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Capacitive Proximity Sensor

Detects a broad range of targets—including many metals, plastics, liquids, and granular products—when they create enough capacitance change at the sensing face.

PLC address%I0.2
SignalMaterial detected
BenchLive + faults
FIELD DEVICE / 24 VDC

01 / Recognize it

What this sensor looks like

Learn the housing, active face, mounting, and connector before you meet it on a machine.

Representative real-world Capacitive Proximity Sensor hardware on an industrial workbench
Representative field appearance · form factors vary by manufacturer

Hardware recognition

Know what to look for

Use the silhouette, active face, and connection style to identify the device before checking its part number and datasheet.

Body and mounting
Often resembles an inductive barrel sensor but may use a plastic threaded body.
Active face
The active face detects dielectric material through a non-metal wall.
Cable and terminals
A sensitivity screw or teach control is commonly placed near the cable end.
Field rule: identify by appearance, then verify the exact wiring, range, approvals, and output type from the device label and datasheet.

02 / Understand the principle

Watch cause become a PLC signal

Follow the physical event through the sensing element and into the exact controller value.

Signal story / live loop

Capacitive Proximity Sensor: cause to controller

Paused

Now showingPhysical event

Product rises past the probe → Capacitance increases → %I0.2 = 1

03 / Test and commission it

Commission it on the bench

Move the process, adjust the setpoint, invert the logic and inject faults. Watch the PLC value respond immediately.

Commissioning bench

Capacitive Proximity Sensor

24 VDC%I0.2
OFF
35 %
55 %

PLC channel

%I0.2

RAW 0

Engineering value

35 %

Material detected

Output logic
Inject a field fault

Channel healthy

Signal is inside the expected operating range

Terminals
Brown +24 VBlue 0 VBlack OUT

Commissioning note: Calibrate with the real product; moisture and wall thickness change sensitivity.

Field guide

A capacitive proximity sensor detects targets by measuring a change in capacitance at its sensing face. The sensor contains a high-frequency oscillator connected to internal electrodes. Many metals, plastics, liquids, wood products, and granular media create a detectable change, but reliable range depends on target size, dielectric properties, distance, mounting, and sensitivity adjustment.

**Capacitive vs inductive proximity sensors** — this is the most important distinction: - An inductive sensor detects only conductive metal targets. It is unaffected by non-metallic objects. - A capacitive sensor can detect many conductive and non-conductive materials when their dielectric contrast and target geometry produce enough signal. Verify the application with the selected sensor rather than assuming every target is detectable. - Use inductive for metal-only detection in oily environments; use capacitive when the target is non-metallic or when you need to sense through a thin non-metallic wall.

**Sensing through containers** is a key application. A capacitive sensor can be mounted flush against the outside of a plastic tank or conveyor guard and detect the presence of liquid or product inside — the material changes the effective capacitance through the tank wall. The sensitivity potentiometer on the sensor is adjusted so the sensor triggers on liquid but not on the tank wall alone.

**NPN and PNP output wiring** follows the same three-wire convention as inductive sensors: - **NPN (current sinking):** the signal switches toward 0 V when active, so the input circuit must source compatible current through the load. - **PNP (current sourcing):** the signal switches toward the positive supply when active, so the input circuit must provide a compatible path to 0 V.

Brown for positive supply, blue for 0 V, and black for the primary output are common IEC-style conventions on three-wire DC sensors, but connector pinouts, cable colors, extra outputs, and indicator behavior vary. Verify the exact sensor and PLC input manuals before wiring; never choose the input common from a naming shortcut alone.

**Sensitivity adjustment** is unique to capacitive sensors — a potentiometer on the body lets you tune the sensing threshold. This is necessary because different materials have very different dielectric constants: water (εr ≈ 80) triggers much more strongly than dry cardboard (εr ≈ 2–4). The adjustment prevents false triggering on condensation, dust coating on the face, or thin container walls.

Use this when…

  • Detecting plastic bottles or cardboard cartons on a conveyor
  • Monitoring liquid level through a non-metallic tank wall
  • Sensing granular material (grain, powder, pellets) in hoppers or chutes

Where you will see it

Beverage filling

Capacitive sensors detect filled PET bottles (water changes the dielectric) through the bottle wall, confirming fill level before capping.

Packaging

A capacitive sensor through a conveyor guard rail detects cardboard cartons — inductive proximity would miss them entirely because cardboard is non-metallic.

PLC wiring reference

Trace the complete electrical path instead of treating the PLC tag as magic. Confirm the device datasheet before wiring real hardware.

  1. 1Brown +24 V
  2. 2Blue 0 V
  3. 3Black OUT

Commissioning checkpoint

Calibrate with the real product; moisture and wall thickness change sensitivity.

PLC address
%I0.2
Expected signal
Material detected

Field questions

Frequently asked questions

What signal does a Capacitive Proximity Sensor send to a PLC?

Material detected is read at %I0.2. The exact electrical connection is Brown +24 V, Blue 0 V, Black OUT.

How do you commission a Capacitive Proximity Sensor?

Calibrate with the real product; moisture and wall thickness change sensitivity.

Next skill

Connect it to PLC logic

Unlock PLC integration challenges

See plans

Free first success

Use the capacitive proximity sensor signal in PLC logic

Apply the wiring and commissioning model in a scored browser exercise, then save your progress and continue through the recommended path.

No installNo credit cardImmediate pass/fail feedback

Technical reference and worked-example guide

Capacitive proximity sensor guide: implementation, evidence and troubleshooting

Direct answer

Capacitive proximity sensor guide becomes useful when it connects target material, dielectric contrast, size, distance, container wall, sensitivity, hysteresis, mounting, nearby metal, buildup, moisture, temperature, output type and switching logic with target approach through electric-field change, oscillator and threshold circuit, transistor output, plc input, logic decision and independently observed presence or level, then proves stable detection and release across representative target, fill and environmental ranges without nuisance switching under normal, boundary, fault and recovery conditions. The objective is a repeatable engineering or learning result, not merely activity inside a page or tool.

This guide is written for automation learners and technicians detecting powders, liquids, plastics, glass or other targets by changes in an electric field. The intended result is specific: the reader can relate dielectric properties, distance, container wall and environment to detection stability and isolate setup, wiring or application faults.

a technician tracing realistic industrial sensors, signal wiring, PLC inputs and measured trends at an instrumentation learning bench while studying capacitive proximity sensing, setup and PLC input evidence
The scene keeps capacitive proximity sensing, setup and PLC input evidence connected to declared conditions, observable behavior, diagnostic boundaries and evidence that another person can reproduce.

System map / 02

Six concepts that control the result

Treat these as connected checkpoints. Each checkpoint has an expected state, an observable state and a boundary to the next part of the system. That structure prevents a software indication from being mistaken for physical proof.

NODE 01observable

Define the operating contract

target material, dielectric contrast, size, distance, container wall, sensitivity, hysteresis, mounting, nearby metal, buildup, moisture, temperature, output type and switching logic. For capacitive proximity sensing, setup and PLC input evidence, record the initial condition, actor, requested change, observable result and stopping condition before selecting a tool or implementation.

NODE 02observable

Map the evidence path

target approach through electric-field change, oscillator and threshold circuit, transistor output, PLC input, logic decision and independently observed presence or level. Separate request, internal state, output or service, physical or user-visible result and independent feedback so each boundary can be inspected.

NODE 03observable

Prove normal operation

stable detection and release across representative target, fill and environmental ranges without nuisance switching. Run more than one cycle from a known state and retain the values, timings or artifacts that demonstrate repeatability.

NODE 04observable

Exercise a boundary case

buildup, moisture, changing density, wall variation, nearby object, temperature drift, vibration, excess sensitivity, lost supply and restart. Choose minimum, maximum, simultaneous, delayed or restart conditions that reveal assumptions hidden by the happy path.

NODE 05observable

Diagnose a controlled fault

a target, dielectric, geometry, environment, sensitivity, mounting, output, wiring, input or interpretation mismatch. Preserve the first symptom, divide the system at a measurable boundary and change one condition only after predicting the result.

NODE 06observable

Transfer and hand over

the sensor trialled with actual product, vessel or package tolerances and intended PLC input. Restore normal state, remove temporary changes, repeat affected checks and document which claims remain limited to the learning environment.

Procedure / 03

A six-step practice and commissioning workflow

Run the steps in order the first time. Later, the same structure becomes a diagnostic loop: define the expected condition, observe the boundary, interpret the difference and choose one proving action.

  1. 01

    Write the acceptance case

    Convert target material, dielectric contrast, size, distance, container wall, sensitivity, hysteresis, mounting, nearby metal, buildup, moisture, temperature, output type and switching logic into initial conditions, one stimulus and observable pass criteria.

    Evidence: Another person can repeat the case without guessing the intended result.

    Avoid: Using page completion or an animation as the acceptance criterion.

  2. 02

    Build the map

    Document target approach through electric-field change, oscillator and threshold circuit, transistor output, plc input, logic decision and independently observed presence or level and name who owns each state or decision.

    Evidence: Every request and result has a source, destination and useful inspection point.

    Avoid: Using the same value as command, status and independent feedback.

  3. 03

    Run the baseline

    Apply stable detection and release across representative target, fill and environmental ranges without nuisance switching from a clean start and record the expected evidence.

    Evidence: Repeated runs produce the same bounded result.

    Avoid: Changing several parameters before a baseline exists.

  4. 04

    Challenge assumptions

    Test buildup, moisture, changing density, wall variation, nearby object, temperature drift, vibration, excess sensitivity, lost supply and restart without changing the acceptance contract.

    Evidence: Limits, timing and restart behavior reach defined states.

    Avoid: Testing only one ideal sequence.

  5. 05

    Isolate one failure

    Introduce or analyse a target, dielectric, geometry, environment, sensitivity, mounting, output, wiring, input or interpretation mismatch and locate the first disagreement.

    Evidence: The proving action distinguishes the leading hypotheses.

    Avoid: Resetting, forcing or replacing before evidence is retained.

  6. 06

    Close the evidence loop

    Complete the sensor trialled with actual product, vessel or package tolerances and intended plc input and repeat the affected regression cases.

    Evidence: Reference use is complete when inputs, assumptions, units or initial conditions are recorded and the result is independently checked at a useful boundary.

    Avoid: Treating an acknowledged message or one successful rerun as handover.

Diagnostic matrix / 04

Symptoms, proving points and next actions

The table is a reasoning aid, not a parts-replacement chart. Preserve the initial symptom, inspect the named boundary and use the interpretation to choose the next controlled test. Site safety procedures and equipment manuals remain authoritative.

Diagnostic symptoms, inspection points, interpretations and next actions for Capacitive proximity sensor guide: implementation, evidence and troubleshooting
Observed symptomInspectInterpretationNext proving action
The expected result is unclearRequirement, initial state, actor, stimulus, units and pass conditionThe technician, programmer and reviewer may be solving different versions of the task.Rewrite one observable acceptance case before continuing.
Internal state changes but the outcome does notRequest, final owner, output or service boundary and independent feedbackA software or interface indication proves intent at one layer, not the complete outcome.Trace the first boundary after the changing state.
Normal case passes but an edge case failsLimits, timing, simultaneous events, reset and restart assumptionsThe implementation contains a hidden assumption exposed by the changed condition.Add the failed boundary as a permanent regression case.
The failure disappears after resetOriginal symptom, histories, diagnostics, timestamps and active causeReset changed evidence or state without proving the initiating cause.Reproduce under a controlled condition and preserve pre/post-event data.
Simulator and target disagreeModel boundary, software version, task timing, I/O behavior, data types and configurationA learning model and the intended target do not share one of the recorded assumptions.Reduce the case and verify against current target documentation.
The result cannot be explainedPrediction, observation, proving action, alternative hypotheses and limitationsActivity occurred but the evidence is not yet transferable or reviewable.Have the learner defend the signal path and repeat a changed case.

Product evidence / 05

What the browser practice can actually demonstrate

The page connects definitions and worked examples to runnable tools, explicit assumptions and repeatable checks so a formula or pattern can be challenged.

Where simulation stops

A generic guide cannot guarantee target detection or hazardous-area suitability; representative tests and current device specifications are required.

Commissioning notebook / 06

Six cases that turn the concepts into evidence

Use these as written briefs rather than click-through instructions. For every case, state the expected condition before acting, retain the first useful observation and explain why the final result proves the requirement. A different program or component choice can still be correct when it produces the same bounded behavior and evidence.

Case 01

predict → observe → prove

Prove define the operating contract

Engineering context. target material, dielectric contrast, size, distance, container wall, sensitivity, hysteresis, mounting, nearby metal, buildup, moisture, temperature, output type and switching logic. For capacitive proximity sensing, setup and PLC input evidence, record the initial condition, actor, requested change, observable result and stopping condition before selecting a tool or implementation. Begin with a written normal condition and identify which request, state, physical result or communication value will provide independent confirmation. Do not begin by changing the configuration; the initial state is part of the evidence and should remain reproducible.

Controlled setup. Use the “Write the acceptance case” stage of the workflow: convert target material, dielectric contrast, size, distance, container wall, sensitivity, hysteresis, mounting, nearby metal, buildup, moisture, temperature, output type and switching logic into initial conditions, one stimulus and observable pass criteria. The acceptance record should show this result: another person can repeat the case without guessing the intended result. Record initial conditions, the exact stimulus and the observation point so another learner can repeat the case without relying on your memory.

Fault challenge. Introduce or analyse “The expected result is unclear” as one bounded deviation. Inspect requirement, initial state, actor, stimulus, units and pass condition The working interpretation is that the technician, programmer and reviewer may be solving different versions of the task. The next proving action is to rewrite one observable acceptance case before continuing. Change only one condition before observing the result, and preserve timestamps or measurements where timing matters.

Review and recovery. The most common trap here is using page completion or an animation as the acceptance criterion. After restoring the cause, repeat the normal case and at least one stop, timeout, disconnect or restart boundary relevant to this topic. Remove temporary forces and bypasses, return the model to a known state and retain the evidence that both operation and recovery are deliberate.

Explain it aloud: What can a capacitive proximity sensor detect? A defensible short answer is: It can detect many conductive and nonconductive materials when their dielectric effect is large enough at the installed distance and sensitivity.

Case 02

predict → observe → prove

Prove map the evidence path

Engineering context. target approach through electric-field change, oscillator and threshold circuit, transistor output, PLC input, logic decision and independently observed presence or level. Separate request, internal state, output or service, physical or user-visible result and independent feedback so each boundary can be inspected. Begin with a written normal condition and identify which request, state, physical result or communication value will provide independent confirmation. Do not begin by changing the configuration; the initial state is part of the evidence and should remain reproducible.

Controlled setup. Use the “Build the map” stage of the workflow: document target approach through electric-field change, oscillator and threshold circuit, transistor output, plc input, logic decision and independently observed presence or level and name who owns each state or decision. The acceptance record should show this result: every request and result has a source, destination and useful inspection point. Record initial conditions, the exact stimulus and the observation point so another learner can repeat the case without relying on your memory.

Fault challenge. Introduce or analyse “Internal state changes but the outcome does not” as one bounded deviation. Inspect request, final owner, output or service boundary and independent feedback The working interpretation is that a software or interface indication proves intent at one layer, not the complete outcome. The next proving action is to trace the first boundary after the changing state. Change only one condition before observing the result, and preserve timestamps or measurements where timing matters.

Review and recovery. The most common trap here is using the same value as command, status and independent feedback. After restoring the cause, repeat the normal case and at least one stop, timeout, disconnect or restart boundary relevant to this topic. Remove temporary forces and bypasses, return the model to a known state and retain the evidence that both operation and recovery are deliberate.

Explain it aloud: Why does a capacitive sensor false-trigger? A defensible short answer is: Moisture, buildup, changing product density, nearby objects, mounting, temperature, cable issues or excessive sensitivity can shift the switching threshold.

Case 03

predict → observe → prove

Prove prove normal operation

Engineering context. stable detection and release across representative target, fill and environmental ranges without nuisance switching. Run more than one cycle from a known state and retain the values, timings or artifacts that demonstrate repeatability. Begin with a written normal condition and identify which request, state, physical result or communication value will provide independent confirmation. Do not begin by changing the configuration; the initial state is part of the evidence and should remain reproducible.

Controlled setup. Use the “Run the baseline” stage of the workflow: apply stable detection and release across representative target, fill and environmental ranges without nuisance switching from a clean start and record the expected evidence. The acceptance record should show this result: repeated runs produce the same bounded result. Record initial conditions, the exact stimulus and the observation point so another learner can repeat the case without relying on your memory.

Fault challenge. Introduce or analyse “Normal case passes but an edge case fails” as one bounded deviation. Inspect limits, timing, simultaneous events, reset and restart assumptions The working interpretation is that the implementation contains a hidden assumption exposed by the changed condition. The next proving action is to add the failed boundary as a permanent regression case. Change only one condition before observing the result, and preserve timestamps or measurements where timing matters.

Review and recovery. The most common trap here is changing several parameters before a baseline exists. After restoring the cause, repeat the normal case and at least one stop, timeout, disconnect or restart boundary relevant to this topic. Remove temporary forces and bypasses, return the model to a known state and retain the evidence that both operation and recovery are deliberate.

Explain it aloud: What should I learn first about capacitive proximity sensing, setup and PLC input evidence? A defensible short answer is: Start with the operating contract and evidence path: target material, dielectric contrast, size, distance, container wall, sensitivity, hysteresis, mounting, nearby metal, buildup, moisture, temperature, output type and switching logic, followed by target approach through electric-field change, oscillator and threshold circuit, transistor output, plc input, logic decision and independently observed presence or level. Add advanced features only after the baseline is predictable.

Case 04

predict → observe → prove

Prove exercise a boundary case

Engineering context. buildup, moisture, changing density, wall variation, nearby object, temperature drift, vibration, excess sensitivity, lost supply and restart. Choose minimum, maximum, simultaneous, delayed or restart conditions that reveal assumptions hidden by the happy path. Begin with a written normal condition and identify which request, state, physical result or communication value will provide independent confirmation. Do not begin by changing the configuration; the initial state is part of the evidence and should remain reproducible.

Controlled setup. Use the “Challenge assumptions” stage of the workflow: test buildup, moisture, changing density, wall variation, nearby object, temperature drift, vibration, excess sensitivity, lost supply and restart without changing the acceptance contract. The acceptance record should show this result: limits, timing and restart behavior reach defined states. Record initial conditions, the exact stimulus and the observation point so another learner can repeat the case without relying on your memory.

Fault challenge. Introduce or analyse “The failure disappears after reset” as one bounded deviation. Inspect original symptom, histories, diagnostics, timestamps and active cause The working interpretation is that reset changed evidence or state without proving the initiating cause. The next proving action is to reproduce under a controlled condition and preserve pre/post-event data. Change only one condition before observing the result, and preserve timestamps or measurements where timing matters.

Review and recovery. The most common trap here is testing only one ideal sequence. After restoring the cause, repeat the normal case and at least one stop, timeout, disconnect or restart boundary relevant to this topic. Remove temporary forces and bypasses, return the model to a known state and retain the evidence that both operation and recovery are deliberate.

Explain it aloud: How do I practise capacitive proximity sensing, setup and PLC input evidence effectively? A defensible short answer is: Use short cases with known initial conditions, a written prediction, one action and an observable result. Then alter a boundary or fault and explain why the evidence changed.

Case 05

predict → observe → prove

Prove diagnose a controlled fault

Engineering context. a target, dielectric, geometry, environment, sensitivity, mounting, output, wiring, input or interpretation mismatch. Preserve the first symptom, divide the system at a measurable boundary and change one condition only after predicting the result. Begin with a written normal condition and identify which request, state, physical result or communication value will provide independent confirmation. Do not begin by changing the configuration; the initial state is part of the evidence and should remain reproducible.

Controlled setup. Use the “Isolate one failure” stage of the workflow: introduce or analyse a target, dielectric, geometry, environment, sensitivity, mounting, output, wiring, input or interpretation mismatch and locate the first disagreement. The acceptance record should show this result: the proving action distinguishes the leading hypotheses. Record initial conditions, the exact stimulus and the observation point so another learner can repeat the case without relying on your memory.

Fault challenge. Introduce or analyse “Simulator and target disagree” as one bounded deviation. Inspect model boundary, software version, task timing, I/O behavior, data types and configuration The working interpretation is that a learning model and the intended target do not share one of the recorded assumptions. The next proving action is to reduce the case and verify against current target documentation. Change only one condition before observing the result, and preserve timestamps or measurements where timing matters.

Review and recovery. The most common trap here is resetting, forcing or replacing before evidence is retained. After restoring the cause, repeat the normal case and at least one stop, timeout, disconnect or restart boundary relevant to this topic. Remove temporary forces and bypasses, return the model to a known state and retain the evidence that both operation and recovery are deliberate.

Explain it aloud: What counts as proof of competence? A defensible short answer is: A repeatable artifact or system result plus an explanation of the signal path is stronger than time spent, screenshots or a copied answer. Physical competence requires separate supervised evidence.

Case 06

predict → observe → prove

Prove transfer and hand over

Engineering context. the sensor trialled with actual product, vessel or package tolerances and intended PLC input. Restore normal state, remove temporary changes, repeat affected checks and document which claims remain limited to the learning environment. Begin with a written normal condition and identify which request, state, physical result or communication value will provide independent confirmation. Do not begin by changing the configuration; the initial state is part of the evidence and should remain reproducible.

Controlled setup. Use the “Close the evidence loop” stage of the workflow: complete the sensor trialled with actual product, vessel or package tolerances and intended plc input and repeat the affected regression cases. The acceptance record should show this result: reference use is complete when inputs, assumptions, units or initial conditions are recorded and the result is independently checked at a useful boundary. Record initial conditions, the exact stimulus and the observation point so another learner can repeat the case without relying on your memory.

Fault challenge. Introduce or analyse “The result cannot be explained” as one bounded deviation. Inspect prediction, observation, proving action, alternative hypotheses and limitations The working interpretation is that activity occurred but the evidence is not yet transferable or reviewable. The next proving action is to have the learner defend the signal path and repeat a changed case. Change only one condition before observing the result, and preserve timestamps or measurements where timing matters.

Review and recovery. The most common trap here is treating an acknowledged message or one successful rerun as handover. After restoring the cause, repeat the normal case and at least one stop, timeout, disconnect or restart boundary relevant to this topic. Remove temporary forces and bypasses, return the model to a known state and retain the evidence that both operation and recovery are deliberate.

Explain it aloud: Why test faults and restart behavior? A defensible short answer is: Because a target, dielectric, geometry, environment, sensitivity, mounting, output, wiring, input or interpretation mismatch or buildup, moisture, changing density, wall variation, nearby object, temperature drift, vibration, excess sensitivity, lost supply and restart can expose assumptions that never appear during ideal startup and steady operation.

Answer surface / 07

Questions people ask about Capacitive proximity sensor guide

These concise answers define the operating, training and product boundaries most often missed in broad summaries. The full workflow and diagnostic table above provide the evidence behind them.

What can a capacitive proximity sensor detect?

It can detect many conductive and nonconductive materials when their dielectric effect is large enough at the installed distance and sensitivity.

Why does a capacitive sensor false-trigger?

Moisture, buildup, changing product density, nearby objects, mounting, temperature, cable issues or excessive sensitivity can shift the switching threshold.

What should I learn first about capacitive proximity sensing, setup and PLC input evidence?

Start with the operating contract and evidence path: target material, dielectric contrast, size, distance, container wall, sensitivity, hysteresis, mounting, nearby metal, buildup, moisture, temperature, output type and switching logic, followed by target approach through electric-field change, oscillator and threshold circuit, transistor output, plc input, logic decision and independently observed presence or level. Add advanced features only after the baseline is predictable.

How do I practise capacitive proximity sensing, setup and PLC input evidence effectively?

Use short cases with known initial conditions, a written prediction, one action and an observable result. Then alter a boundary or fault and explain why the evidence changed.

What counts as proof of competence?

A repeatable artifact or system result plus an explanation of the signal path is stronger than time spent, screenshots or a copied answer. Physical competence requires separate supervised evidence.

Why test faults and restart behavior?

Because a target, dielectric, geometry, environment, sensitivity, mounting, output, wiring, input or interpretation mismatch or buildup, moisture, changing density, wall variation, nearby object, temperature drift, vibration, excess sensitivity, lost supply and restart can expose assumptions that never appear during ideal startup and steady operation.

Can browser practice replace official software or hardware?

No. It can build concepts and diagnostic reasoning. Exact firmware, I/O electrical behavior, networking, safety and commissioning require current official tools, documentation and target equipment.

How should progress be documented?

Keep the requirement, initial state, program or configuration, observed values, fault hypothesis, proving action, recovery result and a concise limitations statement.

Real capacitive proximity sensor footage

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Watch the real browser product respond to the task on this page, then try the same practical workflow yourself. No slides, concept mockups, install, or credit card.

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Capacitive Proximity Sensor — Detect Metal, Plastic and Liquid