Lesson Notes By Weeks and Term v4 - SHS 2

DIGITAL ELECTRONICS

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  2. Lesson Notes By Weeks and Term v4
  3. SHS 2
  4. DIGITAL ELECTRONICS

Subject: Physics

Class: SHS 2

Term: 2nd Term

Week: 16

Grade code: 2.3.3.LI.2

Strand code: 3

Sub-strand code: 3

Content standard code: 2.3.3.CS.1

Indicator code: 2.3.3.LI.2

Theme: ELECTRIC FIELD, MAGNETIC FIELD AND ELECTRONICS

Subtheme: DIGITAL ELECTRONICS

Lesson Video

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Performance objectives

Lesson summary

Welcome, future engineers and scientists! Today, we are diving into a very practical part of digital electronics. Think about your mobile phone, a computer, or even the traffic lights in Accra or Kumasi. All these devices rely on making clear, unambiguous decisions based on electrical signals. These signals are either HIGH (like 'ON' or '1') or LOW (like 'OFF' or '0'). However, sometimes an input to a microchip can be left "floating"—not clearly HIGH or LOW. This is like asking a question and getting no answer; it creates confusion and makes the device unreliable.

Lesson notes

A. The Problem: The "Floating" Input Pin

In digital electronics, a microchip (like the brain of a device) needs to read inputs as either a clear HIGH (usually +5V or +3.3V) or a clear LOW (0V or Ground).

Consider a simple circuit with a switch connected to a microcontroller input pin (MCU Pin): When the switch is CLOSED: The MCU Pin is connected directly to Ground (GND). The microchip reads this clearly as LOW (0V). This is good. When the switch is OPEN: The MCU Pin is connected to... nothing! It is "floating". It's like an antenna that can pick up stray electrical noise from the air (from lights, phones, etc.). The voltage at the pin can drift unpredictably, and the microchip might read it as HIGH, then LOW, then HIGH again, all without the switch being touched. This leads to erratic and unreliable behaviour.

We need a way to give the input pin a default voltage level when the switch is open. This is where pull-up and pull-down resistors come in. B. The Solution Part 1: The Pull-Up Resistor

Evaluation guide