555 Timer 2-Wire Keypad Interface

Ouput1

Rajendra Bhatt explores a method of interfacing with a keypad via a simple 555 timer. The result is a two wire interface for 16 buttons.

There are other methods which do not require as many components, such as Jon Chandler's 10 Keys on One Port Pin. Although Rajendra's approach is not often seen and he gets good points for originality!

Attached are images of the project, including a schematic of the 555 interface.

Circuit-550x2751

The 555 timer has been configured as an astable multivibrator. It's output frequency varies based on the active resistance. With R1 and C as fixed components, each key has a resistor which acts as a switchable load (altering the output frequency accordingly).

The microcontroller can then read the output frequency to determine which key was pressed. The more I think about it, the better Jon's solution sounds. Still, its somewhat original. [via]

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