fairchild ic testing


I’m trying to connect a Joystick/Encoder/Button (Copal Electronics CJ25) to a Raspberry PI. Using gpiotest I managed to get it up and running pretty well – using 3.3V input (though the data sheet mentions 5V).

The problem is, that each of the joystick’s axes has 3 levels of voltage as output. 0x, 0.5x and 1x input voltage. So with 3.3V input there’s 0V, ~1.7V, 3.3V.

Is there a way to simply step up/down voltage (resistors?), so that by using two different GPIOs I can detect all three states with those digital I/Os ?

  • 1
    Ah, the joystick’s tristate logic of 0V, 1.7V, 3.3V is interesting. I immediately think of using an ADC, eg MCP3208 to convert to 2 bit values, 00, 01, 10. Of course for Arduino with ADC pins, we don’t need any external ADC. For Rpi, Low level max is about 0.8V, so 1.7V and 3.3V are above 0.8V will be read as High. Your two different pins way looks good. So if we step down the input to 1/3, then one pin can tell 0V and 1.7V/3 = 0.6V as low. The other pin can tell the non stepped down 1.7V and 3.3V as high. In other words, if one pin says high, the other pin says low. Then it middle. – tlfong01 2 days ago   
  • Thanks! l’d really like to avoid any additional ADC, but by using two GPIOs I’ld need to know what resistor values to use, and that depends not only on voltage but on current, right? As I understand the max overall current is 50mA for all GPIOs. Maybe someone had a hint on how to safely connect that single input to two GPIOs stepping down the voltage by resistors on the 2nd GPIO? Thanks in advance!! – ftw 2 days ago
  • The Pi is pretty irrelevant to this question. – joan 2 days ago
  • 1
    I agree two GPIO pins is better than an external ADC. I said I immediately thought about ADC because I have been using ADC too often. I must admire your two input pins to differentiate 3 logic level is innovative and I have never thought of that. Coming back to the voltage divider using resistors, a very simple rule of thumb is to limit GPIO current to 2mA. So take 3V3 as maximum, a safe total resistor value is 3V3/ 2mA ~= 3k/2 ~= 1.5k. To add a little bit safe margin, I would use total resistance ~= 4k7 to 10K. My calculation is dodgy, not proofread. 🙂 – tlfong01 2 days ago   
  • @ftw, I read about tri state logic but never seen a real device using tri state logic. So I was curious to read the datasheet to see the spec. But I was surprised not to find and logic table showing three states as you said 0x, 0.5x and 1x. Can you confirm if you are really using CJ25, and let me know which logic table shows three states. As I can see the “A” and “B” outputs are only two states. – tlfong01 2 days ago   
  • @joan The Pi is not that irrelevant to this question as I’m asking for a PI-safe way of using it’s 3.3V GPIOs with this encoder/joystick. – ftw 2 days ago
  • @tlfing01: The tristate is described on the 2nd page of the data sheet at “Joystick”->”Output signal”. It states 0, 2.5 and 5 volts, but I tested it with 3.3V and got the equivalents, so the “neutral” state results in half input voltage. I’ll give the resistors solutions a try, thanks! – ftw 2 days ago
  • @tlfong01 … left side of last page … joystick diagram … center position is 2.5V – jsotola 2 days ago
  • @ftw, you could use two comparators to detect the position of one joystick axis … use something like a 555 timer as a voltage comparator ( or use a 556 dual timer ) … set one to trigger at 1V … the other at 2V … the two outputs would be read as 2 bits of data – jsotola 2 days ago
  • @jsotola, Oh my goodness, how could I miss that. I thought hard and have concluded two reasons: (1) Old dog cannot learn new tricks, (2) my eye/brain communication/control/analysis (neural network pattern recognition etc ) is a very little bit weaker/peculiar from average/normal (I knew this when I was 14. More about this later). – tlfong01 2 days ago   
  • Now the old dog is learning new tricks. It starts with the following: (1) JoyStick – Wikipedia en.wikipedia.org/wiki/Joystick (2) Getting started with the Sense HAT Joystick projects.raspberrypi.org/en/projects/… (3) SenseHat python roll [on hold] raspberrypi.stackexchange.com/questions/99763/… – tlfong01 2 days ago   
  • @jsotola, OMG, old dog cannot learn new tricks agn. Have been using 555 many years, always multi frequency band, adj freq osc sig gen. I know 555 can do monostable. So I only know how to use 555 as bistable or monostable, but never as a comparator. I vaguely remember RS/JK/D flip flop principles. But I forgot if bistable means oscillator. So the old dog even forget old tricks! 😦 I embarrassingly admit I was once a Fairchild engineer, testing ICs 555/556/709/741/723/78xx/79xx, cannot remember more names, but mostly noble, shiny, bright, metal cans, not ugly look cheap black plastics. – tlfong01 just now   Edit   


You MAY be able to use the Pi logic levels to detect this, but it would be unreliable. (The trigger point is ~1.3V, but varies from Pi to Pi.)

The only thing you can rely on in <0.8V => LOW; >2.0V => HIGH.

See Electrical Specifications of GPIO and https://www.raspberrypi.org/documentation/hardware/raspberrypi/gpio/README.md

An ADC would be overkill, all you need are voltage comparators – these are cheap, and available in multi comparator packages e.g. LM339 or LM393. See https://en.wikipedia.org/wiki/Comparator

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