# Trying to understand (and mimic) the multimeter's interference with a circuit

I am trying to use a microcontroller to regulate the speed control of a sewing machine, rather than the foot pedal that normally serves this purpose. The pedal cord has 2 wires that connect it to the sewing machine. I analyzed the foot pedal by measuring its resistance (stand-alone, while not connected to the machine) and observed that the resistance varies from 3 ohms to 12 kilohms as it is depressed. If I try to measure the resistance through the pedal when it's attached to the operational machine, the multimeter interferes with the operation of the machine by either slowing the machine down with the probes one direction or speeding it up so much (when I reverse the probes) that the machine runs even when the pedal depression would normally be inadequate to start the machine going.

When the pedal is plugged into the machine the voltage across the 2 wires going into the pedal varies from 12 millivolts (DC) when the pedal isn't depressed up to 4.5 volts when the pedal is fully depressed.

When the system is idle with the pedal connected, the current is 4.64 mA. As the machine starts to sew, the current drops to 1.07 mA and at peak speed (pedal fully depressed), the current is 0.38mA.

I created a system with a microcontroller controling a digital potentiometer to mimic the resistance of the stand-alone pedal and varying the resistance. It only worked when I used the multimeter to measure resistance across the digital potentiometer in the orientation that speeds up the machine. Presumably it's injecting current to measure the resistance.

To simplify the diagnosis of what's happening, I did a simple experiment replacing the pedal the cable with 3 kilohms of resistance plugged into the sewing machine. Alone, the machine does not operate with this setup. When I attach the ohmmeter across the resistors the machine begins to operate, provided I have the probes in place with the "right polarity". As soon as I remove the probes or if I reverse them, the machine stops. This level of resistance corresponds to the pedal being partially depressed and well in the range of where the machine would be expected to operate.

I don't know a ton about what I'm doing, but I'd like to get some understanding of what I might modify in this simple circuit to have the machine operate. I haven't opened up the pedal to see what's going on inside, but my assumption is that it's only varying the resistance and not sending any kind of digital signal. Perhaps that's a bad assumption. Any ideas or suggestions would be most welcome.

• use a voltmeter to measure the voltage between the two probes of the ohmmeter Commented May 15 at 22:28
• be very careful ... the foot pedal may have power line voltage between its wiring and earth ground Commented May 15 at 22:30
• You should neve use an ohmmeter on a powered circuit. The ohmmeter applies a voltage between its probes and measures the resulting current (or vice versa), so any other current will affect the indicated resistance - and, as you discovered, the current or voltage applied by the meter will affect the operation of the circuit you are trying to measure. Commented May 15 at 23:29
• Thanks to all of you for pointing out my error. I'm getting guidance and doing research and I was unaware of this risk. Commented May 18 at 18:09

The pedal cord has 2 wires that connect it to the sewing machine. I analyzed the foot pedal by measuring its resistance (stand-alone, while not connected to the machine) and observed that the resistance varies from 3 ohms to 12 kilohms as it is depressed.

The pedal is likely a potentiometer with one end of the resistor and the wiper exposed and connected to the two pins.

If I try to measure the resistance through the pedal when it's attached to the operational machine, the multimeter interferes with the operation of the machine

Yes, it will do that. Multimeters use a small current source which you then apply over the component under test. The exact current used will depend on the meter.

... by either slowing the machine down with the probes one direction or speeding it up so much (when I reverse the probes) that the machine runs even when the pedal depression would normally be inadequate to start the machine going.

During normal operation, the sewing machine will put current through the pedal's pins - quite possibly in conjunction with another fixed-value resistor inside the machine... this will produce a variable voltage which can then be used to determine the desired speed. This is basically a potential divider.

If you're also injecting current from the multimeter, then the measured voltage will be influenced by the meter's current source. It's generally a bad idea to try and measure resistance on a powered system - at best you'll get an invalid reading, and at worst you could damage something.

If you're keen to measure the system "in action", then measure the voltage across the pedal / digital potentiometer instead - you can then calculate the resistance in effect.

From the figures you've shared, I'd suggest the schematic looks something like this (the numbers mostly line up, though I could be wrong).

simulate this circuit – Schematic created using CircuitLab

• This is extremely helpful and I appreciate your detailed answer. I'm still puzzling over why the machine will not go as the rheostat I have in the Pedal part of your circuit varies. I'm about to do some more measurements based on your suggestions and see if I can get further insights. Commented May 18 at 18:12
• Are you saying that when you replace the pedal with your digital potentiometer, the machine won't run? If so, you need to make sure that A) the configured range your digipot matches that of the pedal (i.e: approx 0Ω to 12kΩ)... and B) one end and the "wiper" of the digipot are wired in place of the pedal. If you use a different range, then the "speed control" voltage will be mismatched to what the machine was designed for, and it'll misbehave. Commented May 18 at 23:54
• Yes, it matches (verified) and no it won't run. The digital version couldn't get the minimum resistance down to 3 ohms so we constructed this with an analog potentiometer and matched the full range of resistance and it still doesn't run. It seems there is some other condition that the machine requires. Short of pulling the pedal apart there are no obvious indicators of what to try. Commented May 20 at 1:41