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Punch Cards

Punch cards use a decimal-based encoding system, not binary, and are read by machines through either electrical contact or light sensors that detect holes punched into the card. Understanding how punch cards work provides foundational context for how early computers processed and interpreted data.

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About this video

Punch cards store data using a decimal encoding scheme rather than binary. Each card contains columns representing the digits zero through nine, and a single hole punched in a column corresponds to that digit's value. This is distinct from binary encoding, which would use powers of two across eight columns to represent data. The decimal approach allowed machines to interpret a wide range of values directly from the physical pattern of holes on each card. Machines read punch cards through two primary methods: electrical contact and optical sensing. In the electrical method, a conductive probe passes over the card, and wherever a hole exists, the probe makes contact with a conductor beneath, completing a circuit and generating a signal. In the optical method, a light source such as a laser is positioned on one side of the card while a photo sensor sits on the other. When the card passes through, holes allow light to reach the sensor, which registers each detection as a signal for the computer to process. In practice, a complete program or set of instructions was encoded across multiple punch cards fed sequentially into the machine. Each card contributed a portion of the overall input, and the machine assembled those signals into actionable commands. Punch cards represent one of the earliest methods of persistent, reusable data input and offer important historical context for understanding how computing hardware and data encoding evolved over time.

What you'll learn

What's covered

Punch Cards

Aligned to

CompTIA Tech+
1.1 Compare and contrast notational systems.
1.3 Illustrate the basics of computing and processing.

Key terms

Punch Card
An early form of data input and storage consisting of a paper card in which holes are punched at specific positions to represent binary instructions or data that a card reader translates into computer input.
Decimal System
A base-10 numbering system using digits 0 through 9, used by punch cards to encode data by designating one hole position per column to represent a single digit.
Electrical Contact
A punch card reading method in which a probe and conductor complete an electrical circuit through a punched hole, sending a signal to the computer to register the encoded data.
Optical Sensing
A punch card reading method that uses a light source and photo sensor to detect holes in a card; when light passes through a hole, the sensor registers it as a signal for the computer to read.

Topics

Punch Cards Data Encoding Decimal Encoding Optical Sensing Computer History Early Computing

Transcript

Punch Cards Are Not Binary

You don't really need to know how to read punch cards or how they really work. One thing to note about punch cards is that they're not binary. If they were binary, we would have eight columns and we'd punch out a 1, 2, 4, 8, 16, those powers of two. But instead, what we see on here is a column of 0 through 9, and then we would punch out one of those 0 through 9. So there's 10 digits there, and this is actually a decimal system.

This card would be read into a machine, and then the machine would see these little punches along here and interpret that into the code for the computer. The primary two methods of reading a card are usually through electricity or through light.

Reading a Card With Electricity

The actual machine is going to look and operate a little differently than this, but this is going to get the point across. What we have here is this conductor, and I also have a probe right here, and when I press these two together it makes a complete circuit and will make a sound. So every time it makes that complete circuit, we're going to hear a sound.

What I have here is a punch card that I punched out holes into. What we could do then is either drag the card or drag the probe across these, and we're going to see that every time it makes a connection, it now sends that signal to the computer. So that's how it would read the different punches into this card.

Reading a Card With Light

Now let's look at how we would use light. What I have here is a laser, and then this is just something that's creating some vapor. This is representative of a photo sensor that would actually detect whether there's light or not, and that's how it's going to read the card. Every time the light shines through, that sensor is going to pick it up and realize that there's a hole there. We're going to drag it across here, and the card is either going to block or shine through one of those holes, creating a signal for the computer to read. So that is how it reads it through light.

Then we would have a number of these cards with all the different programming on the card that we'd feed into the machine to give it instructions.

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