MJLorton Solar Power and Electronic Measurement Equipment Forum
Older Technology => Older Technology => Topic started by: SeanB on December 31, 2015, 08:22:54 AM
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None are what you see these days, but were used in old minicomputer keyboards, where they wanted the ultimate in reliability.
3 different sensing methods used. One used capacitive sensing of a graphite stub on each key, which came close to the round pads, but was separated by a mylar sheel from actual contact, then it was read by a scan board in the main minicomputer body.
The other 2 units use hall effect switches, the 3 pin ones are scanned by powering up each row from a 74154 buffer, then seeing if any open collector outputs went low when a key was pressed. the sensors are those little ceramic boards with the 3 leads, the hall sensor being the black ceramic blob in the middle. The last uses a powered sensor, with 4 leads, so you have an enable input instead of powering up each chip thousands of times a second.