Occupancy Lighting and HVAC From a 24 GHz Radar Module: Switching a Relay With S1, No Gateway

The 24 GHz presence module (R24AVD1) reports whether a space is occupied as a single logic pin: S1 goes High when someone is present and Low when empty. Wire S1 to a relay and you get occupancy-controlled lighting or HVAC with no Wi-Fi, no cloud, no gateway — the radar switches your load directly. This reuses the same transistor/relay topology as the fall-alarm article, applied to a lower-cost 24 GHz part.

What S1 (and S2) are

Key behaviors from the module's protocol:

• Occupied → unoccupied is reported after a 40 s stillness delay; occupied appears in < 0.5 s.
• Sensitivity (DP5, 1–10, default 7) and scene mode (DP6) tune range and false-trigger rejection. Use a lower sensitivity in corridors/parking where curtain or plant sway would otherwise false-trigger.
• Power the module per its datasheet (3.3 V I/O; VCC and current are specified there). The relay/load side uses its own 5 V/12 V supply, kept separate from the module.

The same rule as always

S1 switches a transistor; the transistor switches the load. S1 never touches the load.

The pin is 3.3 V TTL and weak (mA-level) — never drive a relay or lamp directly.

Bill of materials

Wiring

   R24AVD1 (3.3 V)                    Load side (5 V / 12 V)
 ┌──────────────┐
 │ S1 ─┐        │
 │     ├─▱▱─────┤── 10 kΩ pull-down ──► MOSFET gate
 │ GND ├────────┤────────────────────► MOSFET source ──► load supply 0 V
 └──────────────┘    MOSFET drain ──► relay coil ──► load +V
                                 1N4007 (cathode to +V, anode to drain)
                                 NO contacts ──► light / HVAC ──► 0 V

1. Power the module from its specified supply; power the relay/load from its own 5 V/12 V supply.
2. S1 → 10 kΩ pull-down → MOSFET gate.
3. MOSFET source → load-supply 0 V.
4. MOSFET drain → relay coil → load +V; flyback diode across the coil.
5. Relay NO contacts switch the light or HVAC circuit.

When S1 is High, the relay closes and the load runs; when the space goes empty (after the 40 s delay), it opens.

Using S2 to avoid "lights off while still"

S2 is High only when the person is moving. If you drive the relay from S1 alone, a person who sits still keeps the lights on (good). If you prefer "on only while moving," use S2 instead — but then a still person darkens the room. Most installs want S1 for occupancy and leave S2 for UART-based analytics.

Mounting and false-trigger reality

• Ceiling mount, height ≤ 2.75 m, tilt ≤ 3°, long side of the board toward the door. Keep the antenna 3–5 mm from the enclosure's inner surface.
• Known false-alarm sources: fans, pets near the radar, swinging metal curtains, and the internal motor of an air conditioner directly in the beam. Aim the beam away from these.
• Not a medical device. A sensing building block; not for diagnosis or treatment.

Module specifications (R24AVD1 guide)

Every figure below is from the R24AVD1 getting-started guide (MicRadar / 云帆瑞达). Note: this guide does not publish VCC or current — power the module per its datasheet; the I/O levels below are the only electrical values given (all 3.3 V).

Output pins (3.3 V logic):

Data points (protocol):

Body-motion amplitude (DP4): 0% none; 1% static (sleep, breathing only); 2–30% micro motion; 31–60% walk / slow limb; 61–100% run / large close motion.

Sensitivity bands (DP5, default 7): low (1–3) for corridor/parking — large motion only, rejects curtain/plant sway; mid (4–6) normal indoor; high (7–10) low-interference spaces — fine motion, but prone to false "occupied".

Coverage and range (ceiling mount): beam 90° horizontal / 60° pitch. Install height ≤ 2.75 m, tilt ≤ 3°, antenna face 3–5 mm from a non-metal, flat enclosure. Detection at ceiling: motion ≈ 8 m × 12 m, static ≈ 4 m × 6 m, sleep ≈ 3 m × 2 m. Max diameters: motion 12 m, static 6 m, sleep 3 m.

Scene-mode ranges (trigger / static diameter): max-area 12/8 m & 6/4 m; office 10/7 m & 6/4 m; hotel (default) 9/6 m & 6/4 m; living 7/5 m & 6/4 m; bedroom 4/3 m & 6/4 m; area-detect 3/2 m & 6/4 m; bathroom 1.8/1.5 m & 6/4 m.

Interference (from the guide): moving non-living objects — metal/liquid motion, oscillating fans, pets near the radar, swinging metal curtains — cause false triggers. Do not aim the beam directly at an air conditioner (its internal motor false-triggers); aim parallel to it instead.

Layout: keep the radar's components ≥ 1 mm above other PCBA parts; the enclosure face must be non-metal and flat (no curved surface).

Where to go next

• The GPIO/UART split: GPIO vs UART.
• Same topology on the fall module: Local fall alarm.
• Full specs: Module spec sheet.
• Presence tuning: Presence radar guide.

Evaluation units are on Tindie.

Get the R24AVD1 24 GHz presence module on Tindie — evaluation units for gateway-free occupancy lighting and HVAC.