Keypads work by creating unique electrical signals when a key is pressed, allowing the connected device to identify the specific key.
Here's a breakdown of how it works:
Matrix Arrangement
Most keypads, especially those with multiple keys, use a matrix arrangement of rows and columns to minimize the number of input/output (I/O) pins required. A common configuration is a 4x4 matrix for a 16-key keypad.
I/O Lines
The keypad utilizes I/O lines connected to a microcontroller or processor. Some of these lines are designated as outputs, while others are designated as inputs. For a 4x4 keypad, you'd typically have 4 output lines and 4 input lines.
Scanning
The microcontroller sequentially activates each output line, one at a time. This is known as "scanning". When an output line is activated (typically set to a HIGH voltage), the microcontroller then reads the input lines.
Key Press Detection
When a key is pressed, it creates a connection between a specific row (connected to an output line) and a specific column (connected to an input line). If the microcontroller finds a HIGH voltage on a particular input line while a specific output line is active, it knows that the key at the intersection of that row and column has been pressed. A circuit is completed between an output and an input when a key is pressed.
Unique Signal Creation
Each individual keypress creates a unique signal for the host. The combination of the activated output line and the detected input line uniquely identifies the key. The microcontroller uses this information to determine which key was pressed.
Debouncing
Key presses often generate "bouncing" – multiple rapid on/off signals due to the physical contact of the key. To avoid misinterpreting these bounces as multiple key presses, a debouncing technique is used. This typically involves a short delay (milliseconds) after detecting a key press, during which the input is ignored. After the delay, the input line is checked again to confirm that the key is still pressed.
Example: 4x4 Keypad
Consider a 4x4 keypad with rows R1-R4 and columns C1-C4.
| C1 | C2 | C3 | C4 | |
|---|---|---|---|---|
| R1 | Key 1 | Key 2 | Key 3 | Key 4 |
| R2 | Key 5 | Key 6 | Key 7 | Key 8 |
| R3 | Key 9 | Key A | Key B | Key C |
| R4 | Key D | Key E | Key F | Key # |
If Key 6 is pressed:
- The microcontroller activates R2 (sets it HIGH).
- The microcontroller reads the input lines (C1-C4).
- It detects a HIGH voltage on C2.
- The microcontroller determines that Key 6 (R2, C2) was pressed.
Handling Multiple Key Presses
While generally not intended for normal use, some keypads (if the processor allows) are designed to handle the simultaneous pressing of two keys without ambiguity. This requires a more sophisticated scanning and decoding algorithm.
In summary, keypads utilize a matrix structure and sequential scanning to detect key presses and generate unique electrical signals that a connected device can interpret.
