Dehumidifier Touch Sensor Auto Start

I had a small thermo-electric dehumidifier that I wanted to control with a smart wifi plug (I found one with built in temperature and humidity sensor) but the dehumidifier would not auto start when powered on at the wall. I made a transistor based circuit to create a short pulse on start up and used it to trigger a reed relay which triggers the "power on" touch sensor. The sensor still works manually as well. The circuit was designed to work off the 5V supply for the dehumidifier microcontroller.

I've uploaded the Kicad 7.0 project archive here: https://drive.google.com/file/d/1yAaubCoT8H0EjchbGkzXRoTMRqyjwg4p/

See the schematic for components. I simulated the circuit in Kicad which said C3 should be 47uF and C4 33uF, but after testing, I found that 22uF for C3 and 2.2uF for C4 gave a suitable delay and pulse length. C3 controls the delay before the pulse, C4 controls the pulse length.

I did the circuit board layout on stripboard/vero board.

The reed relay I used was a BR-500.

The thin wire for the touch sensor trigger was from an old electric motor winding, 0.16mm diameter according to my calipers.

To make the wire spiral to place above the touch sensor, I wrapped the thin insulated wire around the conical end of a pencil and then squashed it flat. It's enamelled/insulated wire but I added a piece of sticky tape to add a bit of extra insulation and hold it together and in place.

I based the circuit on the "Delay On Timer Circuit" on this website but modified it to make a short pulse. I used Kicad to simulate it but found I had to use quite different value capacitors in practice to get the desired timing. I'm a novice at electronics, I just recently watched a youtube video about how transistor current gain can vary a fair bit between components, so it's possible you might have to play with resistor and/or capacitor values to get the circuit to work for you. I'd suggest simulating it in Kicad to get an idea how changing components affects it.

I 3d printed a couple of mounts for the pcb which I glued to the dehumidifier top cover. I've attached the 3d file.

Hopefully you can figure out the stripboard layout from the photos.