your guide to pinball in Belgium
Description of the BLC board (CONTROL OF LIGHTS)
This board, (which in number of three equips this machine) has the job of controlling the lights of the game, which are the ones that lit in accordance to the progress of the play. They control up to sixteen groups of little lamps.
It is composed of a decoder 4028, eight thyristors C106F, eight resistors and sixteen rectifier diodes. Each thyristor can control two groups of little lamps which are powered from the transformer with two half-cycles opposite to the a.c. voltage.
The control inputs D0, Dl, D2 & D3, come from the device output (8) of the IOS board.
The inputs D0, Dl & D2 are in charge of determining the address to which of the eight outputs must be connected. The fourth input D3 is high when such address must not be connected and low when it must be connected.
The pulse applied to the port of the thyristors is of a very short lenght of time, around 20 microseconds and enough to turn-on the thyristors which, once fired, will not disconnect until the end of the corresponding half-cycle. These pulses repeat in each half-cycle while the corresponding little lamp must be lit.