Electronic Keys & Locks

Key Scanning with a Small Number of Connections Schematic Circuit Diagram

Efficient Scanning Method for Multiple Keys

When dealing with a considerable number of keys, connecting each key directly to the microcontroller isn’t practical. Instead, a matrix arrangement is employed, reducing the necessary port pins to just seven for a set of twelve keys, for instance. Through software scanning of rows and columns, the microcontroller determines the specific key pressed. However, in certain scenarios, there might be limitations such as having a small microcontroller with very few port pins, rendering the economical matrix method unsuitable.

Key Scanning with a Small Number of Connections Schematic Circuit Diagram

Efficient Key Scanning with Minimal Port Pins

By employing a clever technique, the challenge of scanning keys can be tackled effectively using only four port pins. This method necessitates the incorporation of four additional diodes and the ability to configure the pins individually as either inputs or outputs via software. Four column lines (1-4) are organized in a matrix alongside four-row lines (A-D) connected to the microcontroller. Each row line is linked to a column line through a diode (1N4148).

Software Recognition of Pressed Keys

To identify a pressed key, the software applies voltage to each row sequentially while observing the states of the remaining port pins configured as inputs. Thanks to the diodes, a voltage will be detected on only one of the inputs, determined by the pressed key. The software can then assign the appropriate action based on this event.

Replacement Diode: 1N4148W

While the 1N4148 diode has been a trusted semiconductor for many years, Vishay Intertechnology no longer provides it for new applications. Instead, the recommended alternative is the 1N4148W. This component is not only RoHS compliant but also AEC-101 stress test qualified, making it suitable for modern designs.

Leveraging Datasheets and CAD Drawings

To optimize the diode’s usage in fast-switching applications, it’s crucial to support the 1N4148 datasheet with accurate CAD drawings. Platforms like Ultra Librarian offer comprehensive libraries, providing access to datasheets, ECAD models, and simulation and testing data for electronic components, all in one place. Moreover, for streamlined procurement, it’s advisable to check component availability status, price, and models offered by various vendors, preventing supply chain challenges.


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