External On / Off switch for use with QT Py and BFF?

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mmryspace
 
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External On / Off switch for use with QT Py and BFF?

Post by mmryspace »

Hi I am looking to develop a project using the QT Py SAMD21 and the QT Py BFF Charger.

The project will be in a box so I would like to have an external switch to power the Qt Py on and off when it is using either USB or LiPo power. Also, when the external power switch is off, the LiPo could still be charged while the USB is plugged into the QT Py.

I am new to circuit design so I am curious what will be involved in designing this functionality?

If I understand correctly both the QT Py has one 5V pin for input and output and the BFF utilizes a Schottky diode so this seems possible. Are there any examples or schematics that I could look at for reference? Thanks!

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dastels
 
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Re: External On / Off switch for use with QT Py and BFF?

Post by dastels »

The switch on the BFF only controls power when powered by the battery. It's always on when powered by USB. The battery will change whenever power is supplied by USB.

You could solder wires to the "on" side connections of the onboard switch to connect an external one. Left the onboard switch off to let the external on control the power.

This switch won't turn off the power when 5v is coming from USB, though.

Schematic and fab print are at https://learn.adafruit.com/adafruit-qt- ... /downloads

Dave

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mmryspace
 
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Re: External On / Off switch for use with QT Py and BFF?

Post by mmryspace »

dastels wrote: Thu Sep 29, 2022 9:12 pm The switch on the BFF only controls power when powered by the battery. It's always on when powered by USB. The battery will change whenever power is supplied by USB.

You could solder wires to the "on" side connections of the onboard switch to connect an external one. Left the onboard switch off to let the external on control the power.

This switch won't turn off the power when 5v is coming from USB, though.

Schematic and fab print are at https://learn.adafruit.com/adafruit-qt- ... /downloads

Dave
Thanks so much for this info Dave, super helpful! Ok, so it sounds like it is not possible to have a "universal" external power switch for both scenarios, i.e. USB or Battery power. Certainly not a deal breaker especially for my use case but it would be nice if that option were available.

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dastels
 
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Re: External On / Off switch for use with QT Py and BFF?

Post by dastels »

Unfortunately not with the way the BFF works (due to the limited pins available on the QtPy).

Dave

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mmryspace
 
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Re: External On / Off switch for use with QT Py and BFF?

Post by mmryspace »

dastels wrote: Fri Sep 30, 2022 10:16 am Unfortunately not with the way the BFF works (due to the limited pins available on the QtPy).

Dave
To avoid having to solder the external switch on the BFF's exisitng pads, is the Pro Trinket LiPoly backpack a suitable alternative since it has an external switch option? Spec-wise it appears to be compatible with the QT Py: https://learn.adafruit.com/adafruit-pro ... ck/pinouts

I was so focused on the BFF that I forgot about this one but it seems better suited for my use case...

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dastels
 
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Re: External On / Off switch for use with QT Py and BFF?

Post by dastels »

No, it's not compatible. The ItsyBitsy, Protrinket, etc. have 5v (USB) and Vbat pins whereas the QtPy just has 5v. That's the reason for the BFF instead of just using the backpack.

Dave

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mmryspace
 
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Re: External On / Off switch for use with QT Py and BFF?

Post by mmryspace »

dastels wrote: Sun Oct 02, 2022 7:03 pm No, it's not compatible. The ItsyBitsy, Protrinket, etc. have 5v (USB) and Vbat pins whereas the QtPy just has 5v. That's the reason for the BFF instead of just using the backpack.

Dave
Ah - ok - totally missed that. Thank you 🙌

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