Seeed XIAO SAMD21 with PIR 250303

[AllenPitts]

Hello ETO forum,

Have built and installed a half dozen closet lights
that use PIRs (motion detectors) to turn three
12 volt lamps on for a minute.=, schematic attached.

The timer is a microcontroller called XIAO SAMD21

A couple of the closet lights have failed, not because
they won't come on but because they won't go off.

First the PIR was tested to make sure that it is
going from high to low so the PIR wasn't the culprit.
But the PIR, which are very reliable, operates
normally in a test circuit.

The XIAO SAMD21 was then tested and the MC seems
to be normal. Also LED D1 acts as a sort of heartbeat
for the XIAO and LED D1 operates as expected,
blinking on and off every three seconds.

The only other component that might fail is the
MOSFET. Very difficult to test a transistor once
soldered into the PCB. So, the MOSFET was removed
and replaced with a brand new, freshly tested
unit.

Out of desperation a new XIAO was unpackaged
programmed and installed. Same result. The
load lights come on but will ot go off.

It is unlikely that there is a problem with
the sketch since the same program is being
used in four other systems that are operating
normally.

I am at the end of ny troubleshooting
skills rope.

Thanks.

Allen Pitts

 

[rjenkinsgb]

At a guess, the gate voltage from the SAMD21 is not high enough to fully turn on some of the FETs; it's right on the edge of the gate voltage range. The MCU itself is a 3.3V device, not 5V.

That device has a gate threshold (where it barely starts to conduct) in the 1V to 2V range. Devices towards the upper end of that range may still have quite high resistance when on, and being cooked?

They will stand 12V on the gate, so you could some extra circuitry to switch a higher voltage to the gate.

Or find a "logic level" FET that is guaranteed to by fully on at 3V.