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@@ -385,14 +385,24 @@ Here is another excellent article:
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I found the above at https://hackaday.com/2018/09/30/whats-the-cheapest-way-to-scan-lots-of-buttons/
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-#### Notes for the ESP8266
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+### Notes for the Hardware Design
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+
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+#### ESP8266
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I got burned on this. On many boards, the gpio15 has a pulldown resistor on board.
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And gpio16 does not have an on-chip pullup resitor. That makes it difficult to use
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these pins with the techniques above. FWIW.
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+#### Two Diodes
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-
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+The final design above involves two diodes in series, and assumes that will present the
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+GPIO pin with a logic "low". I measured a pair of 1N4148 diodes in series with 100uA going
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+through them, and got 0.5V each, for a total of 1.0V presented to the GPIO. Many microcontrollers
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+will accept that as a logic low, but some may not. (The ESP8266 documents a logic low as
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+any voltage lower than 0.825V, an AtTiny threshold is 1.0 .....both @ 3.3Vsupply). The trouble
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+starts when the abient temperature drops down -60C, each Silicon diode will rise by 0.13V, and the
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+combination of both diodes will take it way out of detection range. So, if your keypad is going
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+to be outside, *definitely* use Shottkey diodes.
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