Modding an Adafruit PIR for 3.3 volts

green circuit board covered in surface mount components. A grey wire has been soldered to the output pin of the SOT-89 package 7133-1 voltage regulator
slightly dodgy soldering of a grey jumper wire to the Vout pin of the PIR’s voltage regulator

Consider the Adafruit PIR (motion) sensor (aka PIR Motion Sensor, if you’re in Canada). Simple, reliable device, but only runs from a 5 V supply. Yes, there are smaller PIRs that run off 3.3 V, but if this is what you have, you need to do some soldering. Annoyingly, the sensor on the board is a 3.3 V part, but the carrier was designed in Olden Tymes when King 5 V ruled.

You can try powering it from 3.3 V, but it’ll go all off on its own randomly as its own power supply won’t be supplying enough voltage. There are a couple of sites on how to modify these PIRs that either describe old kit you can’t get any more, or do it completely wrongly. Just one post on the Adafruit support forum gets it right.

One way of doing this is to provide 3.3 V directly to the output pin of the voltage regulator, and ignore the 5 V power line entirely. The regulator’s a SOT89-3 part that looks a bit like this:

71xx-1 SOT-89 package outline, with three pins at the bottom and one large ground tab (connected to centre pin, but not visible) at the top
wee leggy thing

In the photo above, it’s flipped over. Whichever way it’s oriented, we want to put power directly into the Vout pin. There may be easier points to solder this to than a tiny surface mount pin (almost definitely one of the capacitors) but this has held for me.

How to use it in MicroPython? Like the TTP223 capacitive touch sensors I looked at before, a PIR gives a simple off/on output, so you can use something like this:

from machine import Pin
from time import sleep_ms

pir = Pin(21, Pin.IN)

while True:
    print("[", pir.value(), "]")
    sleep_ms(1000)

value() will return 1 if there’s movement, 0 if not. There are trigger time and sensitivity potentiometers to fiddle with on the board if you need to tweak the output.

line graph showing output signal going from 0 to 1, back down to 0 and ending at one over a period of about 20 seconds
Thonny plotter output showing a couple of movement detections. High output (on my device) stays up for about 4 seconds, so you can be pretty leisurely about polling PIRs

Just remember: don’t connect the 5 V power line if you make this mod. I’m not responsible for any smoke emitted if you do — but I can always sell you a replacement …

MQTT Talk tonight

I’m talking at the Raspberry Pi Toronto Meetup tonight, and if all goes well, the Net-Connected Cowbell will make an appearance:

My slides: MQTT.odp

Links: