Seriously though, everything is a voltage

Most of the electronic devices we use have a specific purpose: phones make calls, toasters toast bread, calculators crunch numbers.  With some electronics background, you can begin to see these gadgets as the sum of many, more basic working parts.  Once you have knowledge of how those individual parts work, you not only gain a greater appreciation of the more complicated gadget, but you can also dream up new ways of combining those working parts to create something new.  Hacking together some new gizmo is really the entire philosophy behind the Arduino boards that this high school classroom is hoping to experiment with.

Let me give two examples of useful circuits that can be understood by taking a look at the simple components they are built from.  The first is a quick-and-dirty hacked together project from my own research.  The second example is really the exact same circuit, but should be much more familiar – a bike speedometer.

Recently for my research projects I had to wrap a wire around a spool 500 times, creating a coil of wire.  (Perhaps I’ll talk about why one would want to do this in a future post).  It would be very tedious to count the 500 turns myself, so I MacGyver-ed together a simple counter using a magnet, a calculator, and a special switch that acts as a magnetic sensor.

A magnet, magnet sensor, and calculator used to count the number of revolutions made by the red-and-blue spool.  You can see that so far, it has gone around 92 times.

I taped the magnet to the spool and positioned the magnet sensor nearby.  You can see that the sensor was wired to a calculator.  In the video below, watch as the magnet passes the sensor, and the counter increments from 71 to 72.

So how exactly does this act as a counter?  I’ll try to explain using the diagram below.  The small magnetic sensor is just a switch with two magnetic wires inside of it.  When a strong magnet is held nearby, those two wires are tugged by the nearby magnet, causing the wires to connect, and the switch to turn ON.  By wiring this switch to the “plus” key of an old accounting calculator, I could “press” the “plus” key whenever the magnet would pass by the sensor (without actually pressing the plus key with my own finger).  You see, when you press the “plus” key of a calculator, you are actually just closing a switch.  When that switch is closed, the voltage from the calculator batteries pushes electrons through a circuit, and the calculator enters “addition” mode.  Lastly, the reason we used an accounting calculator: if you first enter the number “1” on an accounting calculator, every time the “plus” key is pressed after that, “1” is added to the currently displayed number.  Voila, a counter!

The magnetic sensor is normally an open (or OFF) switch.  When a magnet comes nearby, it closes the switch and "presses" the "plus" button, causing the calculator display to increase by 1.

A bike speedometer/odometer works exactly like this.  A magnet attached to a spoke spins past a magnetic sensor once each revolution.  This sensor sends an electrical current up a wire to the display on the handlebars.  The display works just like the calculator, counting the number of electrical pulses it receives.  On each revolution of the wheels, the bike moves forward a distance equal to the circumference of the wheel.  So just by counting the number of wheel revolutions, and multiplying by the circumference of the wheel, the bike odometer can figure out how far you’ve ridden.

The magnet, magnet sensor, and display used to measure a bike's speed and distance are nearly identical to the circuit I used to count revolutions in the above example.

Since the bike computer has a built-in clock, it can also calculate your speed.  That’s just the circumference divided by the time it takes for the wheel to go around once (speed = distance/time).

Once you see how you can use a few simple circuit elements to accomplish a useful task, you can dream up lots of uses for it.  A magnet and sensor switch like this could be used to sense if a door is ajar, or to wake up your laptop when you open the screen.  Counters are used to dispense the right number of bills at an ATM, and to keep track of the number of cars that travel down a given street.  With just a little bit of electronics background, those gadgets and gizmos we make use of everyday start to look less like magical black boxes, and more like useful, understandable, and engineer-able tools.