Keyboard links to microcomputer

Posted on Dec 12, 2012

You can divide a large keypad or keyboard into sections of 12 keys each (Figure 1). Each section connects to a separate comparator circuit, which detects a keystroke and generates a trigger pulse using the monostable circuit of IC4, and you can add more sections in the same manner. Diodes D1, D2, and D3 couple and isolate the comparators` outputs to Pin 2 of IC4. Each keypad section connects the same selection of timing-resistor values to the monostable. Therefore, each keypad section`s output pulse widths occupy the same range: 110 to 1320 µsec.

For the microcontroller to identify an active keypad section, the circuit generates a single, double, or triple pulse, depending on whether the pressed key resides in section 1, 2, or 3, respectively. The microcontroller identifies the pressed key by meas

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