# I Built a Neural Network's First Neuron From Scratch — the 1958 Perceptron

> Source: <https://dev.to/dev48v/i-built-a-neural-networks-first-neuron-from-scratch-the-1958-perceptron-3gfg>
> Published: 2026-06-13 23:51:26+00:00

Before transformers, before backprop, there was **one neuron** — Frank Rosenblatt's 1958 Perceptron. Build it and you understand the atom that every deep network is made of.

This is Day 1 of DeepLearningFromZero: neural nets built from a single neuron up, no framework magic.

Take inputs, multiply each by a weight, add a bias, then apply an activation. The original used a **step**: output +1 if the sum is ≥ 0, else −1.

``` js
let w = [Math.random(), Math.random()], b = 0;
const sum = x => w[0]*x[0] + w[1]*x[1] + b;
const predict = x => sum(x) >= 0 ? 1 : -1;
```

`w₁·x₁ + w₂·x₂ + b = 0`

is the equation of a straight line — the **decision boundary**. One side is class +1, the other is −1. So a neuron's entire "knowledge" is the tilt and position of one line.

Predict each point. If correct, do nothing. If wrong, nudge the weights toward the right answer:

```
for (const { x, y } of data) {
  if (predict(x) !== y) {        // y is the true label, +1 or -1
    w[0] += lr * y * x[0];
    w[1] += lr * y * x[1];
    b    += lr * y;
  }
}
```

Geometrically, that rotates and shifts the line so the misclassified point ends up on the correct side. Repeat for a few epochs:

``` js
for (let epoch = 0; epoch < 50; epoch++) trainStep(0.1);
```

Rosenblatt proved it: **if the two classes can be separated by a straight line, the perceptron will find one in a finite number of steps.** Watch the accuracy climb to 100% and training stop.

A single neuron can only draw **one straight line**. The famous failure is **XOR** — no single line separates it. That limitation nearly killed neural nets in the 1970s.

The fix: stack neurons into layers, swap the step for a smooth activation, and train with gradient descent + backprop. That arc — from this one neuron to a transformer — is the rest of the series.

🌐 Train the neuron live (watch the boundary rotate): [https://dev48v.infy.uk/dl/day1-perceptron.html](https://dev48v.infy.uk/dl/day1-perceptron.html)

Day 1 of DeepLearningFromZero. From one neuron to transformers, built from scratch.