Clothes Drier Window Interlock


My house was built without a penetration for the clothes drier vent. When drying clothes, it is important to first open the window and poke the vent pipe out. Occasionally one of my housemates (or I) forget to do this, causing the laundry room to become a sauna rain indoors (due to condensation on the ceiling). I devised an incredibly simple electrical solution – a circuit that cuts power to the drier whenever the window is shut!


To detect the position of the window, I attached a magnetic reed switch to the window frame and a magnet to the window. Some reed switches are closed when the magnet is present (window shut) which would do the opposite of what we want. I purchased a commonly available double throw reed switch. It has both normally open and normally closed contacts, saving the need for a transistor to invert the signal. The reed switch controls a relay, powered by a switch mode power supply. The relay is able to cut or supply power to the drier.  Here is a schematic of the circuit.

Components list

  • Isolated AC to 12Vdc power supply
  • SPDT reed switch & magnet
  • 2x grommets
  • Relay with 12Vdc rated coil and 10A 250VAC rated contacts
  • Plastic case
  • RGB colour changing LED
  • 560Ω resistor to limit LED current
  • 1N4001 diode to protect reed switch from relay coil back EMF
  • 3.5mm audio plug & socket
  • hookup wire
  • 2M mains extension cord


I started by drilling holes in the case and fitting the grommets. Next, I arranged the relay and power supply, fixing them to the case with machine screws and a drop of thread locker on each nut to avoid them shaking loose with vibration (the finished unit sits on top of the drier). Next, I completed the DC and AC wiring. I used an extension cord as a cheap and quick way of getting power in and out of the box. The neutral and earth wires pass through, while the phase wire is interrupted by the normally open contacts of the relay. The switch mode power supply connects to the mains side of the relay contacts. Finally, I drilled a hole in the lid and used glue to fix the LED in place. A printed label on the front finishes the job.