Output devices such as LEDs, buzzers, motors and solenoids, interfacing them to a control circuit, and calculating the series resistor needed to protect an LED.

What this key area is asking

The SQA wants you to know the common output devices a control circuit drives (LEDs, buzzers, motors, solenoids), how they are interfaced to the processing stage, and how to calculate the series resistor that protects an LED. The output sub-system is the end of the input-process-output chain, where an electrical signal becomes light, sound or motion.

Output devices

The course works with a small set of standard devices:

• LED (light-emitting diode) - converts electrical energy to light. Used as an indicator. Conducts in one direction only and needs a current-limiting resistor.
• Buzzer - converts electrical energy to sound. Used for alarms and alerts.
• Motor - converts electrical energy to rotational (kinetic) motion. An inductive load.
• Solenoid - converts electrical energy to linear motion or a magnetic force (for example to throw a bolt or open a valve). An inductive load.

Motors and solenoids are inductive, so when switched off they produce a back-emf and require a protective diode, as covered with transistor switching.

Interfacing to the control circuit

A processing stage (a logic gate or a comparator) outputs only a small current at a logic level, far too little to run a motor, solenoid or even a bright lamp directly. The standard interface is a transistor switch: the small output controls the transistor, and the transistor switches the larger load current from the supply. For loads that are high-power or that must be electrically isolated (such as mains devices), the transistor switches a relay, whose contacts then switch the load. This is why the output sub-system is rarely just the device: it is the device plus the transistor (and sometimes a relay) that lets the low-power decision drive real power.

The LED series resistor

An LED conducts at a roughly fixed forward voltage (often around 2 V) and then its resistance is very low, so it must never be connected straight across a supply: it would draw a huge current and burn out. A series resistor limits the current to the LED's rated value.

The logic is Ohm's law applied to the resistor: the resistor carries the LED current and must drop whatever supply voltage is not dropped across the LED. Choose the resistor so that the LED runs at its rated current and no more.

Examples in context

A status panel uses LEDs as indicators, each with its own series resistor sized for the supply, so green, red and amber lamps all run at a safe current. A door-entry system uses a solenoid to throw the lock bolt, switched by a transistor from the control logic with a diode across the solenoid for the back-emf. A robot uses motors for its wheels, each driven by a transistor (often in an H-bridge for direction), interfaced from the controller. In every case the output device is matched to the job, and the interface circuit lets the small control signal drive it safely.

Try this

Q1. State the energy conversion in a buzzer. [1 mark]

• Cue. Electrical energy to sound.

Q2. An LED (forward voltage 2 V) runs at 20 mA from a 6 V supply. Find the series resistor. [2 marks]

• Cue. $R = \frac{6 - 2}{0.020} = \frac{4}{0.020} = 200\ \Omega$.

Q3. State why a motor is interfaced to a logic output through a transistor rather than connected directly. [1 mark]

• Cue. The logic output supplies too little current for the motor (and the motor's back-emf would damage it); the transistor switches the larger current safely.