12 Volt DC PIRs for Outdoor Use

I've wanted to automate the operation of some external lights in my home for a long time, and PIR motion sensors are a must. I already have several as part of an intruder alarm system, but they are not rated for outdoor use; electronics that is typically uses components with wider than normal operating temperatures (e.g. -40 to +85 degrees Celsius instead of 0 to +70), and conformal coating is often used to protect circuitry from moisture (condensation).

When I first looked for external PIRs (several years ago), I found that almost every one of them was designed to be powered by the mains, and to switch a mains load (a lamp). But I need to drive the inputs of a microprocessor based control system from the PIR, so I would prefer a contact closure (volt free) output. I did find one or two low voltage DC units, but they were horrendously expensive (nearly 100 GBP).

A more recent search turned up a much more reasonable solution:

12V DC Outdoor PIR Motion Sensor

This unit retails for under 25 GBP from Solar and Wind Power Systems. It is IP44 rated, runs from a 12 Volt (AC or DC) power supply and has a relay that can switch up to 10 Amps. It allows adjustments to on-time, range (sensitivity) and ambient light.

Note that the unit is supplied with a 4 way terminal block that is quite large (appropriate for 10 Amps). I swapped this for a smaller one. Also, the knock-outs molded into the ABS plastic housing are quite difficult to remove - I resorted to drilling them out.

Here's a video of a quick test of one. In this setup, the ambient light adjustment is at maximum (to allow daylight operation), the range/sensitivity is low, and the on-time is at a minimum - corresponding to about 5 seconds. The power supply is 12VDC, and the output is wired to ground the cathode of a blue LED - the anode is wired to 12V via a 470R resistor.

Notice that the unit's output remains on while the PIR is detecting motion, and stays on afterwards for 5 seconds.

I checked the range quickly in this setup, and detection worked reliably at 4 meters in daylight.

Arduino Real Time Clock

I needed an accurate real time clock for an Arduino based home automation project; many are I2C controlled and based on Maxim's DS3231 which is achieves an accuracy of about 1 minute per year (2ppm) through temperature compensation - a potentially useful side effect of this is that the part provides a temperature reading. Most boards also include an I2C EEPROM. I found a seller on eBay offering 3 units for under 5 GBP, including next day postage:

Real Time Clock and EEPROM board

The board provides a header with 6 pins:

• GND - ground
• VCC - power (3.3V or 5V)
• SCL - I2C clock
• SDA - I2C data
• SQW - programmable interrupt/square wave output
• 32K - free running 32kHz clock output

There's a holder for a CR2032 coin cell on the reverse, to provide battery backup. Note that these are typically not included with the boards (there are international restrictions on shipping lithium batteries).

I hooked it up to my Arduino Mega 2560 with male-female flying leads: I wired GND and VCC to GND and 5V on the power header, SCL and SDA to their counterparts on the communication header, and the SQW output to the INT3 input.

I then wrote a quick program in to test the board (I prefer to code in C using Atmel's Studio for AVR microcontroller development, for more on this see here.) This used the nice, stable and simple I2C master driver written by Peter Fleury. Note that I ignored the board's EEPROM.

The program prints the current time, day, date and temperature once a second:

       
RTC test
time: 12:14.44   day: TUE   date: 08/03/2016   temperature: +24.00
time: 12:14.45   day: TUE   date: 08/03/2016   temperature: +24.00
time: 12:14.46   day: TUE   date: 08/03/2016   temperature: +24.00

The program uses the Arduino's USB serial port for console I/O, based on Mika Tuupola's recipe here. I use TeraTerm on a desktop Windows PC for this, connected to the Arduino's COM port at 115200 baud.

You can download a ZIP of the Atmel Studio 7 project from here.