What you’ll need
- 1.2 k resistor
- 2.2 k resistor
- Light dependent resistor
- 1 μF capacitor
- Breadboard and wires
You’ve probably all seen those cheap, solar-powered lamps that you can stick into your garden to try and give it a classy bit of illumination during the night. If you’ve actually got one then you may have found out that they don’t shine very brightly and the plastic stakes can be very flimsy. So why not make your own version? What we’ll show you on this page is the beginning of an array of light-sensitive LEDs using a single LED, so that you can understand how the system works. We’ll use a special resistor called a light dependant resistor (LDR) or photoresistor that changes its resistance based on the levels of light it’s receiving.
There’s no extra software you’ll need for this, so just make sure your Pi is up to date with an apt-get update and apt-get upgrade. Turn it off, unplug it from the power source and get to work wiring up the circuit as shown in the Fritzing diagram below. Take special note of the placement of the components, especially the LDR and the capacitor. What you’ll need to do is measure the time it takes for the capacitor to fully charge between pin three of the GPIO ports and ground. Also, make sure the negative end of the capacitor is hooked to the ground side of the circuit.
Once that’s done, turn your Raspberry Pi back on and download the code we’ve created for this project using:
$ wget http://gadgetmag.wpengine.com/wp-content/uploads/2014/08/dusklight.zip
Unzip it and run it. We’ve made it so the Python shell will print out the values that the circuit returns, with a higher number meaning it’s darker and the capacitor takes longer to charge. See how the circuits react to you covering the LDR with a finger or shining a torch on it in order to get an idea of how your version will work.
To get this working for outdoor use it will require a bit of trial and error. The easiest way is to set it up when it’s getting to about the level of darkness you’d want the light to turn on and record the output from the sensor. Change the if statement so that it activates the LED over a certain number and you’ll be set. With some external power and more LEDs you can have a Raspberry Pi power an entire array of shining lights around your garden.