Digital Output Level Sensor – OS2: The Complete Guide

In this guide: What "digital output" means for a level sensor (and why it is not analog), how NPN / PNP / open-collector outputs work with pull-up and pull-down, why point-level sensors like the OS2 use digital output, how the OS2's infrared total-internal-reflection optics drive its digital signal, wiring to MCU / PLC, digital vs. 4–20 mA analog, applications, and the complete FAQ.

Digital Output Level Sensor – OS2: Quick Answer

A "digital output level sensor" gives a discrete on/off signal - present or absent, high or low - not a continuous value. The OS2 is the Optomax-family digital optical level sensor: an infrared total-internal-reflection tip detects liquid, and the sensor outputs a logic-level digital signal (typically NPN open-collector or PNP, sink/source) that reads HIGH or LOW at a controller - there is no analog 4–20 mA. "Digital output" means the sensor's output transistor switches between two states: an NPN (sink / open-collector) output pulls the line LOW when wet (needs a pull-up resistor), and a PNP (source) output pulls it HIGH when wet (needs a pull-down). Digital output is ideal for point-level alarms and pump/valve control; choose analog (4–20 mA, 0–10 V) only when you need the continuous 0–100% level. The OS2's "digital" is exactly this - a logic-level switch, not a measuring transmitter.

What "Digital Output" Means

Two States, Not a Number

Digital vs. analog output:

A switch, not a gauge: A digital output level sensor reports a binary state - the liquid is at the sensing point, or it is not. The controller sees a logic HIGH or LOW, never a level percentage. That is perfect for "pump on when low" or "alarm when high," but useless if you need inventory volume. An analog output (4–20 mA, 0–10 V, HART) instead reports the continuous level. The OS2 is digital: it is a point-level switch, not a transmitter.

NPN vs. PNP - Sink vs. Source

The Two Digital Output Flavors

How each reads:

Sink pulls down, source pulls up: An NPN (sink, open-collector) output connects the line to ground when triggered, so you add a pull-up resistor to V+ and the controller reads LOW when the sensor is wet (liquid present). A PNP (source) output connects the line to V+ when triggered, so you add a pull-down resistor to ground and the controller reads HIGH when wet. "Open-collector" / "open-drain" just means the transistor's switch node is left open until it conducts - you supply the pull. The OS2 presents its digital state this way; confirm NPN or PNP on the specific part.

Why Point-Level Sensors Use Digital Output

On/Off Is What They Do

Digital fits the job:

The trip is binary: A point-level sensor exists to report "liquid reached this height" - a yes/no event. A digital output delivers exactly that with a simple, cheap, noise-resistant logic line and no ADC, no calibration of a span, no 4–20 mA loop. That is why float, optical, conductive, capacitive, and vibrating-fork point switches almost all use digital outputs. You only move to analog when the application needs the number, not the trip.

OS2: Digital Output From Optics

How the IR Tip Drives the Signal

OS2 signal chain:

Optics become a logic bit: In the OS2, an infrared LED and phototransistor are optically coupled through the sealed tip. In air the IR returns to the phototransistor (no liquid); when liquid wets the tip, the IR escapes and the phototransistor current changes. A comparator with hysteresis decides the state (preventing chatter at the trip point) and drives the output transistor - NPN (sink) or PNP (source) - which presents the digital HIGH/LOW to the controller. The "digital" in OS2 is this output stage: the optics decide, the transistor reports it as a clean logic level. (For the verified Optomax principle, see OS2 Digital Optical Level Sensor.)

Wiring to MCU / PLC

Reading the Digital State

Typical connections:

Match the pull to the type: For an NPN OS2, connect V+ and GND to the supply, add a pull-up resistor from OUT to V+, and the MCU/PLC digital input reads LOW when the tip is wet (liquid present). For PNP, add a pull-down from OUT to GND and read HIGH when wet. Share ground between sensor and controller. Most PLC digital inputs are sink or source type - match the OS2's output polarity to the PLC input type (sink input with PNP source sensor, source input with NPN sink sensor). Confirm the exact OS2 output polarity and supply (Optomax-class 8–30 V DC) on the datasheet.

Digital vs. Analog (4–20 mA)

Pick the Right Output

Trip or tape: Use digital (OS2) for safety trips, pump on/off, and valve control - cheap, simple, no calibration. Use analog 4–20 mA (a capacitive or ultrasonic transmitter, for example) when you need the actual fill percentage or volume for inventory. Many tanks use both: an OS2 (or similar digital switch) for the low/high trip and an analog transmitter for the number. The output type follows the question you need answered.

Applications

Where a Digital OS2 Fits

Installation Notes

Clean Trip, Clean Signal

Confirm before wiring: Know whether the OS2 is NPN or PNP, add the correct pull resistor for an open-collector output, share ground, and rely on the built-in hysteresis (or a controller deadband) to avoid chatter. Keep the optical tip clean so the digital state reflects the real liquid, not a coating. Validate the HIGH/LOW at commissioning by simulating the level.

Frequently Asked Questions

Q1: What does "digital output" mean on a level sensor like the OS2?

"Digital output" means the sensor reports a discrete two-state signal - present or absent, HIGH or LOW - not a continuous level value. The OS2 is the Optomax-family digital optical level sensor: its infrared total-internal-reflection tip detects liquid, and the sensor presents that detection as a logic-level output (typically NPN open-collector or PNP, sink/source) that a controller reads as HIGH or LOW. There is no analog 4–20 mA or 0–10 V; the OS2 is a point-level switch, not a measuring transmitter. Digital output is the right choice when you need an on/off trip (alarm, pump, valve), and analog output is for when you need the continuous 0–100% level.

Q2: What is the difference between NPN and PNP digital output on a level sensor?

NPN (sink, open-collector) and PNP (source) describe which way the output transistor switches the line. An NPN output connects the line to ground when triggered, so you add a pull-up resistor to V+ and the controller reads LOW when the sensor is wet (liquid present). A PNP output connects the line to V+ when triggered, so you add a pull-down resistor to ground and the controller reads HIGH when wet. The sensor's state is the same (liquid present / absent); only the polarity of the line differs. Match the output type to the PLC input type - a sink (NPN) sensor pairs with a source input, and a source (PNP) sensor with a sink input - or use the appropriate pull resistor at the controller.

Q3: Why does the OS2 use a digital output instead of 4–20 mA?

Because the OS2 is a point-level switch, not a transmitter. Its job is to report "liquid reached this height" - a binary event - and a digital output delivers exactly that with a simple, cheap, noise-resistant logic line that needs no ADC, no span calibration, and no 4–20 mA loop. A 4–20 mA analog output would only be useful if you needed the continuous 0–100% level, which the OS2 does not measure. For pump on/off, alarm, and valve control (the typical OS2 duties), digital is simpler and cheaper. If you also need inventory volume, add a separate analog transmitter alongside the OS2 rather than asking the OS2 to output analog.

Q4: How do I wire an OS2 digital level sensor to a PLC?

Match the OS2's output polarity to the PLC input. For an NPN (sink) OS2, connect the supply (Optomax-class 8–30 V DC), add a pull-up resistor from the output line to V+, and connect the output to a PLC sink-type digital input - the input reads LOW when the tip is wet. For a PNP (source) OS2, add a pull-down resistor to ground and connect to a PLC source-type input - it reads HIGH when wet. Share ground between the OS2 and the PLC, and confirm the exact output type and supply on the datasheet before wiring. Use the built-in hysteresis (or a controller deadband) to prevent chatter at the trip point, and validate the HIGH/LOW by simulating the level during commissioning.

Q5: When should I choose a digital output level sensor versus an analog one?

Choose digital when the application only needs an on/off action at one or more set points - alarm at low, pump on at low, shutoff at high, valve open/close. Digital is cheaper, simpler, needs no ADC or span calibration, and is noise-resistant. Choose analog (4–20 mA, 0–10 V, HART) when you need the actual continuous level or volume - inventory tracking, dosing by percentage, or closed-loop control on level. Many systems use both: a digital switch (like the OS2) for the safety/pump trip and an analog transmitter for the number. The decision follows the question: "is the liquid at this point?" → digital; "how full is it?" → analog.

The Bottom Line

A digital output level sensor reports a discrete on/off state - HIGH or LOW - not a continuous value, and the OS2 is the Optomax-family digital optical level sensor that does exactly this: an infrared total-internal-reflection tip detects liquid, a comparator with hysteresis decides the state, and an output transistor (NPN sink / open-collector or PNP source) presents it as a clean logic-level digital signal with no analog 4–20 mA. "Digital output" means the output switches between two states: NPN pulls the line LOW when wet (needs a pull-up), PNP pulls it HIGH when wet (needs a pull-down); open-collector/open-drain just leaves the switch node open until it conducts. Point-level sensors like the OS2 use digital output because their job is a binary trip - alarm, pump on/off, valve - which a simple logic line does better and cheaper than an analog loop; move to 4–20 mA only when you need the continuous 0–100% level. Wire the OS2 by matching NPN/PNP to the PLC input type, add the correct pull resistor, share ground, keep the optical tip clean, and validate the HIGH/LOW at commissioning. Digital answers "is the liquid at this point?"; analog answers "how full?" - the OS2 answers the first.

Last updated: August 2026

Disclaimer: This guide explains the concept of a digital output level sensor using the OS2 (Optomax-family digital optical level sensor) as the example, for educational and specification-reference purposes. The Optomax family and its operating principle are established and documented - e.g., the Optomax Industrial Series part LLC200D324-003 (8–30 V DC, 7.5 mA max, 1 A sink/source, −25 to +80 °C) and the Basic Series LLC200A3SH. The specific "OS2" designation and its exact output polarity (NPN vs. PNP), supply scheme, and parameters could not be pinned to a single public datasheet entry and are presented as Optomax-family / representative class values; confirm the exact OS2 variant against the official SST Sensing / Process Sensing Technologies Optomax datasheet. NPN/PNP, open-collector, pull-up/pull-down, and digital-vs-analog concepts are general electronics principles. This guide is not affiliated with, endorsed by, or sponsored by any sensor or PLC manufacturer.