Convert button to always on

[PizzaViking]

I've bought a cheap dehumidifier from Amazon that I want to convert that its always on.
The idea is to control it with a timer switch and turn on/of remotely. Now the dehumidifier has a button that you need to push to power it on. I wanna convert it to be always on (I'll controll it with the timer on the power supply instead). The thing is it needs to be pushed if you apply the power. Holding the button in won't work and neither does brighong the contacts on the circuit board.

How do I go about to make it come on if I plug in the power cord without pushing the jävla [Swedish] button?

[james_s]

You might need to draw a schematic of that board in order to figure out what it's doing. A simple way is to add a one-shot circuit that effectively pushes the button whenever power is applied but if all the logic for the button is on that little board there it should be easy to just bypass. I don't see a microcontroller, the two 8 pin ICs look like mosfets.


Actually looking closer the one in the center might be a microcontroller, it has more pins connected separately. Can you read the part numbers on the chips?

[PizzaViking]

Nothing written on the components...must be military secrets 

[mariush]

My mind went to a circuit with a 555 IC and a transistor to make the connection for a second or two. See https://www.electronics-tutorials.ws/waveforms/555_timer.html


Or could be a job for a 6-8 pin microcontroller (there's <1$ pics/arduino and 10-15$ programmers).  Program the chip to wait 1-2 seconds from power on, then turn on an output pin for 1-2 seconds then turn off the output. A transistor or mosfet on the output pin would make the connection like you manually press the button.   

Probably could also do it with a RC delay circuit and a schmitt trigger inverter or something (set r and c so that voltage rises slowly to some threshold and once the threshold is reached the inverter flips from off to on, "pressing" the button.  You'd have to figure a way to release the button press.

[james_s]

Well draw a schematic of the button and the stuff immediately connected to it and see if it looks like anything recognizable. Also what does that board do exactly? Does it just switch power on to some other circuit or does it have multiple things connected to it?

[sleemanj]

Educated guesses...



U3 is a voltage regulator.

The tab is ground, the pin closest to the 300R resistor is output voltage and the other is the input

The switch input to the mcu is the pin next to R3, R3 is a pullup

When switch K1 is pushed therefore the indicated MCU Input goes low and things happen.

So.  You could grab a handy arduino out of your parts bin, power it from the voltage regulator which is probably 3.3 or 5v or something similarly useful, connect one of it's pins to the MCU input, and

Code: [Select]

   void setup()
  {
       delay(10000); // wait 10s
       pinMode(whateverpinitis, OUTPUT);
       digitalWrite(whateverpinitis, LOW);
       delay(500); // hold for half a second
       digitalWrite(whateverpinitis, HIGH);
       pinMode(whateverpinitis, INPUT;
  }

   void loop()
   {
      // nothing
   }

Or use one of the other bajillion ways to skin that cat, but a ready to roll arduino pro-mini or nano is probably the quickest solution.

[DavidAlfa]

You can bypass the mcu control, so it works all the time whenever it has power.
Desolder it from the board and enable the mosfet gate:





If you want to keep it, a very simple alternative using CMOS:



After power on, the output stays high for ~13s (idle), then is set low for 260ms, and goes idle again. Exactly like a button would do.

You can use a 4093, 74HC/HCT132, HEF4093B... it must be a CMOS version (TTL gates have lower input impedance), also Schmitt trigger to ensure glitch-free operation.