Page 43 - WilliamsMidway WPC 91 Schematics 16-9289

General Dlumination
The General Il1umination circuit contains five separate strings of up to 18 bulbs each for a
max imum total of 90 bulbs . Each string of bulbs is controlled by a triac that in turn, is controlled
by the microprocessor.
The microprocessor has control of the triacs through a latch that it uses to store the control signals.
The General Illumination circuit can be dimmed . The microprocessor, by using a Zero Cross
Detection circuit , has the abi lity to know when the AC line-voltage is passing through a zero cross .
Dimming is achieved by the microprocessor sending a control signal to the 74LS374 latch which
turns the triac On atsome point after the zero cross has been detected . The longer the delay the
dimmer the bulbs. Once the triac is turned On, the control signal must be removed immediately to
allow the bulbs to turned Off at the next zero cross. The microprocessor then delays the turn On
again. This is repeated for every AC cycle that the bulbs are dimmed . The zero cross circuit uses a
LM339 comparator to detect the point when the AC Iine-voltage crosses zero .
To turn the bulbs On without dimming the processor sends a control signal to the triac and leaves
the signal applied.
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Block Diagram of General illumination Circuit
When point "A" toggles low , then points "B" and "C" are high . This turns On the triac and the
desired General Illuminat ion string lights.