# Action and use of circuit components

Transducers

Transducers are devices that convert one type of energy into another.

Potential Divider Potential divider is simply a variable resistors used as potential divider in which all three terminals are connected. This arrangement is normally used to change voltage, for example to change the brightness of the lamp, or control the volume (loudness) in a hi-fi amplifier circuit. A potential (or voltage) divider is made up of two resistors. The output voltage from a potential divider will be a proportion of the input voltage and is determined by the resistor values. In the arrangement of the circuit in the diagram the value of Vout can be determine by the formula:

Vout = (R2/R1+R2). Vin

The potential difference across the apparatus is directly proportional to the resistance across it.

Thermistor

Thermistors are semiconductors, they could conduct electricity but resistance decreases as temperature increases. Thermistors are often used as current limiting devices that help prevent overheating. Once the current gets to a certain limit (increasing heat lowers resistance, leads to increasing current), the circuit is broken to prevent overheating.  A thermistor is a heat sensor (resistor) which changes its resistance with the change of temperature (heat) around it. Its resistance decreases as the temperature increases which is reverse to the normal conductor. For example:  Icy water 0°C has high resistance, about 12kΩ.  Room temperature 25°C has medium resistance, about 5kΩ.  Boiling water 100°C has low resistance, about 400Ω. Thermistor is called input transducer. It means it changes resistance with the change in environment

1. What is a thermistor?
A thermistor is a thermal-dependent resistor.  It is derived from thermal-resistor.
2. What is relation between the resistance of a thermistor and temperature?
The relation is an inverse relation.  When the temperature is increased, the thermistor resistance decreases and when the temperature is decreased the resistance is increased.
3. What is the main application of a thermistor?
A thermistor is used in the design of control-circuits which require automatic switching that depends on temperature.
4. List applications that uses thermistors
1. Fire alarm
2. Air conditions
3. Refrigerators

Light-Dependent Resistor (LDR)

An LDR is a light sensor (resistor) which changes its resistance with the brightness of light around it. It is made from cadmium sulphide compound (CdS) and its resistance decreases as the brightness of light falling on the LDR increases.  Darkness: maximum resistance, about 106Ω. Very bright light: minimum resistance, about 100Ω. LDR is called input transducer. It means it changes resistance with the change in environment.

Light-dependent resistors exhibit photoconductivity; increasing light intensity promotes conductivity by lowering resistance. LDRs can be used on street lamps as input transducers – the changing light affects the internal resistance, which is used to control a circuit. In this case, when it gets dark, the street lamps turn on. LDR’s can be used in light sensitive switches, for example in street lights. When the light intensity is low the resistance changes affecting other circuitry which turns on the street lights

1. What is a LDR?
LDR is a light dependent-resistor.
2. What is relation between the resistance of an LDR and light intensity?
The relation is an inverse relation.  When light-intensity is increased, the LDR resistance decreases and when then light-intensity is decreased the resistance is increased.
3. What is the main application of an LDR?
An LDR is used in the design of control-circuits which require automatic switching that depends on light.
4. List applications that uses LDR.
1. Counting system
2. Control Street lights in cities
3. Elevators

The Relay

A relay is an electromagnetic switch – it activates when a current is passed through it. It works by having a coil inside. When a current is flowing it creates a magnetic field that pulls one switch contact towards or away from another. Relays can be used to turn on second circuits. The first circuit activates the relay which in turn turns on the second circuit.

Temperature operated alarms work when the change (increase) of temperature takes place and the resistance of the transistor in the circuit decreases, increasing the current in the circuit. The current in the circuit flows through the relay, activating the electromagnetic switch. The switch operates the alarm.

Capacitor  The capacitor is used to store electric charge (or that means electrical energy). A basic capacitor is made up of two conductors separated by an insulator, or dielectric material. The dielectric can be made of paper, plastic, mica, ceramic, glass, a vacuum or nearly any other nonconductive material. When the terminals of a capacitor are connected with the battery the capacitor becomes charged and stored energy. Similarly when the two terminals short circuited, the capacitor discharged. 