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I have a remote controller for a portable heater, and I am working on a small project to be able to control the fan on/off from an arduino with Bluetooh HM-10 module. I thought a solenoid might work by mounting on top of the remote but it doesn't have enough force, I want to remove the mechanical button and figure out a way to control it without the switch but I don't really know what circuit I should use.

Also, there is a 2.3V difference between the two left pads and 0.2V when the switch is pressed on.

Any idea how I could wire this up to arduino via some switch/relay circuit? Any circuit design/schematic would be helpful.

Here are the images of remote and button traces.

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enter image description here

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  • \$\begingroup\$ If the switch is default open, you don't have to de-solder it. Just place a switch/relay/transistor/fet in parallel. \$\endgroup\$ – Oldfart Jan 26 '19 at 12:34
  • \$\begingroup\$ It needs to be controlled by an Arduino, and doesn't have enough room for adding traces in parallel, so not really sure what you mean.. can you explain in a little details, thanks \$\endgroup\$ – condo1234 Jan 26 '19 at 12:36
  • \$\begingroup\$ Do you mean soldering a transistor on top of the two left pads? ... still need to control it from arduino though \$\endgroup\$ – condo1234 Jan 26 '19 at 12:52
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If the button is normally open (no connection when not pressed) you can solder two wires at the bottom to the switch contacts and feed those to the circuit which needs to control the 'button'. The button itself can remain in place. There is no need to de-solder it.

You can use a relay which is the simplest and safest method as that will work independent of how your fan controller works. If the button needs to be pressed for a long time and you want to save power a toggling or latching relay is best but they are bit more difficult to control.

schematic

simulate this circuit – Schematic created using CircuitLab

A transistor or FET may be possible but for that you would need the schematics and details of how the fan controller works.

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    \$\begingroup\$ In this case a selenoid relay is not necessary. It will work but it's overkill on a 3V remote control. A small FET or even directly from the Arduino, assuming there is a common ground connected. The Arduino board could also power the remote controller instead of the batteries, assuring a common ground at the same time. Place 1K resistors between the Arfuino output and the switch connections for safety, at least for testing. \$\endgroup\$ – Fredled Jan 26 '19 at 19:22
  • \$\begingroup\$ @Oldfart I tried your solution but when trying with NC, I press button to turn it on by default, then when I add the wires to NC and COM of the relay I have the button off i.e. turn off the function of the button. When I send a control signal to open the relay it only works first time or randomly will work and then come back on, it seems unreliable not sure why that is? \$\endgroup\$ – condo1234 Jan 30 '19 at 5:31
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Opto-connection from the microcontroller to the remote control avoids any grounding problems.

This circuit is worth a try. You need to connect Q1's collector to the positive terminal of your switch and the emitter to the negative end of the switch.

Very often these devices multiplex all the buttons to save pins so we can't assume that either side is connected to the remote ground (battery negative). The opto-isolated solution doesn't care but there is a small voltage drop across the transistor and it might not short out SW1 adequately. The parts are cheap so it's worth a try.

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  • \$\begingroup\$ Nice solution but it's also worth a try to find a common ground and connect the arduino directly. Optocouplers are extremely sensitive devices and require exact voltage. \$\endgroup\$ – Fredled Jan 26 '19 at 19:27
  • \$\begingroup\$ In my answer I addressed the possibility that there is no common ground and the photo of the PCB suggests that this is the case. "Optocouplers are extremely sensitive devices." Extremely sensitive to what? They are extremely robust devices and used in all sorts of communications and industrial applications. They are insensitive to stray light, electromagnetic radiation, static, etc. They are generally used in digital signal applications and when designed correctly to accommodate deterioration of the LED they will last a very long time in service. \$\endgroup\$ – Transistor Jan 27 '19 at 10:04

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