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I'm a total newbie in electronics, and just started venturing in home automation.

In Brazil there are many different types of tankless water heaters, especially for bathroom faucets and shower. Mine is a somewhat newer model with an LCD display and four buttons: SET, UP, DOWN and POWER.

My goal is to hardwire the UP and DOWN buttons to a dry contact (a SONOFF 4CH Pro) so I can remotely increase and decrease the temperature, much like what you'd want to achieve with an air conditioner. My wife (as most women) loves boiling hot showers, I don't.

I've roughly grasped the concept of NO and NC, and that I would need to wire the two cables coming from the terminals from those two buttons to the NO + COM of two channels in my SONOFF.

The problem is that by looking at the backside of the board, I can't see that second terminal.

Here is a large image of the front of the LCD display, and here's the backside.

The buttons are touch sensitive, and I can see the springs connected to single terminals (R5 103, R6 103, R7 103 and R8 103) as I look at the backside of the board (left side of the image), but I can't seem to figure out that second terminal.

I'm hoping that someone can enlighten me on this one. Cheers!

Edit: I opened up the panel just now and tested by touching both "R5 103" and "C1 223" with either points, one at a time. Every time I did, it would case the sensor to be activated. Any chance to get the same result from a relay?

Leandro Dimitrio
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  • You need NO & COM, not NC. Using NC would make it seem like the button(s) were pressed all the time. – brhans Sep 21 '20 at 02:52
  • Thank you, edited. – Leandro Dimitrio Sep 21 '20 at 03:02
  • What is the purpose of the SET button? – mhaselup Sep 21 '20 at 03:32
  • I think you are saying you have identified one terminal for each of the switches. Can you use a multimeter on the continuity setting to buzz out the other connections local to the "known" terminals when you operate the switch? – mhaselup Sep 21 '20 at 03:40
  • Touch sensitive? Likely capacitive sensors so not your typical buttons you can just connect a relay across. – Passerby Sep 21 '20 at 03:48
  • @mhaselup, the SET button enters a menu. What do you mean by continuity setting? – Leandro Dimitrio Sep 21 '20 at 03:57
  • @Passerby, in what way are capacitive sensors different from typical buttons? – Leandro Dimitrio Sep 21 '20 at 03:57
  • A typical button will have a mechanical operation like a keyboard-you press and it operates a switch. A capacitive button detects the presence of your touch. Do you have to press the front panel buttons or will simply "touching" them suffice? – mhaselup Sep 21 '20 at 04:01
  • Yes, they operate by simply touching them, no pressure needed. By looking at the backside of the board again, I'm seeing a visual correlation between "R5 103" and "C1 223" (top left corner). Does that make sense? – Leandro Dimitrio Sep 21 '20 at 04:05
  • Continuity is the setting where the multimeter bleeps/buzzes indicating a short circuit. It could be used to check a switch-hold the probes on either side on the switch terminal and press the switch to see if it buzzes. – mhaselup Sep 21 '20 at 04:26
  • why are you not mixing the hot water with cold water for a cooler shower? – jsotola Sep 21 '20 at 05:04
  • @jsotola, LPG is way too expensive in Brazil, so the less gas consumption the better. – Leandro Dimitrio Sep 21 '20 at 05:28
  • Okay, new development: I opened up the panel just now and tested by touching both "R5 103" and "C1 223" with either points, one at a time. Every time I did, it would case the sensor to be activated. Any chance to get the same result from a relay? – Leandro Dimitrio Sep 22 '20 at 02:00

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Capacitive touch sensors not have two terminals so you can't directly use a relay to simulate a button push. They work by sensing if there is a sudden but small change in touchpad capacitance (in the order of few picofarads). Since the pushbuttons are capacitive, the whole circuit board may not be mains isolated, so be careful.

It might work if you simulate a finger with some capacitance and resistance to ground, but it is impossible to know what values are good, and even the capacitance of the extra wiring can make the pushbuttons unstable.

Modifying equipment needs good understanding of the modified device, such as schematics, but it may not be enough if the touch sensing algorithm is made in software.

Justme
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