how to bypass a capacite touch button

[e90reborn]

hello

this was taken from an appliance that requires the press of a touch button to turn on.

capacitive touch button uses a spring with a single lead. power button is the first one from bottom to top (spring already removed).

I tried to short the spring lead to ground (as shown in the pictures) with no success. should I try shorting it to VCC ? any idea the best pin to pick ? should I add a capacitor and/or resistor or a single jumper is enough ?

thanks in advance

 

[e90reborn]

shorting it to one of the legs of the LED does work but not as intended. it randomly switches between states (on/off) in a period of a couple of seconds to sometimes a couple of minutes.

 

[Tony Stewart]

If you can't trigger it with your finger on the top or bottom pads, It must have lost calibration somehow.

I suspect it is a CN clone of Microchip's https://4donline.ihs.com/images/Vip...7-1.pdf?hkey=52A5661711E402568146F3353EA87419

Check for shorts. Photo is too fuzzy around your rework on bottom.

 

[e90reborn]

Sorry I wasn’t clear enough. I can trigger it with my finger. That’s exactly the point, I’m trying not to have to use my fingers and let it power on automatically when power cord is plugged in.

If you can't trigger it with your finger on the top or bottom pads, It must have lost calibration somehow.

I suspect it is a CN clone of Microchip's https://4donline.ihs.com/images/Vip...7-1.pdf?hkey=52A5661711E402568146F3353EA87419

Check for shorts. Photo is too fuzzy around your rework on bottom.

 

[Tony Stewart]

I mean only around the switch. Does the sensor have 3 pins + a guard ring?? You'll have to reverse engineer it. Otherwise AC couple the LED pin to the pad you need to bridge.

 

[e90reborn]

I mean only around the switch. Does the sensor have 3 pins + a guard ring?? You'll have to reverse engineer it. Otherwise AC couple the LED pin to the pad you need to bridge.

It’s just one spring (not shown in the picture) with a single pin. I tried to short this pin to multiple different places to no avail. I’m guessing I need some kind of pulse generator with a delayed single pulse. Maybe a RC circuit ?

 

[Tony Stewart]

How did it break? Bad solder joint somewhere? INSPECT CAREFULLY. READ THE DATASHEET
That's about all you can fix.
It's not a resistive/conductive sensor.

 

[augustinetez]

it randomly switches between states (on/off) in a period of a couple of seconds to sometimes a couple of minutes.

On the assumption the posted pdf is close enough to the ic on your device, the seemingly random changes of state are the ic assuming the touch switch has something interfering with it so it goes through one of it's various routines to re-calibrate or compensate for other possible variations such as temperature drift.

Bottom line is that what you are trying to do isn't going to happen the way you want.

How did it break? Bad solder joint somewhere? That's about all you can fix.

It's not broken Tony, he wants to modify it.

 

[e90reborn]

On the assumption the posted pdf is close enough to the ic on your device, the seemingly random changes of state are the ic assuming the touch switch has something interfering with it so it goes through one of it's various routines to re-calibrate or compensate for other possible variations such as temperature drift.

Bottom line is that what you are trying to do isn't going to happen the way you want.


It's not broken Tony, he wants to modify it.

I was thinking something like this in the hope the capacitor would trigger the button ON while charging and then leave it alone for good (open circuit after fully charged).

 

[augustinetez]

The way your pcb works re the touch buttons (again assuming the correct pdf) is that it is looking for the change in state on the touch pad.

So, you touch the touch pad which the IC on your pcb detects as a change of state.

After determining that it is a legitimate touch, the IC causes something to happen i.e. turns on or lights a LED or whatever.

The IC then checks that the state has reverted back to the condition that applied before you touched it.

This is where you need to look at the how that circuit you linked works (RC delay to anyone else looking at this thread) and read the description a little more carefully i.e. "The voltage at the output will after a short delay become the same as the input" - it means the state on the input (touch) pin will not revert to it's original state after the initial turn on until you turn off the power.

By all means try it, but I'm pretty sure you'll find it won't work the way you want.

 

[e90reborn]

The way your pcb works re the touch buttons (again assuming the correct pdf) is that it is looking for the change in state on the touch pad.

So, you touch the touch pad which the IC on your pcb detects as a change of state.

After determining that it is a legitimate touch, the IC causes something to happen i.e. turns on or lights a LED or whatever.

The IC then checks that the state has reverted back to the condition that applied before you touched it.

This is where you need to look at the how that circuit you linked works (RC delay to anyone else looking at this thread) and read the description a little more carefully i.e. "The voltage at the output will after a short delay become the same as the input" - it means the state on the input (touch) pin will not revert to it's original state after the initial turn on until you turn off the power.

By all means try it, but I'm pretty sure you'll find it won't work the way you want.

what if I swap GND and INPUT (VCC) in the picture below ?

 

[augustinetez]

I don't think it will make a difference, but as before, try it and see what happens.

Just be aware the input pin is expecting to see no more than 30pF, assuming the IC on your pcb is as per the above datasheet - are you able to read the numbers on the bigger of the two IC's and post here?

 

[e90reborn]

I don't think it will make a difference, but as before, try it and see what happens.

Just be aware the input pin is expecting to see no more than 30pF, assuming the IC on your pcb is as per the above datasheet - are you able to read the numbers on the bigger of the two IC's and post here?

I am sorry for the delay.

Bigger one:
KW23K
KB27554

Smaller one:
AiP650EO
51BT252

 

[augustinetez]

I am sorry for the delay.

No problem.

Part# of interest for the big one is KW23K and like you, no information on the manufacturer or datasheet available that I have been able to find.

The small one is a seven segment decoder ic, possibly SPI or i2c driven.