The GRAK Photo board contains 4 separate photocell pre-processing circuits and 4 analog comparators. The front-end circuitry filters and buffers the perceived light intensity. Both this signal and a time-averaged version are brought out to the edge connector. Each analog comparator can be set to compare one particular light channel's intensity to a fixed threshold, the time-averaged value of the channel, or the level (or time-averaged level) of another channel by appropriate jumpering of the edge connector. The comparison threshold for each channel can be varied using the the trimpots on the back of the board. The outputs from the photocell portion of the board are 4 binary comparison lines which can be tri-stated by bringing the "Read" input high.

The GRAK Photo board also contains two independent "monostable" circuits. These can be used to stretch the duration of a digital event, or generate a specific length pulse in response to a single logic-level transition. The time-constants associated with each unit can be varied using the trimpots at the back of the board. As with the comparator signals, the monostable outputs can be tri-stated using a separate "Read" input.

Block Diagram

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