Run the zones of a building: people, light, climate and alarms

It treats each zone as a small loop with the same parts: someone is there or not, the light is enough or not, the air is warm, cold or stale, and something may be wrong. Every minute, the PLC runs these rules for each zone:

1. Presence. A presence sensor on a digital input says if there is movement. The zone is in use until the off delay passes with no movement.
2. Light. In use, the lights go on, and a light sensor measures the daylight on the desk. The PLC dims the lights with a 0-10 V output so the desk gets 500 lx, not more.
3. Climate. In use, the comfort set points apply: heat below 21 °C, cool above 24 °C. Empty, the zone falls back to 17 and 28 °C. An open window stops the fan coil.
4. Air. The room sensor measures CO2. Over 1,000 ppm, the PLC asks the air handling unit for more fresh air for that zone.
5. Alarms. After hours, the floor arms by itself. Movement without a valid access starts the siren and an alert. The call button of the rest room calls the team at any time.

The same rules repeat for each zone with its own sensors and set points, so a new zone is a new set of parameters, not a new program.

Run the An office day scenario and follow the open office and the meeting room.

A presence sensor does not count people. It sees movement. A person who reads or types without moving much can go unseen for a few minutes. That is why the lights stay on for an off delay after the last movement.

A short delay saves energy in rooms that empty often, but it switches the lights off on people who sit still. Set Lights off after the last movement to 2 min and watch the events table: each Lights off with people inside is a person who has to wave an arm.

Daylight dimming saves most where the daylight is strong. The open office has large south windows, so at noon on a sunny day its lights may need only a few percent. The reception and the rest room get little daylight and save little. The case of the smart office uses the same idea: lights by presence, dimmed by the light from outside.

A zone that nobody uses does not need 21 °C. With setback set points, the fan coil works less in empty rooms. The price is time: when people come in, the zone needs a while to reach comfort again.

A schedule keeps every zone in comfort from 07:00 to 19:00, used or not. A meeting room that stays empty all morning is heated all morning. Presence saves that energy, but the first minutes of a meeting may be cold. The stat person-minutes out of comfort counts it: one person for one minute outside 20 to 25 °C, 1 °C beyond the comfort set points, so the normal swing of the fan coil does not count. Compare both choices in the An office day scenario.

A window contact stops the fan coil of that zone. Heating or cooling against an open window wastes energy. The PLC keeps a frost guard: if the room falls under 8 °C, it heats anyway.

The fan coils need water from a plant, and the fresh air comes from an air handling unit. Here the PLC tells the unit if the floor is in use and when a zone needs more air, over Modbus TCP. The unit has its own controller for its fans, coils and dampers, as in our case of the HVAC OEM.

Run the A full meeting room scenario and watch the CO2 of zone 2 and the window.

With the same presence sensors and the access reader. In this example, after 19:00 the PLC waits until no zone has seen movement for 30 minutes, then arms the floor. A valid badge or face at the reception disarms it. Movement with the floor armed starts the siren and sends an alert to the guards. The reception gets an entry delay of 30 seconds, so a person with a badge can disarm before the siren sounds. The other zones trigger at once.

This is the logic of the smart office case: biometric access by day, an alarm fed by motion sensors at night. For an intrusion system that must meet EN 50131 or the rules of an insurer, use a certified alarm panel for the detection and let the PLC read its state and drive the lights and the messages. Fire detection belongs to its own system under EN 54. The PLC can read its alarm contact but does not replace it.

Run the Night and intrusion scenario and look at the alarm state in the building panel.

The rest room has a call button, like the one in our case of the hospital room alarm. One press turns on a buzzer and a light at the door and sends a message to the team on Telegram. A second press at the button silences the buzzer. The light stays on 2 more minutes, so people in the corridor see that the call was answered. If nobody answers in 5 minutes, the PLC sends a reminder.

The buzzer and the light work in the PLC with no network. If the network is down, the messages wait in a queue and go out when the link is back. This is an assistance call for an office, not a certified nurse call system for a hospital (DIN VDE 0834, for example).

A room sensor can fail too. The PLC polls each room sensor over Modbus RTU. After 3 polls with no answer, it marks the sensor as failed, stops the fan coil of that zone except the frost guard, boosts the fresh air of that zone to be safe and sends an alert.

Run the Calls and faults scenario and follow the rest room and the network.

1. The off delay of the lights

Long enough that people who sit still keep their light, short enough that empty rooms go dark. Corridors and toilets take short delays, offices and meeting rooms longer ones.

2. Presence or schedule for the climate

Presence suits rooms with irregular use, like meeting rooms. A schedule suits open offices that are always used in working hours. Many buildings mix both: a schedule for the open areas and presence for the rooms that are booked.

3. Fresh air on demand

A fixed base rate is simple, but a full meeting room goes over 1,500 ppm of CO2. Asking for more air only where CO2 is high keeps the air fresh without ventilating every room at the peak rate.

4. Where the logic lives

Keep the zone rules, the alarm and the call in the PLC, so they work if the network or the dashboard stop. Put the screens, the history and the messages to the phones on top, on the same Raspberry PLC or on another computer.

5. Relays and loads

The relays of the Raspberry PLC 57R are rated 4 A at 230 Vac with a resistive load. LED drivers take a high inrush current when they switch on, so switch each lighting circuit through a contactor. Fan coil valves with thermal actuators are small loads and can go on the relays. A fan coil fan takes a small motor current: check its rating, or switch it through an interface relay.

This simulator puts together three of our case studies in one floor. The values, zones, times and faults here are examples, not data from those projects.

• Smart office. Raspberry PLCs control biometric access, lights by presence and daylight, the temperature from a mobile app and an intrusion alarm. Read the smart office case study
• Hospital room alarm. A PLC turns on an audible and a visible alarm and sends the call to the nurses through a Telegram bot. Read the hospital room alarm case study
• Air handling unit controller. An HVAC manufacturer builds an M-Duino 58 into its air handling units, with a Modbus TCP map for the building management system. Read the air handling unit case study

To see how a PLC polls Modbus devices and serves a map to a building management system, try the Modbus Network simulator.

These examples cover the parts of this floor on Industrial Shields hardware.

• A Modbus TCP register map for a plant, for the link with the air handling unit and the building management system
• Telegram alerts and remote control, for the call and the intrusion messages
• An alarm history in MySQL with Node-RED, for the events table
• Email alerts from a Python monitoring server, for a second channel of alerts

More examples: PLC Solution Library · all simulators.

Example I/O for this floor

This example uses 14 of the 18 inputs, 20 of the 24 relays, 4 of the 9 analog outputs and one of the two RS-485 ports. The room sensors share one RS-485 bus, each with its own Modbus address. The UPS lets the PLC close its files when the power drops. For more zones, add a second PLC per floor and link them over Ethernet. Check your loads and levels against the Raspberry PLC family datasheet. Full specs: Raspberry PLC 57R FLEX.