# Computing stuff tied to the physical world

## LED discharge – it’s analog!

In Hardware on Dec 29, 2010 at 00:01

The LED discharge circuit presented yesterday can be used for a number of experiments. Yesterday, I asked what this sketch does:

Here’s what happens:

(Not sure this video will work properly with all browsers – here are MP4 and MOV links)

It’s a little bit of a trick: pin 6 is set to a “1” digital output (i.e. 3.3V) for one second, and then to an input for 5 seconds. Being an input means it no longer supplies power (or rather: a negligible trickle through the pull-up). So running the sketch is like periodically connecting a 3.3V power supply for one second and then disconnecting it for the next 5 seconds:

If you look at the circuit schematic again, you can see that pin 6 (i.e. DIO3) is connected to VIN. When it it at 3.3V, it will charge capacitor C1 through resistor R1.

The JP3, JP4, and JP5 pins are not used yet (analog 1 and 2 are inputs). The LED and resistor R2 are powered by the same voltage as what is currently present on the capacitor C1:

• when pin 6 is high, C1 charges, and the voltage over C1 increases
• the LED draws some current, and lights up
• when pin 6 “floats”, C1 discharges, and the voltage over C1 decreases
• the LED still lights up, but it gets dimmer as the voltage over C1 drops

Here’s a graph of the LED brightness over time, which matches what is shown in the above video:

There’s a lot of hand-waving in there: can’t actually know excatly what’s going on until we measure it, right?

Let’s start by measuring the voltage over the capacitor C1. Note that the bottom is tied to ground, i.e. it’s 0V by definition. The top side of the capacitor is tied to VHIGH, i.e. analog 2 (AIO3). So all we need to do is measure while waiting for time to pass:

Sample output:

Oh, wait. That’s not terribly useful. The analog converter reports values from 0..1023, corresponding to voltages 0..3.3V – so why not convert it to millivolts first? (I prefer to use ints, floating point isn’t very convenient on small 8-bit microcontrollers). This is the improved version:

And here’s some new output over a longer period of time, rearranged for brevity:

A quick copy of these values into a spreadsheet produces this graph:

As you can see, the measurements follow a very clear and regular pattern. But what’s going on? Why are these lines curved? Why doesn’t the voltage go all the way up to 3.3V? Why doesn’t it drop to zero? How does this graph explain the LED’s brightness changes? How can we get a constant brightness?

Questions, questions. Welcome to the real world, which – in case you hadn’t noticed – is mostly analog!

The neat bit about all this is that not only can we just play around and hook up components in all sorts of funky ways, we can in fact even explain exactly what is going on. Those graph shapes, for example, are fully predictable exponential curves, and there’s a very simple reason why they are this way.

I’ll go into that in a future post, but first let’s find out how to create an arbitrary voltage using nothing but one digital output pin. C’ya tomorrow!