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Old 29th Oct 2018, 10:11 pm   #116
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,482
Default Re: MK14 schematic revisions

To further illustrate the above:

Image #1: Circuit diagram of external keypad and its connections to the MK14 edge connector. This also shows which 'fingers' of the edge connector need to be joined together for any desired keypress. I should say that this diagram relates to the keypad edge connector of an original MK14, modern replicas may or may not have the keypad edge connections in the same order.

Image #2: Optocoupler output circuit which replicates the above, to enable an external device to 'type' programs into the MK14. This is the scheme used by my interface, but it requires one optocoupler per keypad switch position. Tested, works, but complicated to build.

Image #3: Simplified scheme proposed by TonyDuell using 12 optocouplers. In this method the controlling micro turns on one column opto and one row opto simultaneously to 'press' a given key. Not tested, but I can't see why it would not work. You could drive the 12 opto LEDs either with 12 individual outputs from a Pi or Arduino - this is simple to visualise and to wire up and programme for, or, to save some micro I/O lines for other purposes the 12 opto LEDS could be arranged in a matrix and driven by 3 + 4 output lines.

Image #4: Same idea implemented using a pair of back to back 4051 ICs. Also untested. Each IC has an 'on' contact resistance of 120R so with 2 ICs in series the 'on' contact resistance therefore rises to 240R. However, the original MK14 key contacts were made from conductive rubber sheet which also had a fairly high 'on' resistance, so the combined series resistance of two 4051s should still be low enough to work. The controlling micro outputs a 3-bit column select value to the first 4051 and a 3-bit row select value to the second 4051, then takes the enable pins low momentarily to 'press' the selected key. This method only uses 7 bits of the controlling micro's I/O port so it is doable even with the most basic model of Pi or Arduino. Although the Pi outputs 3V logic levels, experience with Pis driving other 5V logic has shown that the 3V logic-1 output level from a Pi pin is high enough to be seen as a logic 1 input by a 5V logic device.
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