limiting resistors were selected to protect the Raspberry PI GPIO pins and normalize the brightness of the LEDs.
Two ground pins are included on each interface board so that a single ground pin on the Raspberry Pi can be daisy-chained to each of the interface boards, one for each of the four parking spaces..