Feeding a PLC From a 60 GHz Fall Module: GP1 as an Opto-Isolated Dry Contact

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

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

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:

1. Power the module from 5 V (4.5–5.5 V, ripple ≤ 100 mV); radar GND is its own domain.
2. GP1 → 220–330 Ω → opto LED anode; LED cathode → radar GND.
3. Opto phototransistor collector → PLC 24 V (or through a 4.7 kΩ pull-up to 24 V, depending on the input).
4. Opto phototransistor emitter → PLC digital input (Ix).
5. 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.

Get the R60AFD1-V 60 GHz fall-detection module on Tindie — evaluation units for PLC and local-alarm builds.