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AT42QT1012 Voltage Drop
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AT42QT1012 Voltage Drop

by krou54 on Wed Apr 28, 2021 8:55 pm

I am looking to use the AT42QT1012 capacitive switch as a simple switch to power a micro blower. my power supply comes into the switch at 4.5V (3-AAA batteries) but the voltage getting going to the fan is only 3.06V. Im losing ~1.5V over the switch. My power supply is wires to the switch through the GND and VDD. My micro blower is wired to GND and OUT. Am I wiring this up incorrectly?
I have also found the switch will turn on when touched but will not always turn off when touched again.

Please Advise.

krou54
 
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Re: AT42QT1012 Voltage Drop

by adafruit_support_bill on Wed Apr 28, 2021 8:58 pm

The output of the AT42QT1012 is a logic-level signal. It is not a power switch. To power motors like fans, you need something like a transistor circuit to switch the current:
https://learn.adafruit.com/transistors- ... -2998941-9

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Re: AT42QT1012 Voltage Drop

by krou54 on Mon May 17, 2021 1:38 pm

Thank you, this was very helpful. I purchased a few TIP120 transistors for this application.
However, I am still seeing a 0.8V drop from the power source to fan, why would this be?
Also, the capacitive sensor will turn the fan on, but won't turn it off. Once the load is removed from the capacitive sensor, it works properly again (turns on and off). What is causing this? I have attached a photo of the wiring for reference.

(I realized these transistors may be a bit overkill and will likely use the Bipolar Transistor Kit - 5 x PN2222 NPN and 5 x PN2907 PNP moving forward.)
Attachments
Screen Shot 2021-05-17 at 1.31.00 PM.png
Screen Shot 2021-05-17 at 1.31.00 PM.png (787.92 KiB) Viewed 301 times

krou54
 
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Re: AT42QT1012 Voltage Drop

by adafruit_support_bill on Mon May 17, 2021 1:51 pm

It is normal to have a voltage drop between the collector and emitter of a BJT transistor. It varies depending on the type of transistor and the current flowing through it. But 0.8v is not unusual for a TIP120. MOSFETs are typically more efficient. https://www.adafruit.com/product/355

Here is a nice comparison of a TIP120 vs an N-channel power MOSFET in the same circuit: https://forums.parallax.com/discussion/ ... -vs-mosfet

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Re: AT42QT1012 Voltage Drop

by krou54 on Mon May 17, 2021 9:32 pm

Good information, thank you.

The capacitive sensor will turn the fan on, but won't turn it off. Once the load is removed from the capacitive sensor, it works properly again (turns on and off). What is causing this? I have attached a photo above of the wiring for reference.

krou54
 
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Re: AT42QT1012 Voltage Drop

by adafruit_support_bill on Tue May 18, 2021 5:26 am

Your circuit is missing a resistor to limit the current through the base of the transistor. That would be a resistor in series between the output of the sensor and the base pin of the TIP120. Something in the 200 to 1K ohm range should work.

If using a MOSFET, a base resistor is not necessary.

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Re: AT42QT1012 Voltage Drop

by krou54 on Mon Jun 07, 2021 3:24 pm

Thank you, that worked beautifully.
I have been using the TIP120 and it has worked great but I feel may be a bit overkill for 3-AAA batteries running a micro blower. Would I be able to use the PN2222 NPN or PN2907 PNP instead? If so, with the intent of the capacitive switch closing the circuit once the switch is activated, thus powering the fan, would the PNP or NPN work best? I have done some reading on your website about the difference but still don't fully understand.
Lastly, would still need the resistor? I have been using a 300ohm.

You have been very helpful through all of this and it is greatly appreciated.

Thanks,

krou54
 
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Re: AT42QT1012 Voltage Drop

by adafruit_support_bill on Mon Jun 07, 2021 4:00 pm

DC motors in general are inductive loads and require a protection diode in parallel with the load to protect the transistor. The TIP120 has one built-in. Most simple transistors like the PN2222 do not. https://learn.adafruit.com/transistors- ... -2998941-9

The base resistor is needed to limit the current through the base. It is necessary for BJT type transistors. It is generally not needed for MOSFETs.

Image

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Re: AT42QT1012 Voltage Drop

by krou54 on Sat Jun 19, 2021 12:59 pm

This circuit is working beautifully and the Adafruit products are working great. I am looking to have an automatic off after ~30 seconds. After reading through the data sheets, I've discovered I would likely need a 0.01uF capacitor, I was unable to find one of these on Adafruit's website. Does Adafruit offer such a product? Or is there a better way of going about creating the automatic off after ~30 seconds? I'm operating at ~3.8V.

As always, your help is appreciated.
Thanks,

krou54
 
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Re: AT42QT1012 Voltage Drop

by adafruit_support_bill on Sun Jun 20, 2021 5:32 am

We have only a limited selection of capacitors. The larger electronics suppliers such as Mouser or DigiKey carry full selections in all the different form-factors.

The other way to create a controllable delay would be to add a small microcontroller such as a Trinket M0 to the circuit. You could then program in any delay you like.
https://www.adafruit.com/product/3500

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Please be positive and constructive with your questions and comments.