Micro Limit Switch Guide: Lever, Roller, NO or NC, and Wiring

A micro limit switch is the small clicky switch that tells a 3D printer, CNC router, gate or lid where a moving part is. They all share a 20mm body, so the real choices are how many pins you need, what sort of arm the moving part pushes on, and which contact you wire. This guide covers those three choices, then wires the switch to an Arduino or ESP32, a printer or CNC endstop plug and a 12V circuit.

In this guide

1. 2-pin or 3-pin: COM, NO and NC
2. Lever, roller or no lever
3. Which limit switch do I need?
4. Wiring to an Arduino or ESP32
5. 3D printer and CNC endstops: why NC
6. Using a limit switch on 12V
7. Mounting and adjusting the trip point
8. Industrial and optical limit switches
9. Troubleshooting
10. Frequently asked questions

2-pin or 3-pin: COM, NO and NC

Inside every micro limit switch is one spring contact called COM (common). At rest it touches NC (normally closed). Press the lever past the click and it snaps across to NO (normally open). Let go and it snaps back. A 3-pin switch gives you all three tabs. A 2-pin switch only has COM and NO, so it is open at rest and closed while pressed.

Our lever, roller and 2-pin switches have 1, 3, 2 moulded under the tabs. Hold the numbered face towards you with the lever pivot on the left: pin 1 at the pivot end is COM, pin 3 in the middle is NO and pin 2 at the far end is NC. The 2-pin switch has tabs at 1 and 3 only. The KW11-3Z with no lever has COM, NO and NC moulded above its tabs, with COM at the plunger end.

Pick the 3-pin switch when you are not sure. It does everything the 2-pin does, and the NC tab is the one you want for a machine endstop. For more on contact names, see our momentary vs latching switch guide.

Lever, roller or no lever

The arm on top is called the actuator, and you will also see it called the limit switch arm or head. It decides how the moving part has to meet the switch.

A longer arm needs less force to press and gives more room for error, but it trips a little later and less exactly. The bare plunger trips at the most repeatable point, with the least travel, so it suits a Z axis or a lid that closes squarely.

Which limit switch do I need?

Wiring to an Arduino or ESP32

A limit switch on a microcontroller needs two wires and no resistor. Wire COM to GND and the other tab to an input pin, then turn on the board's own pull-up resistor with INPUT_PULLUP. The pin reads HIGH while the contact is open and LOW while it is closed. Because the switch only ever connects the pin to ground, the same wiring is safe on a 5V Arduino and a 3.3V ESP32.

For a machine, wire it the safer way instead: COM to GND and NC to the input. Now the pin reads LOW at rest and HIGH when the lever is pressed or when a wire falls off, so the code treats both as stop. This sketch does that. It reads the pin twice, 5 milliseconds apart, so the few milliseconds of contact bounce don't print false changes. Our push button guide has a fuller debounce routine if you are counting presses.

// Micro limit switch test, wired NC: pin 1 (COM) to GND, pin 2 (NC) to D2.
// On an ESP32, use a pin such as 27 and change LIMIT_PIN to match.
// At rest the input reads LOW. Pressing the lever, or a wire coming off,
// makes it read HIGH, so both count as "stop".
const int LIMIT_PIN = 2;
int lastState = -1;

void setup() {
  pinMode(LIMIT_PIN, INPUT_PULLUP);  // the board's own pull-up resistor
  Serial.begin(115200);
}

void loop() {
  int first = digitalRead(LIMIT_PIN);
  delay(5);                          // read twice, 5 ms apart, to skip bounce
  int state = digitalRead(LIMIT_PIN);
  if (state != first) return;        // still bouncing, try again

  if (state != lastState) {
    Serial.println(state == HIGH ? "TRIGGERED: stop" : "clear");
    lastState = state;
  }
}

Upload it, open the Serial Monitor at 115200 baud and you should see clear. Press the lever and you hear a click and see one TRIGGERED: stop. Let go and it prints clear again. Pull the NC wire off and it prints TRIGGERED: stop, which is exactly what you want a machine to do.

ESP32 pins to use and avoid

• Use GPIO 25, 26, 27, 32 or 33 on an ESP32-WROOM-32 board. They have internal pull-ups and play no part in start-up.
• Avoid GPIO 34 to 39. They are inputs only and have no internal pull-up, so with INPUT_PULLUP the pin floats and reads at random. If you must use one, fit a 10k resistor from the pin to 3.3V.
• Avoid GPIO 0, 2, 5, 12 and 15. The chip reads them at power-up, and a switch held closed at that moment can stop the board starting or uploading.
• On an ESP32-C3 SuperMini, keep switches off GPIO 2, 8 and 9 for the same reason. Our ESP32-C3 SuperMini guide has its pin table.

3D printer and CNC endstops: why NC

An endstop tells the controller an axis has reached home or the end of its travel. Wire it NC and a broken wire, a crushed plug or a solder joint that cracks after a few thousand moves all read the same as a pressed switch. The machine stops, or refuses to home, instead of driving the carriage into the frame. Wired NO, the same broken wire reads as "not pressed" and the axis keeps going.

1. Use two pins of the plug. Most printer and CNC boards have a 3-pin endstop plug marked S (or SIG), GND and V+. Wire pin 1 (COM) to GND and pin 2 (NC) to S. Leave V+ empty: the switch needs no power, and a switch wired across V+ and GND shorts the board's supply every time it closes.
2. Check the plug order on your board. The order of S, GND and V+ changes between boards, so read the labels printed beside the socket rather than copying the old cable's colours.
3. Tell the firmware which way the switch reads. On Marlin 2.1, NC wiring suits X_MIN_ENDSTOP_HIT_STATE HIGH (the default) and NO wiring needs LOW; older Marlin calls the same setting X_MIN_ENDSTOP_INVERTING. In Klipper, NC wiring is endstop_pin: ^PA1 (your pin name), and NO adds a ! to make it ^!PA1. In GRBL, NC wiring needs $5=1; NO wiring uses the default $5=0.
4. Test by hand before the motors move. Send M119 on Marlin or QUERY_ENDSTOPS on Klipper. Each endstop should read open at rest and TRIGGERED while you hold the lever down. If it reads the other way, change the firmware setting, not the wire.

Using a limit switch on 12V

The rating moulded on these switches, 5A at 125/250VAC, is for AC. On 12V DC, use the switch to send a signal and let something built for the current do the heavy work. When DC contacts open, the arc between them has no zero crossing to put it out, so a motor or a strip of lights switched directly burns the tiny contacts quickly.

1. Fuse the switch feed. Take +12V through a small fuse to pin 1 (COM). A 1A fuse covers a relay coil.
2. Switch the coil. Wire NO to relay terminal 86 to switch the load on when the lever is pressed. Wire NC instead to switch it off when pressed, which is how a winch or linear actuator stops at the end of its travel.
3. Ground the coil. Relay terminal 85 goes to negative.
4. Fit a diode across the coil. A 1N4007 from 85 to 86, with its silver band towards 86, soaks up the voltage spike the coil throws when it turns off. Skip it if your relay already has a diode or resistor printed on its case.
5. Run the load through the contacts. +12V goes through a fuse sized for the load and its wire to terminal 30, and terminal 87 goes to the motor, pump or lights. Press the lever and you hear the relay click and the load start.

The crimp terminal guide covers the spade terminals that fit a relay base. To add an on/off switch to a 12V panel as well, our rocker switch guide covers hole sizes, ratings and wiring. Keep this to low-voltage DC. For a switch inside mains equipment, fit it in an enclosure and have a licensed electrician do the mains wiring, as NZ law requires for most mains work.

Mounting and adjusting the trip point

• Screws: two M2 screws through the body. Our switches have two mounting holes about 9.5mm apart, which matches most printer endstop brackets made for this 20mm switch size. Larger 28mm switches use a wider hole pattern.
• Solid stop: set the switch so the moving part presses the lever just past the click, then meets something solid. If the switch body is the only thing stopping the axis, the lever bends and the trip point wanders.
• Adjustment: drill or print the bracket with slotted holes, so you can slide the switch 2 to 3mm before tightening the screws. That is how you adjust a micro limit switch. Leave the lever unbent, because bending it changes the force and the trip point.
• Soldering: hook the stripped wire through the hole in the tab, then solder quickly with a hot, clean iron. Holding the iron on a tab for 5 seconds or more softens the plastic, and the switch can stick or lose its click.
• Strain relief: slide heat-shrink over each joint and tie the cable to the frame 20 to 30mm from the switch, so the cable flexes there and not at the solder joint. Thin 26 AWG hook-up wire is plenty for a signal.

Industrial and optical limit switches

Search for "limit switch" in NZ and you will also find much bigger switches. Here is where each type fits.

We stock the micro limit switches and Hall sensors. For a gate or machine out in the weather, an industrial limit switch from an electrical wholesaler is the right part.

Troubleshooting

Frequently asked questions

What is the difference between a micro switch and a limit switch?

A micro switch is the snap-action switch itself. It becomes a limit switch when you mount it where a moving part presses it at the end of travel. Our micro limit switches are micro switches with the lever, roller or plunger already chosen for that job.

Can I use a 2-pin switch as a printer endstop?

Yes. Wire its two tabs to S and GND, then set the firmware for NO wiring. It works, but a broken wire won't stop the axis, so the 3-pin SC7301 wired NC is the better endstop.

Do I need a resistor with an Arduino or ESP32?

Not on a short wire. INPUT_PULLUP turns on the board's built-in resistor. On runs longer than about a metre, or near motors, add a 10k pull-up and a 100nF capacitor at the board.

Will it fit the bracket from my old endstop?

Measure the hole centres on the old switch. If they are about 9.5mm apart on a 20mm body, two M2 screws drop straight in.

Can it switch my 12V motor directly?

Use a relay. The switch drives the relay coil, and the relay carries the motor current, as in the 12V wiring above.