Technology comparison

Four detection technologies are available for KNX presence sensing. Each has a different physical principle, which determines its strength and weakness for a given application. Selecting the wrong technology is the single most common cause of occupancy control complaints.

PIR detectors

PIR (passive infrared) detectors sense the change in infrared radiation as a warm body moves across detection zones created by a Fresnel lens. The key word is movement — a person sitting still at a desk will not be detected once the initial motion stops. This makes PIR unsuitable for office workstations but ideal for corridors and stairwells where occupants are always in transit.

High-frequency (HF) detectors

HF detectors emit a 24GHz radar signal and measure the Doppler shift returned from moving objects. At 24GHz the wavelength is short enough to detect chest movement from breathing — this means a person sitting still and working at a keyboard will continuously trigger the sensor. HF is the correct technology for any space where seated occupancy must be detected.

The main weakness of HF is sensitivity to HVAC air movement and to any vibrating mechanical object in the coverage zone. Sensitivity must be correctly configured during commissioning — too high and HVAC airflow causes false triggers; too low and minor movement is missed.

Dual-technology (PIR + HF)

Dual-technology detectors combine PIR and HF in a single unit using a two-stage logic: the PIR triggers the initial occupancy event (capturing entry into the space), and the HF sensor sustains the presence signal as long as any movement — including breathing — is detected. Both sensors must agree before generating a false trigger, which dramatically reduces nuisance switching.

Dual-technology is the recommended default for meeting rooms and offices where occupants alternate between active movement and seated concentration. The additional cost over single-technology units is justified by eliminating the main occupant complaint (lights switching off mid-meeting).

KNX group objects

KNX presence detectors expose multiple group objects depending on the product. Understanding which object to use for each function prevents misconnections in ETS.

1/0/1   → Presence (DPT 1.001)    ← MDT SCA-PD55.01 Output Ch1
                                    → Gira 2118 10 Switch actuator Ch1 (lighting)

1/0/2   → Brightness (DPT 9.004)  ← MDT SCA-PD55.01 Lux output
                                    → DALI gateway brightness controller input

1/0/3   → HVAC presence (DPT 1.001) ← MDT SCA-PD55.01 Output Ch2
                                      → MDT AKH-0400.02 Fan coil controller

1/0/10  → Disable (DPT 1.002)     → MDT SCA-PD55.01 Disable input
                                    ← Push-button (cleaning staff override)

Hold time configuration

Hold time is the delay between the last detected movement and the transmission of the absent telegram (DPT 1.001, value 0). Setting the hold time correctly is critical — too short causes lights to flicker off and on during brief stillness; too long wastes energy in genuinely empty spaces.

Lux threshold linking

Most KNX presence detectors include an integrated lux (illuminance) sensor. Combining the presence signal with the measured lux level enables daylight-linked switching: lights are only switched on when both conditions are true — occupant present AND daylight below threshold.

Inputs:
  GA 1/0/1  → Presence (DPT 1.001)       value = 1 (occupied)
  GA 1/0/2  → Measured lux (DPT 9.004)   value = 320 lux (measured)

Parameters (ETS logic block or detector internal):
  Lux threshold = 500 lux  (above this = sufficient daylight)

Logic:
  IF presence = 1 AND measured_lux < 500:  send ON to lighting GA
  IF presence = 1 AND measured_lux >= 500: send nothing (daylight sufficient)
  IF presence = 0:                         send OFF to lighting GA (after hold time)

Result:
  On a sunny day (>500 lux at desk): no artificial light even if room occupied
  In the evening (<500 lux):         lighting switches on at occupancy

The MDT SCA-PD55.01 implements this logic internally — the lux threshold is configured in ETS parameters, and the detector sends presence telegram on Output 1 only when lux is below threshold. No external logic block is required for basic daylight linking.

Coverage planning

Coverage area depends on ceiling height and detector mounting angle. For ceiling-mounted 360° detectors the effective detection diameter on the floor is approximately 1.5× the ceiling height for PIR and slightly wider for HF.

Common mistakes