When most engineers think about connecting a sensor to a controller, they picture a wire. Pull a cable from the sensing point to the panel, land it on the input terminals, and you’re done. Clean, reliable, familiar.
But what happens when the sensing point is moving?
That’s exactly the challenge one of our customers brought to us at IOthrifty. They needed to monitor and control temperature and humidity on a machine that was constantly in motion. The sensor had to ride with the machine. The controller had to stay off it. A hard-wired connection between the two simply wasn’t an option.
The Problem: Sensing on a Moving Machine
The application itself was straightforward: measure temperature and humidity, compare those values to setpoints, and trigger outputs to maintain the desired conditions. That kind of closed-loop control happens in thousands of facilities every day - cold storage rooms, cleanrooms, drying ovens, environmental chambers.
What made this one different was the motion. The machine moved continuously during operation, which ruled out:
• Wired sensor connections — cables would tangle, fatigue, and ultimately fail
• PLCs or industrial computers — the customer wanted to avoid the cost and complexity of programming a full automation platform just for two control outputs
• Wi-Fi or Bluetooth — range limitations and IT network dependency were concerns in their facility environment
What they needed was a simple, self-contained wireless sensing and control loop — reliable enough for industrial use, affordable enough to justify for a two-output application.
The Solution: A Three-Device Wireless Control Chain
We engineered a solution using three complementary devices that together bridge the gap from a moving wireless sensor all the way to a physical relay output - no PLC, no programming environment, no IT infrastructure required.
Device 1: RHT-Air-WM — Wireless Temperature & Humidity Transmitter
The RHT-Air-WM is a battery-powered wireless transmitter that measures both temperature (−40 °C to 100 °C) and relative humidity (0–100% RH) and broadcasts those readings over a self-contained wireless network based on the IEEE 802.15.4 standard.
Key features that made it the right fit here:
• Long wireless range — up to 1 km line-of-sight, well beyond what was needed on this application
• IT-independent network — operates on its own dedicated wireless protocol, completely separate from corporate Wi-Fi or Ethernet infrastructure
• Battery powered — no power wiring needed on the moving machine; estimated battery life of approximately 2 years at a 10-minute update interval
• Configurable update interval — transmit as frequently as every 15 seconds or as slowly as every 10 minutes
• AES-128 encrypted transmission — data security built in
The sensor mounts directly on the machine and transmits readings wirelessly as the machine moves through its range of motion.
Device 2: AirGate Modbus — Wireless to RS485 Modbus Gateway
The AirGate Modbus is a gateway that receives the wireless sensor data over the IEEE 802.15.4 protocol and converts it to RS485 Modbus RTU - a wired industrial communication standard used by thousands of devices worldwide. Think of it as the translator in the system: on one side, it speaks wireless; on the other side, it speaks wired Modbus.
Key specs:
• Supports point-to-point, star, and tree network topologies — scalable as needs grow
• 128-bit AES encryption on the wireless link
• Powered by 10–30 VDC
• Configurable via USB using software
• Rated for operating temperatures from −10 °C to 70 °C
Device 3: STR571 — RS485 Modbus Panel Meter with Relay Outputs
The STR571 is a 96×48 mm panel-mount Modbus display and controller. It reads variables from Modbus slave devices on its RS485 port — in this case, the temperature and humidity values arriving from the AirGate Modbus — and displays them on its 2.42-inch OLED screen.
But the STR571 does more than display. It includes:
• 2 relay outputs (2A, 250 Vac) — used directly for control, turning equipment on and off based on sensor readings
• Configurable control setpoints — set high and low thresholds for each variable; the relays respond accordingly
• Up to 8 Modbus variables — temperature and humidity only needed two, leaving room to expand
• Intuitive front-panel interface — operators can view readings and adjust setpoints without any software
No PLC. No ladder logic. No HMI license. Just a panel meter with built-in control logic and relay outputs, fed by a Modbus network that happens to start with a wireless sensor on a moving machine.
System Architecture at a Glance
IOthrifty configured all three devices for the customer — setting Modbus addresses, communication parameters, alarm setpoints, and relay logic — so the system arrived ready to install and run.
Why This Approach Works — and When to Use It
This solution is elegant precisely because it avoids over-engineering. For applications that need:
• Two or more control outputs based on sensor readings
• A wireless sensor due to motion, distance, or cabling difficulty
• A simple operator interface with setpoint adjustment
• A self-contained system that doesn’t touch the IT network
…this three-device system hits a sweet spot of capability, simplicity, and cost that a PLC-based approach simply can’t match at this scale.
Not Just for Temperature and Humidity
It’s worth emphasizing: the RHT-Air-WM is one of many sensors that can feed this kind of system. The AirGate Modbus is a general-purpose wireless Modbus gateway, by using two you can create a wire free connection between any Modbus RTU devices.
And for sensors that don’t already have a Modbus RTU output? That’s not a dead end. IOthrifty carries converter products that can take virtually any sensor signal — 4–20 mA loops, 0–10 V analog outputs, thermocouple inputs, RTD inputs, pulse outputs — and convert those signals to a Modbus RTU format which can be used with the AirGate. That means the same AirGate Modbus + STR571 architecture can be adapted to control based on:
• Pressure from a pressure transmitter on a moving platform
• Flow rate from a flow meter in a hard-to-wire location
• CO₂ or gas concentration from a remote sensor
• Weight or force from a load cell in a mobile application
• Any 4–20 mA sensor using a wireless analog input transmitter
The architecture is the same. The wireless-to-Modbus translation is the same. The relay-based control logic in the STR571 is the same. Only the sensor changes.
Ready to Solve Your Wireless Sensing and Control Challenge?
If you’re facing a similar application — or something completely different that still involves remote sensing, wireless communication, or low-cost control — contact the team at IOthrifty. We’ll help you identify the right combination of products and configure them for your specific needs.
We stock and support the full line of sensors, converters, gateways, and panel controllers described in this article, and we can configure devices to your specifications before they ship.