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AT42QT10101 Momentary Capacitive Touch Sensor
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AT42QT10101 Momentary Capacitive Touch Sensor

by GoldstreamSolar on Sun Apr 18, 2021 7:54 pm

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This is in regard to the power consumption of this breakout switch. I am working on a battery-powered device and I'm very interested in getting this switch into the low-power mode, which is supposed to run on 50 uA while it waits for someone to touch the sensor. According to the Adafruit tutorial, pad A is in electrical contact with the solder blob in the middle and pad B is not in electrical contact with either. The overall response is supposed to be 'fast' and the current draw is about a half a milliAmp. I hooked a 3.7V battery to the VCC pin and, through an ammeter, to the GND pin and that's pretty much what I saw. When you touch the sensor the current goes up to a couple mA because the LED comes on. So far, so good.

The problem is that I'm interested in the low-power "slow" mode. It's hard to see the trace (red arrow above) but I scraped away until there was no more electrical contact between pad A and the center, then I solder blobbed the center contact to pad B. When I do this the current drops to about .25 mA, which is nearly as large as the .5 mA I started with, and about 5 times greater than the 50 uA I'm supposed to see. Is there something I'm missing here?

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Re: AT42QT10101 Momentary Capacitive Touch Sensor

by mikeysklar on Tue Apr 20, 2021 1:27 pm

I think you will need to additionally cut the jumper between the two pads for the LED located above the power pads. Also a reminder this low power mode will be limited to momentary only (not toggle).

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Re: AT42QT10101 Momentary Capacitive Touch Sensor

by GoldstreamSolar on Tue Apr 20, 2021 8:23 pm

Maybe I'm totally turned around on this 'low power' thing. I was thinking that a momentary switch would spend most of its time in the off state because the only time it's on is when someone is touching it. That being the case, the switch itself is going to consume way way more energy pulling a small current in the off state for a very long time, compared to the energy it consumes pulling a couple milliamps (mostly the LED, assuming the jumper isn't cut) for the second or two that your finger is on the switch. This is why I assumed a 'low power' mode would reduce the current draw for the hour or day or week that goes by before a person touches the sensor again.

I thought about cutting the LED jumper in case the LED was pulling current even when no one is touching the sensor. I guess I'll give it a try now and see if the off-state current draw drops down to the 50 uA that the docs claim.
Last edited by GoldstreamSolar on Tue Apr 20, 2021 8:51 pm, edited 1 time in total.

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Re: AT42QT10101 Momentary Capacitive Touch Sensor

by GoldstreamSolar on Tue Apr 20, 2021 8:50 pm

OK, I just gave it a shot. Cutting the LED jumper has no effect on the current draw of this switch.

I should describe in a little more detail how the current behaves. [Again, I have cut the Mode trace between the center pad and pad A, and then soldered the center pad to pad B.] If I put an ammeter in line with a 3.7 V battery that is connected to VDD and GND of the momentary capacitive switch I see a time-varying current - period is a few seconds - where the current cycles between about .13 mA and .26 mA.

I don't understand why the current draw varies with time, but perhaps that's a clue for someone smarter than me to use to figure out why the switch is pulling hundreds of uA instead of the 50 uA it's supposed to pull in 'low power' mode.

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Re: AT42QT10101 Momentary Capacitive Touch Sensor

by mikeysklar on Wed Apr 21, 2021 12:08 pm

@GoldstreamSolar,

Thanks for tryign the LED jumper cut.

Looking at the datasheet it explains a bit of your last question. It looks like low power mode last for 80ms chunks so you see a pulsing of lower power mode to fast mode. It would probably be easier to observe the actual draw with a scope than a ammeter in this case as the current draw is not constant.

https://ww1.microchip.com/downloads/en/ ... 01946A.pdf

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Re: AT42QT10101 Momentary Capacitive Touch Sensor

by GoldstreamSolar on Wed Apr 21, 2021 10:42 pm

Well, that was a really shrewd bit o' advice, Mickey. Thanks very much for that.

I took a look at an unmodified Mom Cap sensor with an o-scope and saw a reasonably constant current draw (untouched sensor so diode is off). Then I looked at a Mom Cap sensor that had been modified for 'low power mode' and saw exactly what you described: an on/off duty cycle that reduces the average power draw when you aren't physically touching the sensor. The problem is that the duty cycle is such that the amount of off time isn't anywhere near enough to reduce the average current from .5 mA down to 50 uA. I am seeing, as before, an average current of about 200 uA.

I am running this sensor with a (nominal) 3.7 V battery that varies from 3.7 V to 4.1 V depending on the charge. Is is possible that the duty cycle works properly at 5.5V (i.e. average current drops to 50 uV) but doesn't work properly at 4V and lower? If that's the case, the Adafruit docs oughta say so.

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Re: AT42QT10101 Momentary Capacitive Touch Sensor

by mikeysklar on Thu Apr 22, 2021 3:29 pm

Thank you for taking the time to verify the datasheet low power mode duty-cycle.

I think the duty-cycle will be the same regardless of the input voltage (3v vs 5v). I'm in agreement with you that advertising LP mode was 50uA is a bit confusing when it will always appear from a ammeter perspective as .13mA - .26mA. This is less of an Adafruit misrepsentation than that of the vendor overstating the low power mode abilities.

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Re: AT42QT10101 Momentary Capacitive Touch Sensor

by GoldstreamSolar on Thu Apr 22, 2021 4:06 pm

Whelp, it is what it is. I'd like to thank you for helping me understand what was going on.

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Re: AT42QT10101 Momentary Capacitive Touch Sensor

by GoldstreamSolar on Thu Apr 22, 2021 5:46 pm

Just for the sake of completeness, let me post the traces I get for this sensor (alas, I don't have a digital scope).

Out of the box, I get something that looks like this:


trace 1.jpg
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When I reconfigure for 'low power mode,' I get this:



trace 2.jpg
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So 'low power mode' has a duty cycle that is 100(34/80) = 42% of fast mode.

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Re: AT42QT10101 Momentary Capacitive Touch Sensor

by GoldstreamSolar on Fri Apr 23, 2021 11:12 am

Uh, what was I thinking? I'da noticed this at the time if I'd gone to the trouble to draw the trace to scale.

'Low power mode' has a duty cycle that is 100 ( 34 / (34+80) ) = 30% of fast mode.

< hangs head >

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