3

The video intercom in my apartment has started ringing on its own at random intervals.

The outdoor panel doesn't have a physical doorbell button: the designer opted for a trendy touch sensor that lasted all of six years before these intermittent failures.

This is a common, known problem for this line of intercoms, and my specific model has been discontinued years ago. I'm not eligible for a replacement, and changing the outdoor and indoor units costs $500+.

I've isolated the problem to the (resistive?) touch sensor itself: if I disconnect it, the random ringing disappears and the rest of the unit works perfectly.

The picture shows the sensor: it has five leads, the central one is connected to comb-like traces interleaved with the other four. The doorbell can be configured for a single user, two users (the upper and lower half become two independent buttons) and four users.

enter image description here

My question is: can I modify the circuit so that the touch sensor becomes less sensitive? As in, a finger touching it would trigger it, but it will not react to temperature / humidity / phase of the moon changes?

I was thinking of simply adding a resistor in series to the central lead, but I have no idea if this would work, nor how to select an appropriate resistor.

Vorbis
  • 143
  • 4
  • 1
    One option is to replace the resistive sensor with a plain old pushbutton. – Kuba hasn't forgotten Monica Aug 01 '22 at 13:28
  • @Kuba, I had thought of that, but drilling the outdoor unit to kludge a pushbutton would ruin the aesthetic and I'll keep it as last resort. – Vorbis Aug 01 '22 at 13:32
  • Have you tried wiping down the sensor with some isopropyl alcohol and seeing if the issue persists? That may resolve it, if the issue is surface contaminants changing the impedance. – Willa Aug 01 '22 at 15:42
  • Are you sure the sensor is resistive and not capacitive? Disconnect it and probe with an ohmmeter while touched/not-touched to see. – Mark Leavitt Aug 01 '22 at 16:06
  • Can you follow the circuit from the touch pad to the touch controller and post part numbers and pics? – Bryan Aug 01 '22 at 17:22
  • start by placing various thicknesses of a plastic film over the sensor and test operation ... it is possible that a cover layer has worn off the sensor array ... a piece of electrical tape would be the easiest to try – jsotola Aug 01 '22 at 18:52
  • @john, I've followed your suggestion - looking at the picture I posted (taken with a bright flash, and contrast-enhanced to better show the tracks) I noticed some crud near the copper tracks. Carefully lifting the sensor (it's glued to the plastic support) there was an additional brown blob on the underside, across the two lower tracks; it can be noticed in the picture. I've cleaned it and re-assembled the thing; I'll let you know if it worked. – Vorbis Aug 03 '22 at 07:31

2 Answers2

2

The "false alarm" problem you're having is probably caused by moisture and/or contaminants in the sensing panel. Yes, it's possible adding resistance in series could stop the false alarms temporarily -- and it's unlikely to damage anything -- the underlying problem will probably progress. You also may cause the opposite failure -- no response when someone pushes the button.

With custom flex PCBs now available (OSHPark, others) at reasonable cost, it's possible you could design and fabricate a replacement sensing panel. It's a somewhat ambitious project, but if you have a way to communicating with other owners (or shops that service these intercoms), you might be able to sell them to cover the cost of development.

Mark Leavitt
  • 5,073
  • 1
  • 8
  • 15
  • good idea, but I forgot to mention that the touch sensor is transparent: there are white LEDs behind it to backlight a label with my name. – Vorbis Aug 01 '22 at 14:54
  • The flex PCBs from Oshpark https://docs.oshpark.com/services/flex/ have a substrate and soldermask that is clear (well, not optically clear, but not opaque). Naturally the copper pads are opaque, but you could design them as a perforated mesh and plenty of light should penetrate. But admittedly it's still a challenging project. – Mark Leavitt Aug 01 '22 at 16:03
2

You could attach a thin sheet of clear material over the top of the touch sensor.

That'll reduce the touch screen sensitivity, just like it does with a mobile phone or tablet screen.

Cut up a phone screen protector, or a tablet one if it needs something larger. They're pretty cheap and they work with the capacitive sensing of mobile phone screens so it should work with your sensor.

If it's a resistive sensor, choose a plastic sheet rather than glass and it'll certainly work with it. Mind you, a resistive sensor wouldn't be exhibiting the over-sensitivity you've describing anyway.

TonyM
  • 21,742
  • 4
  • 39
  • 62