Night Light

Here we're making a simple Night Light that turns on when it gets dark, and off when it's light.

On the 2400mAh battery used it should last around 4 months assuming the LED is on for 4 hours a day.

You might also want to check out the Puck.js Night Light which glows a colour to show you the temperature.

You'll need

• A MDBT42Q Breakout board
• A LED and resistor of around 300 Ohms (you can use the on-board
• A Battery
• A ping-pong ball and a case (details below)

Wiring

Simply:

• Wire up the battery using either the JST connector or directly
• Wire the LED between GND and and IO pin, with the resistor in series. We're using pin D15 but any pin will do.

Software

var light = 0, on = false;

setInterval(function() {
  light = analogRead(LED2);
  if (light < 0.16) on = true;
  if (light > 0.18) on = false;
  D15.write(on);
}, 2000);

// Only use this code if you want to make the light
// invisible on Bluetooth until the button is pressed
NRF.sleep();
setWatch(function() {
  NRF.wake();
}, BTN1);

Just upload the code above to flash the MDBT42Q. Once uploaded it'll disconnect from Bluetooth and will be a usable night light.

However, you'll probably want to tweak the numbers for when the LED turns on and off. Simply do this by removing the last bit of code, then while connected type light into the Web IDE's console to find out what the light value is at different brightness levels.

Improvements

As noted in the video, the usage of hysteresis can help to stop the light from flickering on and off, however there are some other additions that can really help.

• Replace light = analogRead(LED2); with D15.reset(); light = analogRead(LED2); D15.write(on);. Suggested by Akos Lukacs in the video comments, this turns the LED off very briefly in order to take a light reading, then turns it back on. There's a barely perceptible flicker.
• Flip the MDBT42 PCB over, so the LED used for sensing is facing away from the LED producing the light.
• Blank off the bottom of the ping-pong ball so light doesn't shine directly down onto the sensor.

3D Printed Case

The case was designed with OpenSCAD - it is simply a hollow cylinder:

difference() {
  translate([0,0,-1.5]) cylinder(r=36/2,h=60);
  cylinder(r=33/2,h=60);
}