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P2 Taqoz 2.8: Read 6 Keypad Switches with one ADC Pin — Parallax Forums

P2 Taqoz 2.8: Read 6 Keypad Switches with one ADC Pin

Christof Eb.Christof Eb. Posts: 1,072
edited 2023-04-03 16:10 in Forth

Hi,
well this should be very simple, but still it might be useful. At least I needed quiet some time to work it out.
It reads a keypad with 6 key switches.
Resistor ladder R=220
GND-R-SW1-R-SW2-R-SW3-R-SW4-R-SW5-R-SW6-3V3
All Switches connected to keysPin=P40, which has an internal pulldown 15k combined with an ADC on the same pin.
So if no switch is closed, adc reads 0V.
If SW6 is closed, adc reads 3v3.
( Yes, 1690 - 499 + is silly. And yes, it could be done without local value. :-) )

Have fun, Christof

Comments

  • How would you know which switch is closed given the connection described as GND-R-SW1-R-SW2-R-SW3-R-SW4-R-SW5-R-SW6-3V3 (these are all serially connected ?) and all R being equal ? i don't quite grasp the circuit, evidently.

  • Oh, sorry for the unclear description.
    One pin of each switch is connected to the multitap voltage divider. The other pin of each switch is connected to the port pin.
    Better?

  • Here's a thread Chip started on doing this same thing: https://forums.parallax.com/discussion/172874/multiple-pushbuttons-on-one-pin-using-adaptive-adc

    I remember reading somewhere here about how to scan a whole matrix keypad using just a single pin and maybe a 555 timer or something, but I can't seem to find it now.

  • One pin of each switch is connected to the multitap voltage divider. The other pin of each switch is connected to the port pin.

    Thank you, I get it now and answers my question too.

  • Christof Eb.Christof Eb. Posts: 1,072
    edited 2023-04-03 06:46

    @Electrodude said:
    Here's a thread Chip started on doing this same thing: https://forums.parallax.com/discussion/172874/multiple-pushbuttons-on-one-pin-using-adaptive-adc

    I remember reading somewhere here about how to scan a whole matrix keypad using just a single pin and maybe a 555 timer or something, but I can't seem to find it now.

    Thanks for this link!
    It is interesting, because I started also with the current source and a similar circuit like Chip. But due to variing resistances of the switch contacts, I then moved to switch low current for low influence of these variations. Perhaps Chip had better quality buttons. I have short cables and do only need one button pressed at a time. I also do not have all these resistor values. My project is about audio and I don't want to have sources of clicks, so there are different solutions.

    Edit: I am somewhat surprised, that Chip's solution works with such high speed. With my setup and buttons the time for stable results must be much longer although I need far less precision of voltages.

  • jmgjmg Posts: 15,140

    @"Christof Eb." said:
    I have short cables and do only need one button pressed at a time.

    If you do not need to sense any/all combinations things can be simpler for just any-single-button press.
    With 5 switches the simpler non linear approach Chip uses can decode 5 bits, to sense any/all, but it needs detection precision as good as 0.818%, and even then struggles to manage a DIP switch use where a cold start can be any combination.
    With keypad buttons, you can assume they are open at power up/ calibrate phases, that also helps.

  • New Source File appended in post #1.

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