PLCs and building-management systems in care homes and commercial buildings do not speak MQTT or JSON. They want a dry contact — a switch that closes. The 60 GHz fall module (R60AFD1-V) gives you exactly that opportunity through GP1 (active-high: High = fall detected). With one optocoupler you turn GP1's 3.3 V logic into an isolated contact the PLC can read, with no microcontroller and no network in between.
This is the module, not the finished Wi-Fi radar. The finished unit has no exposed GPIO.
What GP1 is, and why isolation matters
| Pin | Level | Meaning |
|---|---|---|
| GP1 | High = fall detected, Low = clear | Active-high fall alarm |
| GP2 | High = someone present, Low = empty | Occupancy (optional) |
Two properties drive the design:
- GP1 is a 3.3 V TTL logic output, weak (mA-level). It cannot source a PLC input directly, and it must never see the PLC's 24 V.
- The PLC side is typically 24 V and a different ground domain. Connecting the two grounds directly invites ground loops and a 24 V fault that destroys the 3.3 V module.
An optocoupler solves both: the radar's GP1 drives the opto's LED; the opto's phototransistor switches the PLC input. The two sides share no electrical connection.
Bill of materials
| Qty | Part | Notes |
|---|---|---|
| 1 | R60AFD1-V module | 60 GHz fall module, UART + GPIO |
| 1 | Optocoupler, e.g. PC817 / LTV-817 | NPN phototransistor output, VCEO ≥ 35 V |
| 1 | Resistor 220–330 Ω | Limits LED current from GP1 (3.3 V) |
| 1 | Pull-up 4.7 kΩ – 10 kΩ | For the PLC side, if required by the input type |
| 1 | 5 V supply, ≥ 500 mA | Module peaks at 300 mA; ripple ≤ 100 mV |
| — | PLC digital input (24 V), wires | Sinking or sourcing per your PLC |
The phototransistor in a PC817 comfortably handles the few mA a PLC digital input draws, and its 35 V rating covers 24 V.
Wiring (sinking PLC input — the common case)
Most PLC digital inputs are sinking: they sink current from an external 24 V through the field device.
Radar side (3.3 V) PLC side (24 V, isolated)
┌──────────────┐
│ R60AFD1-V │
│ GP1 ─┐ │
│ ├──▱▱───┤─── opto LED anode (220–330 Ω)
│ GND ├──────┤─── opto LED cathode
└──────────────┘ │
opto phototransistor
collector ───► 24 V (via 4.7k pull-up if needed)
emitter ───► PLC digital input (Ix)
PLC COM / 0V ───► 0 V
Plain-text steps:
- Power the module from 5 V (4.5–5.5 V, ripple ≤ 100 mV); radar GND is its own domain.
- GP1 → 220–330 Ω → opto LED anode; LED cathode → radar GND.
- Opto phototransistor collector → PLC 24 V (or through a 4.7 kΩ pull-up to 24 V, depending on the input).
- Opto phototransistor emitter → PLC digital input (Ix).
- PLC COM / 0 V → 0 V.
When GP1 goes High, the LED lights, the phototransistor conducts, and the PLC input energizes — your ladder logic sees a fall event and can trigger a call system, lighting, or HVAC. When the alarm clears (GP1 Low), the contact opens. The two grounds never connect.
If your PLC inputs are sourcing instead, swap the polarity: emitter → 0 V, collector → PLC input, and the PLC provides the 24 V internally. The opto still isolates.
Behavior and tuning
- Boot: allow ≥ 30 s after power-on before trusting the output.
- On a fall: GP1 rises; the PLC input turns on within milliseconds.
- Alarm duration: fall report defaults to 15 s; for production use ≥ 60 s to reject someone who stumbled and recovered (configurable 5–180 s over UART).
- Static-dwell (optional): a second stage after long stillness; default 300 s, production ≥ 30 min.
Add GP2 to arm the contact
Feed GP2 (High = someone present) into the loop so the PLC only registers a fall when a person is actually in the coverage area — preventing a stale trigger with the room empty.
Caveats
- Power quality first. A weak or noisy 5 V supply is the #1 cause of unstable detection.
- False alarms happen. Moving non-living objects (fans, curtains, flowing water) can trip it; a side-lying person may be missed. Use GP1 for a fast local/open-contact signal and UART/MQTT for logging; design redundancy.
- Not a medical device. A sensing building block, not a life-safety system; not for diagnosis or treatment.
Where to go next
- The GPIO/UART split: GPIO vs UART.
- Local relay/buzzer from GP1 (no PLC): Local fall alarm.
- Full specs: Module spec sheet.
- Fall tuning: Fall radar guide.
Evaluation units are on Tindie.
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