Tag Archives: 555


555 Contest Entry: “Whack a Mole” style game

Jim Chen made a very interesting LED chasing game that uses six 556 timer chips. This is his second entry to the 555 contest which is recently closed. There are nine LEDs in the game. Any of them could glow randomly. The player has to turn off the LED by touching an electrode next to the LED. While the player continue playing the game the time available for the player is less and less. When you missed to turn off an LED within the provided time frame, the game is over. Here’s how the game works. Read more

2-Wire Keypad Interface Using a 555 Timer

Board

Keypads are a very commonly used input device in microcontroller-based systems. In a keypad, multiple switches are arranged in rows and columns so that they could be interfaced to a microcontroller with a minimum number of I/O pins. For example, a 12-key keypad is arranged in a 4×3 format, which allows to interface the 12 keys to a microcontroller with only 7 connections. The location of each key on the keypad is defined by two coordinates: the row and the column. When a key is pressed, it connects its row with its column. The microcontroller must scan all the rows and columns to find out which key has been pressed. This is the most common way of interfacing a keypad to a microcontroller. There are tons of resources on the internet regarding this technique and so I am not going to discuss it here.

Today, I am going to share with you about a new keypad interfacing technique that uses only two I/O pins of a microcontroller: one for signalling the microcontroller when a key is pressed, and the other is to read the key information. It is based on a 555 timer IC which is configured as an astable multivibrator. I am not sure if anybody has ever tried using a 555 IC for keypad interfacing, but this technique really works. I am going to demonstrate this with a 4×3 keypad (a standard telephone dial pad). A PIC16F628A microcontroller will read the output of a 555 timer IC, determine what key has been pressed, and display it on a character LCD module.

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