Sensor manipulator: giving an old boiler a remote control
Rather than touching the boiler's electronics, a small box alters the resistance of the outdoor sensor — three states, two wireless relays. The enclosure is laser-cut acrylic with engraved lettering.
An older heating system has no Wi-Fi, no app and no schedule you could shift from the sofa. The obvious route would be to intervene in its control logic — expensive, tied to the manufacturer, and back to square one when the system is replaced. This project takes the detour and leaves the boiler completely untouched: a small box sits between the outdoor temperature sensor and the heating system and alters the reading. Report a high outdoor temperature and the system throttles back by itself; report a low one and it puts in more work. The boiler notices nothing — it does exactly what it was built to do.
This works because the boiler does not measure temperature at all; it measures ohms. The outdoor sensor is a resistance sensor, and the system converts the measured resistance into an outdoor temperature. Two points were read off this particular sensor: 825 Ω at 0 °C and 979 Ω at 20 °C. Resistance therefore rises with temperature, by roughly 7.7 Ω per degree. That is exactly where the manipulator goes to work, with two potentiometers and two switches. One sits in series with the sensor and raises the resistance: the boiler sees warmer weather and throttles back. The other sits in parallel and lowers it: the boiler sees colder weather and puts in more work. That gives three states, worked through beforehand in an online circuit simulator — unchanged at 824 Ω, in parallel at 810.6 Ω, in series at 1.1 kΩ. The round figures there are examples; how many degrees they amount to depends on the sensor and on the system's characteristic curve, and is set on the potentiometers.
The whole thing is built on a piece of perfboard, deliberately simple: soldered wire links, two trimmer potentiometers in the middle — up to 500 Ω for the series branch, up to 50 kΩ for the parallel one — and four screw terminals leading out: two to the switches, one to the outdoor sensor, one to the boiler. The switches are two Shelly modules working as volt-free relays and drawing their 230 V from the mains; they are set up over Bluetooth and Wi-Fi and then run through the manufacturer's app. The laser supplies the enclosure: dark tinted, translucent acrylic, joined at the edges, with an engraved wordmark and the labels Sensor, Heizung and Power 230V /50Hz. It hangs on two brackets on the boiler-room wall — nothing has to be chased into the plaster there, and the potentiometers could be trimmed with everything connected while the temperature values were read straight off the boiler controller. One sentence belongs with all this: it involves 230 V and an intervention in a heating installation. That is a job for people allowed and qualified to do it — what is manufactured here is the enclosure, not the electrics.
Frequently asked questions
Can I build this myself?
Only with the necessary knowledge — and at your own risk. The manipulator is wired into the sensor line of a heating installation, and the wireless relays sit on the 230 V mains. Both are jobs for people permitted to carry out electrical installation work properly. The laser supplies nothing here but the enclosure; board, components and wiring are self-built.
How can a resistance control a boiler?
Because the boiler does not measure temperature at all; it measures ohms. The outdoor sensor is a resistance sensor, and the system converts the measured resistance into an outdoor temperature. Change the resistance on its way to the boiler and you change its idea of the weather. This sensor read 825 Ω at 0 °C and 979 Ω at 20 °C — resistance therefore rises with temperature, by roughly 7.7 Ω per degree.
What are the three states?
Unchanged, colder, warmer. At rest one relay bridges the series resistor and the parallel branch is open: the boiler gets the true sensor value, 824 Ω in the simulation. Switch the parallel branch in and the value drops — to 810.6 Ω in the example — so the boiler sees colder weather and puts in more work. Open the bridge and the series resistor is in circuit and the value rises — to 1.1 kΩ in the example — so the boiler sees warmer weather and throttles back, to practically off.
Why potentiometers instead of fixed resistors?
Because the simulation only shows the principle, and the round figures in it are examples. How many degrees an intervention amounts to depends on the particular sensor and on the system's characteristic curve. So the board carries two trimmers — up to 500 Ω in the series branch, up to 50 kΩ in the parallel one. Final adjustment was done on the running system: turn, read off the boiler controller, turn again.
How is the circuit board built?
As perfboard, deliberately simple. The connections are soldered wire links and the two trimmer potentiometers sit in the middle. Four screw terminals lead out: two to the wireless relays, one to the outdoor temperature sensor and one to the boiler. Screw terminals because they let you work in the boiler room without a soldering iron.
What do the two Shelly modules do?
They are the switches. Both work as volt-free relays — they only switch the sensor circuit and put no voltage on it — and are supplied with 230 V from the mains. They are set up over Bluetooth and Wi-Fi during commissioning and then run through the manufacturer's app. If you like, you can hang additional room sensors into the same logic and have the states switch automatically.
What about frost protection?
That is the most important point about the third state. When the boiler sees a markedly too warm outdoor temperature it goes practically off — and in winter that must not end with frozen pipes. The system therefore needs reliable frost protection of its own. If it has none, that protection belongs in the control logic of the wireless relays, together with a real room or flow sensor.
What is the enclosure made of?
Laser-cut acrylic, dark tinted and translucent — you can see the board and the relays shimmering behind it. The sides are joined at the edges and the lettering is engraved: the wordmark on the front, Sensor and Heizung at the cable entries, Power 230V /50Hz at the bottom. A box like this is one of the simplest drawings there is — rectangles with finger joints, a few holes, done. To change the dimensions or the lettering, redraw it in the modeller or upload your own vector file.
- Customer
- Formulor — our own project
- Project
- Enclosure for a sensor manipulator between the outdoor sensor and the boiler
- Material & process
- Acrylic, dark tinted and translucent — laser-cut, lettering engraved
More: How the modeller works · How it works
Design, build and photographs: Formulor. Circuit diagrams, measurements and photos come from the original project documentation.