General Illumination (GI): contains five separate strings of up to 18 bulbs per string, with a maximum of 90 bulbs. Each string is controlled by a Triac, which is controlled by the microprocessor. The microprocessor has control of the triaces through through a latch that it uses to store control signals. The GI circuit can be dimmed. The microprocessor has the ability 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 after the zero cross has been detected. The longer the delay, the dimmer the bulbs.
To turn the bulbs on without dimming the microprocessor sends a control signal to the triac, and leaves the signal applied. When the 74LS374’s output (point A) goes low, the collector of the 2N4403 (point B) and the triac (point C) go high. This turns the triac on, which turns on its general illumination string.