# Computing stuff tied to the physical world

## LED discharge circuit

In Hardware on Dec 28, 2010 at 00:01

Yesterday’s post ended with the suggestion to play with some electronics bij hooking up some components to a JeeNode (or Arduino). This is really very useful (and oodles of fun, what a bargain), since it brings together so many aspects of physical computing:

So let’s do it!

Here’s the initial plan, let’s see how it pans out:

• set up a simple yet interesting circuit
• hook it up to a JeeNode (an Arduino wold work just as well, but using 5V)
• write a little test sketch to drive the circuit
• use two analog input pins as a pair of DIY multimeters
• report the results graphically
• interpret the results and try out some variations

The circuit I’m going to use, looks as follows:

If you’re not familiar with solderless breadboards, note that those central holes in the picture are vertically interconnected in groups of five. I.e. the brown wire is connected to one side of a brown-black-red resistor on the right, as well as to one of the two wires coming out of that large black capacitor on the left. And so on.

One LED, one capacitor, two resistors, a few wires, and a JeeNode. That’s all. But as you will see, it’s enough to explore PWM and digital-to-analog conversion, and it illustrates how you can create your very own electronics workbench to explore “RC filters”, charge curves, discharge curves, pulse generators, timers, and even create a very simple oscilloscope to understand what’s going on in a dynamic electronic circuit.

The first thing I need to do is clear things up a bit. While that above picture is a fully functioning circuit, it’s pretty awkward to see exactly what is hooked up to what. Some of these components are polarized, so there’s in fact more to it than “which wire goes where”.

Here’s the same circuit, is schematic form:

See if you can match everything in the schematic up to that picture above.

First thing to notice, is that there’s a lot more info here, and it’s a lot more precise. The components use a standard notation, and their values are also indicated.

Now, although it was a fun exercise for me to draw this by hand and scan it in, it’s a bit tedious if I make mistakes while drawing or when things change. Fortunately, there are computer-aided design (CAD) software packages which make it simple to draw and edit such schematics on-screen. Here’s the same schematic using EAGLE:

It has the same layout as the hand-drawn version, but now all the components also have names: R1, R2, C1, LED1, etc. I’ll refer to these names from now on.

Ok, all nice and well, but what does the circuit DO ???

Well, first of all, since there is no energy source in there: nothing at all until you feed it from an external power source. Doh.

But this is where the fun starts. We could just hook up a battery to it (+ to VIN and – to GND), and there would in fact be some interesting behavior. But we can do a lot better than that: we can put the entire circuit under computer control! The digital output pins on an ATmega are terrific controllable power sources, as long as we only need a few milliamps at 3.3V or 5V. And if we need more… well, that’s where transistors come in (to be described in a future post).

The other great property of an ATmega (many MPUs, for that matter), is that they also have analog and digital inputs, so they can be used to measure various aspects of the circuit under test at the same time.

This is why the above circuit is hooked up to DIO3, AIO2, and AIO3 (digital 6, analog 1, and analog 2 on the Arduino, respectively).

I still haven’t told you what the circuit does. But if you’ve played around a bit with this stuff before, you should be able to predict what this little sketch does:

Stay tuned…

1. Hey… A LED-tester !!! Now just add an LDR and you could also see if the LED does actually glow at the given voltage.