Electronics is the study of how to control the flow of electrons. It deals with circuits made up of components that control the flow of electricity. Electronics is a part of physics and electrical engineering.

Electrical components like transistors and relays can act as switches. This lets us use electrical circuits to process information and transmit information across long distances. Circuits can also take a weak signal (like a whisper) and amplify it (make it louder).

Most electronic systems fall into two categories:

• Processing and distribution of information. These are called communications systems.
• Conversion and distribution of energy. These are called control systems.

One way of looking at an electronic system is to separate it into three parts:

1. Inputs – Electrical or mechanical sensors, which take signals from the physical world (in the form of temperature, pressure, etc.) and convert them into electric current and voltage signals.
2. Signal processing circuits – These consist of electronic components connected together to manipulate, interpret and transform the information contained in the signals.
3. Outputs – Actuators or other devices that transform current and voltage signals back into human readable information.

A television set, for example, has as its input a broadcast signal received from an antenna, or for cable television, a cable.

Signal processing circuits inside the television set use the brightness, colour, and sound information contained in the received signal to control the television set’s output devices. The display output device may be a cathode ray tube (CRT) or a plasma or liquid crystal display screen. The audio output device might be a magnetically driven audio speaker. The display output devices convert the signal processing circuits’ brightness and colour information into the visible image displayed on a screen. The audio output device converts the processed sound information into sounds that can be heard by listeners.

Analysis of a circuit/network involves knowing the input and the signal processing circuit, and finding out the output. Knowing the input and output and finding out or designing the signal processing part is called synthesis.

Now let’s start with the sensors..

1 LDR sensor

What is an LDR (Light Dependent Resistor)?

An LDR is a component that has a (variable) resistance that changes with the light intensity that falls upon it. This allows them to be used in light sensing circuits.

A typical LDR

Variation in resistance with changing light intensity

The most common type of LDR has a resistance that falls with an increase in the light intensity falling upon the device (as shown in the image above). The resistance of an LDR may typically have the following resistances:

You can therefore see that there is a large variation between these figures. If you plotted this variation on a graph you would get something similar to that shown by the graph shown above.

Applications of LDRs

There are many applications for Light Dependent Resistors. These include:

Lighting switch

The most obvious application for an LDR is to automatically turn on a light at a certain light level. An example of this could be a street light or a garden light.

Camera shutter control

LDRs can be used to control the shutter speed on a camera. The LDR would be used to measure the light intensity which then adjusts the camera shutter speed to the appropriate level.

Example – LDR controlled Transistor circuit

The circuit shown above shows a simple way of constructing a circuit that turns on when it goes dark. In this circuit the LDR and the other Resistor form a simple ‘Potential Divider’ circuit, where the centre point of the Potential Divider is fed to the Base of the NPN Transistor. When the light level decreases, the resistance of the LDR increases. As this resistance increases in relation to the other Resistor, which has a fixed resistance, it causes the voltage dropped across the LDR to also increase. When this voltage is large enough (0.7V for a typical NPN Transistor), it will cause the Transistor to turn on.

Projects1

LDR Circuit to turn on LED in dark

Component used
9V battery
LDR sensor
50K, 1K Ohm resistor
BC547 Transistor

So when sufficient light falling on LDR the resistance of LDR in very low, as a result all current is flowing from resistor R2 AND LDR so LED D1 is not glowing,
When there is dark and no light is falling on LDR, so the LDR resistance became very high so current will flow ro the base of transistor Q1 BC547, so transistor became turn on, and LED D1 glow.

LDR Circuit with adjustable sensitivity using potentiometer

Component used
9V battery
LDR sensor
10K Ohm potentiometer
1K Ohm resistor
BC547 Transistor

In this circuit we can adjust the sensitivity of darkness sensor by using potentiomter,

LDR Circuit to glow 230V AC load

Component used
9V battery
5V DC relay
230V AC Lamp
50K, 1K Ohm resistor
1N4007 Diode
BC547 Transistor

50K, 1K Ohm resistor
BC547 Transistor

This Circuit is to operate 230V AC bulb or light on LDR circuit.
This will use in our house, garage, factory, building, street light to turn on lights in dark.

Here also the working principle is same just led is replaced by a DC relay to operate high voltage load,
When light is not falling on LDR the relay will operate and the contact of relay changeover, in this way 230V lamp get connected with 230V supply and start glowing.
A flyback diode 1N4007 is used to save circuit from back EMF generated by relay while de-energizing.