Turn the knob on a traditional dimmer and you’re rotating the shaft of a potentiometer – a humble three-terminal variable resistor that has dimmed UK light fittings for sixty years. Turn the knob on a Samotech SM323 smart dimmer, by contrast, and you’re spinning a rotary encoder – a digital input device that hands its reading to a microcontroller. The two look identical on the outside, but they enable wildly different things.

Quick answer

A potentiometer is an analog variable resistor – the knob position physically changes resistance, which in a dimmer sets the firing angle of a TRIAC and so the brightness of the bulb. A rotary encoder is a digital input that outputs pulses as you turn it, with no physical link to the dimming state; a microcontroller reads the pulses, decides what to do, and dims the light. Potentiometers are simple and cheap; rotary encoders enable smart-home control, scenes, voice commands and remote operation.

What is a potentiometer?

A potentiometer is a passive three-terminal device with a resistive track and a sliding contact (the “wiper”). As the shaft rotates, the wiper moves along the track, and the resistance between the wiper and the two ends changes proportionally to position.

You’ll find potentiometers everywhere in analog electronics – guitar amplifier tone knobs, radio volume controls, car heater controls and, most relevantly here, traditional light dimmers. The key characteristics are:

• Analog output – resistance varies continuously between 0 Ω and the maximum value
• Fixed travel – typically about 270° of rotation between two physical end stops
• Position = state – the knob’s physical position always indicates the current setting
• No power required – purely passive, no electronics needed to read the value
• Mechanical wear – the wiper rubs against the track every time it’s used, so eventually the track wears out

How a potentiometer dims a light

Inside a traditional UK rotary dimmer like the Samotech Standard 2-Way Rotary Dimmer, the potentiometer is part of an RC trigger circuit for a TRIAC (Triode for Alternating Current). Here’s the simplified sequence on every half-cycle of the 50 Hz mains:

1. The mains AC voltage rises from zero towards its peak.
2. A small capacitor connected through the potentiometer charges up at a rate determined by the pot’s resistance.
3. When the capacitor reaches a threshold (typically around 32 V for a UK diac), it triggers the TRIAC.
4. The TRIAC switches on and lets current flow to the bulb for the rest of that half-cycle.
5. At the next zero crossing, the TRIAC turns off, and the cycle repeats.

Rotating the knob alters the potentiometer’s resistance, which alters how quickly the capacitor charges, which alters how long into each half-cycle the TRIAC waits before firing – and that controls how much of the AC waveform reaches the bulb. Less waveform = dimmer light.

This is called leading-edge phase-cut dimming (the cut is at the start of each half-cycle) and it’s how the cheapest, simplest dimmers have worked since the 1960s. It’s a beautifully economical design: the potentiometer plus a few passive components controls a few hundred watts of mains directly. See our leading edge vs trailing edge dimmer guide for full detail on the dimming method.

Limitations of potentiometer dimmers

For straightforward swap-in dimming, a potentiometer-based dimmer like the Samotech standard 2-way is perfectly adequate. But it has fundamental limitations that no amount of polish can fix:

• The knob is the state. If you turn the lights off at the wall switch and back on later, the dimmer comes back to whatever level the knob was last left at. There is no memory.
• No remote control. The only way to change the dim level is to physically rotate the knob.
• No automation. You can’t tell a potentiometer to dim at 9pm.
• No scenes. You can’t recall “movie night” with one button press.
• Mechanical wear. The carbon or cermet track inside the pot wears every time you rotate the knob. After several years of daily use the wiper develops scratchy contact, manifesting as audible “zips” and visible flicker when the knob moves.
• Limited travel. The pot has fixed end stops, so the knob has a definite “full off” and “full on” position. You can’t add more brightness or sensitivity once it’s mounted.
• Not networkable. The dimmer has no idea what other lights are doing in the house. Each pot dimmer is its own isolated island.

These aren’t bugs – they’re consequences of an analog design where the knob position physically controls the resistance. To overcome any of them, you need to break the direct mechanical link between the knob and the lamp current, and that means moving to a different sensing technology.

What is a rotary encoder?

A rotary encoder is a digital input device that produces a sequence of electrical pulses as the shaft rotates. Unlike a potentiometer, a rotary encoder doesn’t have a “position” in the analog sense – it tracks movement. Each click of the shaft generates a pulse pattern that a microcontroller can interpret as either “step up” or “step down”.

The most common type used in smart dimmers is the quadrature encoder, which has two output channels (often labelled A and B). The phase relationship between A and B reveals the rotation direction:

• If A leads B by 90°, the shaft is turning clockwise (step up)
• If B leads A by 90°, the shaft is turning anticlockwise (step down)

Key characteristics:

• Digital output – discrete pulses, not continuous values
• Continuous rotation – no end stops, the shaft can rotate forever in either direction
• State stored in software – the microcontroller keeps track of the cumulative position
• No mechanical contact wear on a resistive track (optical encoders are completely contactless; mechanical encoders still have contacts but produce digital signals instead of analog ones)
• Push-button often integrated into the shaft for combined rotate-and-press control

How the SM323 series uses a rotary encoder

The Samotech SM323, SM323-MT and SM323-WF all use the same physical rotary encoder hardware paired with a Zigbee, Matter or WiFi microcontroller respectively. When you turn the knob, the microcontroller reads the encoder pulses and:

1. Adjusts the internal dim level variable (0 – 100%)
2. Sends a PWM signal to the dimming output stage
3. Reports the new dim level back to the smart-home hub over the radio
4. Optionally triggers other linked actions (scenes, paired lights, automations)

Press the shaft (it’s also a push-button) and the microcontroller toggles the light on or off without losing the dim level – it just remembers the last brightness and recalls it on the next press.

What this enables

• Memory of last level – the dimmer remembers the brightness setting even when the bulb is off
• Remote control – change the dim level from your phone, voice assistant or any room in the house
• Scenes – “Reading mode” sets multiple lights to specific dim levels with one tap
• Schedules and automations – dim lights at sunset, brighten in the morning, off at bedtime
• Linked devices – the SM323 in the kitchen can dim the SM309 in the hallway in parallel
• Voice control – via Home Assistant, HomeKit, Alexa, Google Home, depending on the model
• Firmware updates – new features and bug fixes can be pushed over the air
• Energy and event reporting – the dimmer can report how long it’s been on, how much energy it’s used, and trigger automations on state changes

None of this is possible with a potentiometer, no matter how high quality the pot is, because the knob’s position is the state – there’s no software layer in between.

Potentiometer vs rotary encoder – side by side

Feature	Standard rotary dimmer (potentiometer)	Samotech SM323 series (rotary encoder)
Knob travel	Fixed ~270°	Continuous (infinite rotation)
State storage	Physical knob position	Microcontroller memory
Remote control	No	Yes – Zigbee / Matter / WiFi
Smart-home integration	None	Home Assistant, HomeKit, Alexa, Google, SmartThings
Memory of last level when off	Tied to knob position	Stored in firmware
Scenes / presets	No	Yes
Multi-device sync	No	Yes
Voice control	No	Yes (via hub)
Firmware updates	N/A	Over-the-air updates supported
Push-button on/off	Usually a separate rocker	Built into the rotary shaft
Lifespan failure mode	Carbon track wears out	Encoder is largely solid-state
Price (UK)	~£20 – £35	~£40 – £60
Best use case	Direct like-for-like replacement of an old halogen dimmer	Smart home, automation, future-proof installs

Which type should you choose?

Choose a standard potentiometer dimmer if…

• You’re directly replacing a 1960s – 90s halogen dimmer and don’t need any smart features
• You don’t have or want a smart-home hub
• You’re on a tight budget and only need basic dimming on one circuit
• The dimmer position is somewhere you’ll always interact with manually (a bedside lamp, a workshop)

The Samotech Standard 2-Way Rotary Dimmer is purpose-built for this scenario: trailing-edge, LED-compatible, fits a UK back box, works with the same wiring as the dimmer it replaces, and pairs cleanly with the SM323 in 2-way installs where one end is smart and the other is a backup.

Choose an SM323-series rotary encoder dimmer if…

• You want voice control, scheduling or scenes
• You already have or are planning a smart-home hub (Home Assistant, HomeKit, SmartThings, etc.)
• You want the dimmer to integrate with motion sensors, sun position, or other smart devices
• You want to control multiple lights as a group from one rotary knob
• You want future-proofing – the SM323 family supports OTA firmware so new features can be added over time

The trick is that the SM323 also works as a standalone dimmer with no network – press to toggle, rotate to dim. So even if you never connect it to a hub, it works exactly like a smart-looking rotary dimmer. Add the network later, and it transforms into a fully smart device without any rewiring.

Combining the two in a 2-way circuit

One of the most common Samotech installations is an SM323 at the “supply” end of a 2-way circuit paired with the standard potentiometer dimmer at the “load” end. This gives you smart control from the main switch position while keeping a simple rotary dim at the other end for backup. The wiring details are in our switch-to-a-dimmer wiring guide.

Frequently asked questions

Why doesn’t the SM323 knob “click” at full off or full on?

Because it’s a rotary encoder, not a potentiometer – there are no physical end stops. The knob can rotate forever in either direction. The microcontroller caps the internal dim level at 0% and 100%; rotating past either end has no effect.

Does the SM323 wear out from repeated use?

The rotary encoder itself is rated for tens of thousands of rotations. The microcontroller, power supply and dimming stage are entirely solid-state. There’s no carbon track to wear out, no contact bouncing, no analog drift.

Can I tell which type of dimmer I have without opening it?

Yes. Try rotating the knob past where it “should” stop. A potentiometer has physical end stops and won’t rotate past them. A rotary encoder spins continuously. Also, if the knob also acts as a push-button (you can press it to toggle on/off), it’s almost certainly a rotary encoder.

Do all smart dimmers use rotary encoders?

No. Some smart dimmers use a slider, a touch panel, or a press-and-hold dim approach with no rotary control at all. The Samotech SM323 family is unusual in pairing a true rotary encoder with a no-neutral smart dimmer, giving you the tactile feel of a traditional dimmer with the capabilities of a modern smart device.

Can I retrofit a rotary encoder into an old potentiometer dimmer?

Practically, no. The internal control circuitry is completely different. The TRIAC trigger circuit in a potentiometer dimmer relies on the pot’s variable resistance; replacing it with an encoder means you also need a microcontroller, an output stage, a power supply, and the radio – essentially, you’ve built an SM323. Replacing the whole dimmer is much simpler and cheaper than the retrofit.