In Chapter 1 we talked about computers in the abstract. Now we get to the fun part, building one with electronics…

# Circuit

Modern computers use electricity to communicate and store information, so let’s take a moment to go over the electrical circuit.

First, to power our circuit we’ll use batteries. Batteries are little chemical packets waiting to carry out an electrochemical reaction. Each battery has a “+” and “-” terminal connected to compartments of positive and negatively charged electrolyte.

For the chemical reaction to move toward equilibrium, the “-” terminal must release excess negative charge in the form of electrons which travel to the “+” cell.

Connecting our electronic components in a closed loop allows electrons to flow in the direction of least resistance, from negative to positive. This flow of charge is what we call electrical current!

Electricity acts like a skateboarder traveling down a ramp. The higher the ramp, the more potential energy they have and the further they can travel. A circuit’s potential is measured the same way, *voltage* is the electrical potential difference between negative and positive terminals.

What happens if we use three AA batteries connected in series? Each battery has 1.5 volts of electrical potential, so that’s like stacking three ramps on top of one another to get a big, 4.5 volt ramp.

The positive and negative terminal should never meet each other directly, this is called *shorting*.

*Without a ramp the drop is too high.*

Now let’s make a circuit that lights up!

We’ll use the breadboard, a prototyping tool for quickly testing out circuits before soldering them into fixed hardware. We can add components to the circuit by pressing the metal leads into the holes of the breadboard which are connected in rows by conductive copper underneath the plastic.

The light emitting diode (LED) lights up when an electrical signal travels up one lead and down the other, like hitting a jump.

Now whenever the LED lights up we know our circuit is complete.

# Current & Resistance

Electrical components each have their own tolerance for electricity, so we’ll need to make sure we have the right amount of resistance to protect ourselves and our circuits. LEDs aren’t made to handle so much on their own, so we should add another component to share the load. We can do this with a resistor.

Resistors act like rough surfaces that slow down our skateboard to a safe speed.

But resistors come in all strengths, so how much resistance should we add? Let’s use Ohm’s Law to figure that out.

First we’ll need to understand a little more about electrical current. If we think about voltage as the height of the ramp, current is like the number of friends we have skating with us.

Current is measured in amps and our LED says it’s rated for 20 milliamps at a 2.5 volt drop. What this means is only a certain number of people can safely skate together at one time (0.02 amps), and that all of us should hit the jump at 2.5 volts. If we have 3 batteries stacked up to 4.5V, we’ll need to slow down.

**4.5V - 2.5V = 2V** decrease in potential

Now we can use Ohm’s law to calculate how much resistance will slow us down by 2V if we have 20 milliamps of traffic.

Ohm’s Law:

**Voltage = Current x Resistance**

or

**Voltage / Current = Resistance**

so

**2V / 0.02A = 100Ω**

Which means we should add a 100Ω resistor to our circuit to be at the right speed to hit the jump.

# Switch

Now that our LED is happily lighting up with the right amount of voltage, let’s make a circuit where we can turn the LED on and off. We can select the input for our circuit by using a switch. The switch below is called a SPDT (single pole double throw), which allows us to alternate between two states.

The middle pin always stays connected, but sliding the handle from side to side swaps the connection between the outer two pins. If we connect the right pin to the negative terminal and the middle pin to the positive terminal, then moving the switch to the right will complete the circuit.

When all of the parts are connected on the breadboard, we can switch the LED on and off. This is like “Hello, World!” for electronics, where our input exactly matches the output.

# Spaces of Difference

We’re starting to see how signals and components come together to make an electrical system, which means we’re one step closer to becoming hackers! Whenever we add a new component, we designate its role in the larger circuit. This is an empowering act, rewiring the system into something new.

The city is a space where many circuits and many systems are layered onto one another. Some of these circuits are analog, others are digital. Analog circuits allow for variation and nuance, while digital circuits represent binary spaces, with clear divisions between inside and outside, normal and abnormal.

Most importantly, there are also *spaces of difference* that blur the two. A space of difference is one where inclusivity and exclusivity coincide, like a ramp connecting different planes.^{1}
In these spaces there exists a possibility for transgression.^{2}

A skateboarder refigures modernist architecture as a site for new modalities.^{3}
So too can spaces of difference offer a chance to claim accessibility and inclusivity for those considered other.

# Transistor

Now that we’ve covered the basics, let’s do some tricks.

This is a transistor…

Transistors let us to do something called *negative logic*, which is another way of saying that turning one switch off automatically turns another switch on.

The transistor is an important component which has the ability to switch electronic signals. There are few varieties of transistors, the most common is the NPN (Negative Positive Negative) transistor, which also has three leads, emitter, base, and collector. The NPN transistor, just like the SPDT switch, alternates between two states, except it switches without any moving parts.

The way it does this is by allowing current to flow in one direction only, from emitter to collector, and then controlling that flow with a small amount of voltage at the base. When there’s no signal at the base, the path between emitter and collector is blocked and the current can’t pass through. We can see the NPN transistor uses negative logic because the input at the base is the inverse of the emitter’s output.

In 1937 Claude Shannon first explored the idea of using switching circuits like these for computation.^{4}
Shannon is often credited with formalizing the relationship between communication and uncertainty, but he was also an eccentric scientist who enjoyed unicycling around the laboratory. I think Shannon embodied a connection between computation and play, experimenting with physical relationships like current and momentum through his own system of repitition and abstraction. Ten years after Shannon introduced his theory of information, another event at Bell Telephone Laboratories, the discovery of a new semiconductor material suitable for implementing Shannon’s switching circuit (now called the transistor), would go on to become one of the most significant moments in the history of computing.

# Skating the Circuits

Skating and making circuits are similar in many ways. Once you get the hang of it, you can go anywhere you want. Everyone skates their own way and the same is true for building circuits. There’s a freedom of movement I find appealing about both, a sense of jouissance. A joy of free exploration, improvisation, and play that’s not so much about complicated tricks, but inventing ways to get from one place to another and the hidden corners we discover along the way. Building circuits is about finding your own poetic computation and sharing it with others.

Taeyoon Choi is an artist, educator, and activist based in New York and Seoul. His art practice involves performance, electronics, drawings, and installations that often form the basis for storytelling in public spaces. His projects were presented at the Whitney Museum of American Art and Los Angeles County Museum of Art. He co-founded the School for Poetic Computation where he continues to organize sessions and teach classes.

“Skating the Circuits” is the second of seven chapters from *Handmade Computer*, a book written by Taeyoon Choi and edited by Sam Hart. Chapters will be released bi-weekly during the summer of 2017.

Ramps and skateboarding metaphors inspired, in-part, by Sara Hendren’s

*Slope Intercept*. I collaborated with Sara Hendren and Alice Sheppard to lead a*Ramp and Accessibility Mapping*workshop at Uncertainty School in 2016.^{↥}I lead a skateboarding tour around my studio in Downtown Brooklyn with Damon Rich and the Van Alen Institute.

^{↥}More on Skateboarding vs Modernism.

^{↥}Claude Shannon,

*A Symbolic Analysis of Relay and Switching Circuits*, 1938.^{↥}