MotionMote R3 introduces the ability to use ultra low power Panasonic PIR sensors. These sensors are available in several variants (WL & VZ series, 5-12m range, 1-6uA idle current). The kit offers the EKMB1201111 5m range sensor (5m range). The active detection current of this sensor is ~30uA (during motion), idle current is 2uA.
When assembled with the Panasonic PIR, the total idle current of the unit will be around 10uA!
The default PIR motion sensor in the kit is the HC-SR501 as before. This uses around 60uA in idle, and active detection current is ~200uA.
Hacking the HC-SR501 for lower power
The default kit PIR sensor (HC-SR501) has a supply voltage range from about 3.9V to 24V. That is great but we’re operating from a LiPo battery that runs down from 4.2V when it’s fully charged. It will still last a long time without any modifications to the PIR sensor. When the battery voltage drops below 3.8-2.9V the PIR sensor will trip continuously. We don’t want that. We need the PIR sensor to work down to the 3.3V that will mark the battery discharged state. You will notice the PIR sensor has a diode and regulator.
- The diode has a whopping forward voltage of 0.6V so at a minimum you will need to remove this diode. You can also short it (solder a wire to the 2 terminals of the diode.
- The regulator has another 0.1-0.25V voltage drop so to get the absolute lowest voltage input range for the PIR you can remove the regulator as well (the regulator is linear so it will be very slightly inefficient the higher the input voltage).
The diode (and regulator) should come off easily if you repeatedly heat the terminals. Replace them with solder bridges as shown below.
Tip for easy desoldering: these are SMD devices and could be a bit challenging to desolder if you’ve never done it before. Luckily there’s a pretty easy trick: melt a bunch of solder to your iron tip – this will increase the thermal mass of the tip. For the diode heat one side then move to the other quickly, then back and forth until the diode just comes off. If this is a really small diode (some PIRs have small some larger diodes) then you can just bridge/short the diode terminals with a small jumper wire without having to desolder it.
If you want to make the PIR even lower power, then you need to take the regulator off and bridge the regulator VIN to VOUT. With a bunch of melted solder on your iron tip, touch the 3 leads of the regulator and it should come right off, then drop the melted solder on the center and right pads where the regulator was (as seen in the photo above).
Please note: once you make these mods, the PIR will take up to 5V, and will need to be wired correctly (there is no reverse diode protection any more). This is accounted for on the MotionMote PCB so no worries there. But be careful if you ever want to use it somewhere else.