Lesson Notes By Weeks and Term v4 - SHS 3
ELECTRONIC COMPONENTS AND CIRCUITS
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ELECTRONIC COMPONENTS AND CIRCUITS
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Subject: Applied Technology
Class: SHS 3
Term: 2nd Term
Week: 14
Grade code: 2.5.2.LI.2
Strand code: 4
Sub-strand code: 2
Content standard code: 2.5.2.CS.1
Indicator code: 2.5.2.LI.2
Theme: ELECTRICAL AND ELECTRONIC TECHNOLOGY
Subtheme: ELECTRONIC COMPONENTS AND CIRCUITS
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This lesson introduces one of the most versatile and widely used integrated circuits (ICs) in the world: the 555 Timer. In Ghana, we see the effects of timers everywhere – from the traffic lights in Accra and Kumasi that control the flow of 'tro-tros' and cars, to the beeping of a microwave oven in our homes, to the flashing lights on security systems. Understanding how to control time in an electronic circuit is a fundamental skill for any aspiring technician, engineer, or innovator. The 555 Timer is a simple, cheap, and reliable building block that allows us to create these timing circuits and many more.
A. What is an Integrated Circuit (IC)?
Think of an IC, or "chip," as a miniature city of electronic components. Inside that small black plastic block are thousands of transistors, resistors, and diodes, all connected to perform a specific, complex task. The 555 Timer is an IC specifically designed for timing and oscillation. B. The 555 Timer IC: Pinout and Functions
The 555 timer most commonly comes in an 8-pin package. It's crucial to know what each pin does.
*(Teacher should draw this on the board)* Pin 1 (GND): Ground. This is the 0V reference point for the circuit. It connects to the negative terminal of the power supply. Pin 2 (TRIG): Trigger. This is the "start" pin for the timer. In Monostable mode, when the voltage here drops below 1/3 of the supply voltage (Vcc), it starts the timing cycle. Pin 3 (OUT): Output. This is where the result comes from. The output can be either HIGH (close to Vcc) or LOW (close to 0V/GND). This pin can drive small loads like an LED or a relay. Pin 4 (RESET): Reset. This is an override pin. If you apply a low voltage (GND) to this pin, it stops whatever the timer is doing and forces the output LOW. To allow the timer to operate normally, this pin is usually connected directly to Vcc (+). Pin 5 (CTRL): Control Voltage. This allows for advanced control over the timing, but for most simple circuits, we do not use it. It is good practice to connect it to GND through a small capacitor (e.g., 0.01µF or 10nF) to prevent electrical noise from affecting the timer's stability. Pin 6 (THRES): Threshold. This is the "stop" pin. When the voltage at this pin rises above 2/3 of Vcc, it ends the timing cycle and makes the output go LOW. Pin 7 (DISCH): Discharge. This pin acts like an electronic switch connected to GND. When the output is LOW, this pin is internally connected to GND, allowing it to discharge a capacitor connected to it. When the output is HIGH, this pin is disconnected. Pin 8 (VCC): Power Supply. This is where you connect the positive terminal of your power source. The 555 can work with a wide range of voltages, typically from +4.5V to +16V. A 9V battery is a common choice for projects. C. Modes of Operation