The Curmudgeoclast

Thoughts, projects, and ramblings of Dave Astels

Sun 02 April 2017

SmartHome: Node Hardware

Posted by dastels in project   

Nodes are the metaphorical eyes, ears, and hands of the smarthome system. There will be many of them in a home. Even in my modest apartment I will probably have about a dozen. So they need to be unobtrusive, reliable, and fairly inexpensive.

I went through several progressively more refined prototypes, first in the form of an Arduino Uno shield and a several versions based on the Arduino Pro Mini before having a reasonably final design .

When I thought I had it, I breadboarded it to make final tweaks. Here's the current design on a breadboard. Because it looks cool.

Eventually, I had a design I was happy enough with to make a custom PCB.

Over time some elements have remained constant:

  • Atmega 328 MCU
  • photoresistor for measuring ambient light level
  • an RGB LED (the current design supports common anode and common cathode)
  • an nRF24L01+ radio
  • a PIR motion sensor

Some things, however, have evolved:

  • moving from an Arduino Uno shield to an embedded Arduino Pro Mini to a completely custom board
  • changing from through-hole components to exclusive use of surface mount for everything other than connectors, headers, and power capacitors
  • changing from a DHT temperature/humidity sensor to an Si7021 sensor
  • along with the previous point is more focus on using I2C sensors.
  • dropping battery power in favor of a wallwart (I decided that radio range and stability was more important)
  • adding an MCP23017 port expander to read configuration and provide additional digital IO
  • Adding standard support for a VL53L0X laser proximity sensor
  • Adding standard support for a string of 8 or 60 neopixels

The circuitry

Let's go through the circuitry, once functional block at a time.


This is very much like the standard Arduino Uno power circuit. The big difference is that there is no USB power, and the 3.3v regulator can supply a full 1A of power compared to the 50mA from the Uno's 3.3v regulator. This additional capacity is required for the nRF24L01+ radio.

There is a 47µF capacitor on both the 5v and 3.3v supplies, and a handful of 100nF capacitors scattered around the board between 5v and ground.

Power is usually supplied via the power connector using a 9v wallwart, exactly as the Uno. 5v can also be supplied via the FTDI connector when the MCU is being programmed or by the ICSP connector when the bootloader is being flashed.

You can use any 7v-12v DC supply; I've been using a 9v 1A wallwart from Adafruit.


Here we have the heart of the node: the ATmega328 MCU. You can see the 16MHz crystal with its capacitors, the reset circuit and the FTDI connector. For some reason the schematic part in Fritzing doesn't agree with the PCB part. The PCB part is correct. The connections are there, just in the wrong order. Odd.

To the right you can see connectors for the various standard sensors:

Photo connects to a photoresistor for measurement of ambient light. This is a simple CdS cell which acts as a voltage divider with a 10kΩ resistor. The more light, the lower the resistance , and the higher the voltage read by the analog input of the '328.

PIR connects to a PIR motion detector for, naturally, motion detection. There are pins for power, ground, and the signal from the sensor. Be careful to plug it in the right way around.

RGB connects to an RGB LED. The current limiting resistors are on the board so all that is required is the LED, itself. Note that both power and ground are on the connector (which is keyed) so either a common anode or common cathode LED can be used. This is one of the onboard configuration settings.

Neopixel lets you connect a string of neopixels. The firmware and configuration selection supports either an 8 or 60 pixel strip. For example, my coffee station has a unit with an 8 pixel stick used as a spot light to illuminate the work area when I'm standing at it. I have a 60 pixel strip used to provide hands free under counter lighting on a node in the kitchen.

Finally you can see the I2C and SPI connections toward the top and bottom, respectively. These connect to the respective subsystems described next.


I2C is an amazing interface standard. You can get all kinds of sensors and other useful bits & pieces that you can connect to an MCU using a simple two wire bus - in addition to the usual ground and power connections. A node has two builtin I2C devices, and connections for three more. Note that these use 5v levels, if you want to connect 3.3v devices you'll need to add a 3.3v regulator and some level shifters to the connected device. See Adafruit's Si7021 breakout board for an example.

The first is an MCP23017 at address 0x20 which provides two 8-bit ports. One of these ports is used for configuration. Each bit is normally held high by the internal pull-ups, but can be set low by using a jumper. Currently 7 of the bits are used:

  • 0: high -> a PIR motion detector is connected, low -> it isn't
  • 1: high -> a photoresistor is connected, low -> it isn't
  • 2: high -> a VL53L0X is connected, low -> it isn't
  • 3: high -> an Si7021 temperature/humidity sensor is connected, low -> it isn't
  • 4: high -> the LED is common anode, low -> it's common cathode
  • 5: high -> neopixels are connected, low -> they aren't
  • 6: high -> 60 neopixels, low -> 8 neopixels
  • 7: unused

The other port on the 23017 is used to provide 8 digital I/O bits. these can be used to connect actuators (e.g. a relay to control a fan) or sensors that provide a simple digital output.

The other standard I2C device is an Si7021 temperature and relative humidity sensor at address 0x40. The actual chip is rather daunting to hand solder, so I've designed the board to use Adafruit's breakout board. This approach also gives the option of not having a temperature/humidity sensor if it isn't needed. Despite all that, the final version of the node may well have the Si7021 directly on the board.

I had originally been using DHT11 temperature/humidity sensors but they were bulky and used more cpu cycles to interact with than I liked. The Si7021 is more accurate and since it uses I2C, much easier to interact with.


The part of the circuit is fairly simple: an NRF24L01+ breakout board, and the ICSP connector used to flash the bootloader. The only thing that warrants comment is that you will want to hack the nRF24L01+ breakout to improve the stability of the power. I have had success with this by soldering 100µF electrolytic and 100nF disk capacitors across the power (3.3v) and ground pins, DIRECTLY ON THE BREAKOUT BOARD. If/when you do this, take into account the case you are putting the node into. I find having them stick out parallel to the breakout rather than up and at a right angle to it works best.


Now let's look at the other pieces that connect to a node.


This is a very basic and simple component. It is a resistor whose resistance is inversely proportional to the amount of light falling on it. Combined with a resistor on the board it makes a voltage divider between 5v and ground. The more light shining on it, the lower the resistance, the higher the voltage at the midpoint of the divider, and the larger the value returned by the analog to digital converter it is connected to.

The photoresistors I have used are standard cadmium sulphide (aka CdS) photocells. These are cheap and simple and work quite well if all you need is a general reading on the amount of light. For my purposes this is fine; all I need is an idea of how bright the room is to help decide if more lighting is required. Basically "Is the room dark?" A problem with CdS photocells is that they are not RoHS compliant. You can use a phototransistor as a RoHS compliant replacement.

Adafruit (as usual) has a great tutorial on CdS photocells.

You can get CdS photocells on Amazon for slightly over 10 cents CDN each, whereas I've seen phototransistors for about 60 cents each. For hacking and prototyping I'd use (as I have) CdS cells where they are available/legal but for production versions I would use phototransistors to avoid having my products banned in areas requiring RoHS compliance.

PIR Motion Detector

A PIR (which stands for Passive InfraRed) sensor is a staple in anything that senses motion. More specifically, a warm body that is moving since it looks at changes in the infrared distribution of what it is looking at (i.e. heat). They're really good at detecting humans moving about. Alas, they are also quite good at picking up cats moving about. Pro tip: cats move about A LOT. Since cats tend to move around on the floor, and these sensors see in a cone extending out from the sensor (approximately a 120 degrees), you can mount them high on the wall and/or add some blinders to limit their view of the floor. As usual, Adafruit has a decent tutorial on PIRs.

PIRs are all mostly the same, and look like this:

There are some differences between models. Specifically the presence of a jumper to select retriggerable mode or not, and a sensitivity adjustment. You want both. Look for units like the one below. You can see the jumper at the top left and the two adjustable resistors at the top (the orange twisty things).

I find that retriggerable mode works best. In non-retriggerable mode, a motion detection starts a single, fixed width, high pulse on the output. In retriggerable mode, each motion detection will restart the output pulse timeout. The result is that as long as motion is being detected, the output will be high. This has proven to be far more useful. The diagrams below illustrate the difference.

Non-retriggerable mode

Retriggerable mode

The time adjustment controls the width of the output pulse (or when retriggerable, how long it lasts after the last motion detection). The sensitivity adjustment adjusts, well, how sensitive the motion detector is. Fiddle with them to get the behaviour you want.

The PIR is connected to digital pin 2, and a change in it's output triggers an interrupt. This is allows the node to signal the control system that motion has started/stopped as soon as possible.


An RGB LED is simply three LEDs (one each red, green, and blue) in a single component with either their anodes (common anode) or cathodes (common cathode) connected together. The node sets the color of the LED in response to a command from the control system. To make life easier, one of the configuration jumpers sets whether the LED is common anode or common cathode. The RGB connector has both +5v and ground in addition to red, green, and blue. As mentioned, the current limiting resistors are on the board so all that is required is the actual LED.


I just love neopixels.

You can connect a bunch of them (limited by the processing power of your MCU and the amount of current you can provide) without having to have dedicated PWMs for each, or some multiplexing scheme. I designed the node to handle both an 8-pixel stick as well as a meter of 60-pixel/meter strip. I'm using these as undercounter lighting. The 8-pixel stick is being used to light my coffee making station. Yes, I have a dedicated space for making coffee. It's just how I roll.

The meter long strip will be lighting under one section of kitchen cabinets. These will require a tweak to the power circuitry since they need 3.5 amps peak current. To accommodate that, I have a 5v-4A wallwart that I'll use to power that unit, with the 5v regulator removed and it's input & output connected to allow the wallwart to provide 5v directly. I'll also be increasing the smoothing capacitor on the 5v line significantly to deal with potentially noisier power.

To learn more about them, see Adafruit's excellent tutorial.

Laser distance sensor

I mentioned connecting a distance sensor to the board for the under counter lighting nodes. I had started using an Si1145 Digital UV Index / IR / Visible Light Sensor with an IR LED to measure proximity via bounced IR. Adafruit has this on a breakout board for eash connection. I found that the returned values were inconsistent between devices, and it seems very sensitive to ambient light.

I switched to a laser time of flight distance sensor, using Adafruit's VL53L0X breakout board and am very happy with how it works. It provides a precise distant measure.


For my initial purposes (i.e. getting a system that works) I am less concerned with the look of things. This less than pleases my flat-mate. My plans are to switch to 3D modeling and make some nicer cases once the system is up and running and once I get a 3D printer.

Until then, I found a nice case from Adafruit designed for Arduino shaped boards. This is the prime reason for designing the node board to match the Arduino footprint. One especially nice feature of this case is that a 7cm x 3cm piece of proto board can fit perfectly in either an end or the rectangular face cutout. The PIR sensor also fits perfectly in the more square cutout (with a touch of glue).

A vertically mounted node.

A horizontally, under-shelf mounted node. Note: there are resistors on the face plate since this is an older prototype; not one of the custom PCBs.