Simulating 0.5V button push with Arduino 5V output

Question

I've been tasked with modernizing the controls on an ancient cnc sewing machine. In order to access some of the controls in the quickest and simplest way I need to simulate button presses on the onboard programmer pcb. I have 12 buttons I need to access with an Arduino program that will convert embroidery gcode to appropriate button presses.

My current method is to solder leads over the NO buttons so I can close the circuits without losing the ability to use the buttons manually. The problem is that the voltage of those circuits is only 0.5VDC.

It's probably a simple answer and I'm overthinking it but how could I short over the buttons while somehow using the Arduino 5VDC outputs. I've went through a bunch of relay/transistor examples and I haven't quite found something that fits my needs with my limited understanding.

All in all, I'm hoping to switch a bunch of 0.5VDC with 5VDC at a fast rate (machine can do 400 stitches per minute), and still hold up for 8hr cycle times 5 days a week.

Answer 1

The multimeter in DC mode will average the voltage being measured if it is not DC. So, the 0.5V measurement could simply be due to the buttons being multiplexed, so the voltage across them is not constant but pulsed instead. You can check for this by setting the multimeter in frequency mode. If it does measure a frequency, then the buttons are multiplexed. You can also check how they're wired: if there are less traces going to the microcontroller than buttons, and if several traces go to several buttons, then they are multiplexed.

If it is multiplexed, you won't be able to just connect an arduino output to the buttons. This would be a bad idea anyway: if the arduino is powered and the machine is not, you don't want the arduino outputs to inject current into the machine's micro.

The 4066 analog switch solution will work in all cases (multiplexed or not) so it is the simplest. It also provides some form of isolation, not galvanic isolation but it will prevent the arduino from sending current into an unpowered machine.

74HC4066 is an easy to find 4-switch chip. Its internal resistance is about 50 ohms, that should work fine. Just wire one analog switch across each button. You need 3x about 50c chips for your 12 buttons, so it's not going to break the bank.

You must connect the sewing machine micro's ground with the arduino and analog switch ground. It's best to keep wiring short and tidy. Add a decoupling cap on the board with the 4066's.

Check the power supply for the sewing machine microcontroller. If it is lower than 5V, you're good to go. If it is higher than 5V (unlikely) then the analog switches should run from the higher voltage supply, with voltage translation. These switch chips can't handle voltage higher than their own power supply.

If you steal 5V power from the sewing machine to power your arduino, add a schottky diode to avoid powering the machine's microcontroller from the arduino's 5V, and make sure the power supply can deliver the required current.

Have you searched for an IO port on the machine? Serial perhaps... that could also be useful.

Answer 2

Without going into this weird 0.5V control voltage (is this a diode drop or transistor base or something?) you could do it using 3 CD4066 analogue switches with the control inputs run from the Arduino and the analogue switch elements strapped across the switches. Presuming the switches don't pull some massive current that is. Have you measured the switch current?

Answer 3

IF the button are not multiplexed (it's quite likely they are multiplexed, as it would reduce the number of pins required on the microcontroler of the sewing machine).

And IF the buttons are normally open and connect the "output" to ground when closed (another thing that has to be checked).

THEN, there is a very simple solution : you directly connect an arduino pin to the "output" of each button (ie the side not connected permanently to ground). When you want to "push" the button, you put the pin in output mode and write à 0. When you want to put the button in "released" mode, then you put the pin as INPUT (high impedance), so that it has no effect on the pin (so by default the button is released, but you still can press the button manally if you want). Be carefull : NEVER write a 1, you would probably distroy your sawing machine.

NB : this solution corresponds only to one very specific use case (which yields a far simpler solution). In any other case, you will need either to use the general solution (relay, optocouplers, ...) or to tailor something to your specific case