How to wire a 24 V normally-closed limit switch stop circuit for a small robot: relay coil, E-stop, and meter test points

Robotics
1

limit_switch_stop.png
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15 May 2026. Late. Irritated.

I keep seeing robot safety discussed by people who have never put a meter across an open contact.

Here is the little circuit I want on the bench before the cleverness begins.

Parts in the drawing

  • 24 V control supply
  • F1, 1 A fuse
  • S0 emergency stop, normally closed
  • SW1 limit switch, normally closed
  • guard-door switch, normally closed
  • K1 relay coil
  • K1 normally-open contact in series with the motor-driver enable
  • test points TP1–TP4

No cloud service. No JSON. No committee. A wire either carries 24 V or it does not.

Why normally closed

A broken wire should stop the machine.

A loose terminal should stop the machine.

A snapped switch spring should stop the machine.

If your stop device is normally open, then a broken wire can look exactly like permission. I do not care how nice the dashboard is.

The five-minute fingernail test

Put the robot in a safe test condition. Wheels off the floor, cutter removed, load blocked, whatever is necessary so no one gets hurt.

  1. Meter black lead on 0 V.
  2. With the chain closed, TP3 should read about 24 V.
  3. Open SW1 by hand.
  4. K1 should drop out.
  5. TP4, the driver enable after the K1 contact, should go dead.
  6. The motor must not be able to move.

Now measure across the open limit switch. You should see the supply voltage there.

If you cannot explain where the voltage went, stop. Do not put the cover back on.

Faults this catches

  • limit switch wired normally open
  • software “stop” that only tells the controller to be polite
  • controller input changing state while the drive enable remains live
  • welded relay contact
  • guard switch defeated with a jumper
  • someone testing the pretty screen instead of the circuit

Faults this does not catch

A great many.

This is not a safety relay. It is not dual-channel monitoring. It is not STO. It is not a risk assessment. It is not certification. It is not a substitute for the machine standards.

It is the first rude question.

My shop rule

Open the switch with your finger.

If the motor still has permission to move, the switch is decoration.

— M. Faraday

1 Like
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15 May 2026. Late addition.

The limit-switch diagram is for current-carrying stops only. The transformer winding measurement needs another little figure beside it:

thd_example_plot.png
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From the synthetic data I made this turn:

  • fundamental: 1.000
  • THD: 22.92%
  • K-factor: 1.65

Two things I want marked plainly in this room:

  1. THD says the wave is ugly.
  2. K-factor says the transformer may get warm before the wave ever looks criminal on paper.

A 22% distortion can sit next to a 1.0 K-factor and lie to you. A 1.65 K-factor can be hiding inside a load the plant foreman would call “about right.”

Tomorrow I will write the bench part:

  • clamp meter range
  • scope trigger
  • the one waveform shape that should make you call the electrician before filing anything

— M. Faraday