How to check proxymity sensor with voltmeter?

Here’s a reliable way to test most proximity sensors using only a DMM (voltmeter) and a DC supply.

0) Identify what you have

• Wire colors (typical IEC): brown = +V, blue = 0V, black = output, white = NC/teach.
• Type: 3-wire DC NPN (sinks), PNP (sources), 2-wire DC (series type), or analog (0–10 V / 4–20 mA).
• Supply range: usually 10–30 V DC. Use 12 V or 24 V bench supply.

1) What you’ll need

• DMM set to DC volts
• DC supply (12/24 V)
• Test load resistor: 10 kΩ (¼ W) for logic tests; for 2-wire, 1–2.2 kΩ (½ W)
• A target (metal for inductive, dielectric for capacitive, magnet for Hall, white card for photoelectric)

2) 3-wire DC sensors (NPN or PNP, NO/NC)

Wire power first: brown → +V, blue → 0 V.
Add a test load so the output isn’t floating:

NPN (open collector, sinks): add 10 kΩ from black → +V (pull-up).
Measure black ↔ +V.

• Target absent (NO): ≈ +V
• Target present (NO): ≈ 0–1 V (NC is the opposite.)

PNP (sources): add 10 kΩ from black → 0 V (pull-down).
Measure black ↔ 0 V.

• Target absent (NO): ≈ 0 V
• Target present (NO): ≈ +V (NC is the opposite.)

Tip: Many sensors have an LED that mirrors output—use it as a sanity check.
If you test without the 10 kΩ, a high-impedance meter may show “ghost” volts due to leakage.

3) 2-wire DC sensors (series type)

These behave like a switch with internal current.

Hookup: +V → sensor → load resistor 1–2.2 kΩ → 0 V.
Measure across the load.

• ON (target present for NO): ≈ supply across load
• OFF: near 0 V across load (a few volts may remain due to leakage)

If you only put a meter across the sensor (no load), it may never switch correctly—always include a load.

4) Analog proximity sensors

• 0–10 V output: power as above, measure black ↔ 0 V; move target → voltage should vary smoothly.
• 4–20 mA output: place 250 Ω from output → 0 V and measure across it (expect 1–5 V for 4–20 mA).

5) Expected quick readings (cheat sheet)

• NPN NO: black–+V ≈ +V (OFF), ≈ 0 V (ON) with 10 k pull-up.
• PNP NO: black–0 V ≈ 0 V (OFF), ≈ +V (ON) with 10 k pull-down.
• 2-wire NO (with load): load sees ≈ 0 V (OFF), ≈ supply (ON).
• Analog: varies with distance/target (0–10 V or 1–5 V across 250 Ω).

6) Troubleshooting

• No change: wrong NPN/PNP assumption or NO/NC; add the 10 kΩ load; verify supply polarity/range.
• Always “some” voltage when OFF: normal leakage—use the 10 kΩ to bleed it.
• Board wiring fights programming pins: isolate output with 100–330 Ω if other circuits are attached.
• AC sensors: require an AC load—don’t test on mains unless you’re qualified; use an isolated source.