Analogue vs. Digital Signals

In modern building services and control systems, you'll encounter two fundamental types of signals: analogue and digital. Understanding the difference between them is essential for anyone specifying, installing, or maintaining control equipment.

How They Work

Digital signals are binary: 1 or 0, 24V or 0V. They're either "pump running" or "pump stopped" – no middle ground.

Analogue signals vary; they represent a percentage. A pressure transducer might be reading 8 bar out of a maximum of 10 bar, that will send a signal of 80%. There are various ways analogue signals can be sent but a common one is 0-10V - in this example it will send 8V to represent 80%.

Low level input: a digital signal (it's either in low level or it's not), and transducer: an analogue signal (between 4-20mA).

The Core Difference

Analogue = Variable (like a dimmer switch), ranging 0-100%

Digital = On or off only (like a standard light switch)

A low-level float switch is digital – the tank is either at low level or it isn't. A level transducer is analogue – it sends a varying 4-20mA signal to tell you exactly how full the tank is.

Pros and Cons

Analogue

• Best for: Precise control – maintaining exact setpoints, monitoring gradual changes

• Drawbacks: Sensitive to interference; distance limited; needs some kind of controller to read it

Digital

• Best for: Status indicators, alarms, on/off control; longer cable runs; simpler wiring

• Drawbacks: Can't show graduated values – only yes/no states

Voltage Drop

Both types can suffer if cables are too long or thin. Digital signals might fail to "switch" if voltage drops too low, while analogue signals gradually become more inaccurate.

Types of Analogue Signal

Three analogue standards dominate building services and industrial automation: 0-10V, 4-20mA, and PT100 (there are many, many more but this blog post keeps it simple for the majority of use cases).

0-10V suits local control only. This two-wire voltage signal drives dimming, valve positions, or motor speeds within a cabinet or across a few metres. It is simple to implement but suffers voltage drop over distance and picks up electromagnetic noise easily. Use it for short runs where simplicity is more important than precision.

4-20mA is the industrial standard. Rather than varying voltage, it varies current through a closed loop, making it immune to electrical noise and capable of reliable transmission over hundreds of metres. The 4mA "live zero" provides instant fault detection—if current drops to 0mA, you have a break. Most field instrumentation (pressure, flow, level) outputs this signal, and receiving devices convert it to a voltage reading via an internal load resistor. Use 4-20mA for any sensor signal travelling significant distances or operating in noisy environments.

PT100 is a resistance temperature detector, not a transmission signal. Its platinum sensing element provides 100Ω resistance at 0°C, changing in proportion to temperature for precise readings (±0.1°C) across -200°C to +600°C. Raw resistance cannot travel far without degradation, so PT100s often connect to a local transmitter that converts the reading to 4-20mA for transmission to the controller.

The information provided in this blog post is intended for general knowledge and guidance only. It does not constitute professional advice. Please consult a qualified professional for advice specific to your situation before making any decisions based on this information.