Automatic Night Light using LDR

Here is the simple Automatic Night Light circuit using LDR.

Components Required:

1) Breadboard

2) 470 ohm resistor
3) 22 Kilo ohm resistor

4) LED (Any Color)
5) 15 Kilo ohm Light Dependent Resistor (LDR)
6) 3904 NPN Transistor

6) 9Volt Battery 

Procedure:

• Connect the circuit as shown in the circuit diagram below.
• During day time the brightness will be more, which lowers the resistance of the LDR.
• Therefore the current is grounded, as current prefers only low resistance path.
• Hence there is no base current to forward bias the 3904 NPN transistor and the LED remains OFF.
• During night time the brightness goes down, which increases the resistance of the LDR.
• Therefore the current will not be grounded and prefers a alternate path to flow.
• Hence there is enough base current to forward bias the 3904 NPN Transistor and the LED glows.

Circuit Diagram:

Figure 1: Automatic Night Light using LDR Circuit simulation made in Multisim

Figure 2: Automatic Night Light using LDR

Simple Remote Control

Here is the simple example of a Television remote control working used in our day to day life.

Components Required:

1) Breadboard

2) 470 ohm resistor

3) LED (Any Color)
4) Phototransistor

5) 9Volt Battery 

Procedure:

• Connect the circuit as shown in the circuit diagram below.
• The Phototransistor remains OFF until a IR light is incident on it.
• Hence the LED connected in series to it also remains OFF.
• Now take your television remote which consists of a IR LED.
• Pointing towards the phototransistor press any button on the remote.
• When the button is pressed the remote emits IR and forward biases the phototransistor and lights the LED.
• Based on the input from the remote you can visually see a rhythmic pattern in the LED.
• Now check with some other button inputs, LED glows in various patterns and with some small variations in timings.

Circuit Diagram:

Figure 1: Simple Remote Control Circuit simulation made in Multisim

Explanation:

They are digital pulses sent from the remote to the receiver in the television. Based on those digital pulses received in the receiver end the corresponding changes in channels or volumes happens. For every ON pulse produced by the remote the LED glows and for every OFF pulse the LED dims. 

Pulses produced by the remote are based on timings. When the LED blinking frame rate is greater than 150FPS (Frames per second) our normal human eye cannot detect its blinking and recognizes it as a ON pulse even though it blinks. When it ranges in the normal frame rate our eye detects its blinking. In some cases when the frame rate is nearer to 150FPS we may only see the dimming & brightening of the LED. 

Phototransistor

All transistors are light sensitive. Phototransistors are similar to ordinary transistors but doesn't have a base terminal. The Light input on the phototransistor, forward and reverse biases the transistor and hence switches ON or OFF the phototransistor. They are developed to react to specific wavelength of light. They consist of a opening at the top covered with glass through which the input light is received.

 

Figure 1: Phototransistor Symbol

 

Figure 2: Phototransistor

Transistors

Transistor is a semiconductor device used to amplify and switch electronic signals and electrical power. It is composed of semiconductor material with at least three terminals. Transistors are the building blocks of the digital revolution.

They consist of three terminals namely Emitter(E), Base(B) and Collector(C). An arrow in the transistor is commonly used to represent the direction of the current towards the ground and is always on the emitter leg.

The two basic types of transistors are

• NPN
• PNP

 

Figure 1: NPN and PNP Transistor Symbols

Arrow on the emitter leg facing outwards represents NPN transistor and facing inwards represents PNP transistor.

NPN Transistor:

NPN Transistor acts like a normally open push button but has no moving parts.Only when the base of the transistor is provided with the necessary voltage and current it turns ON the transistor else it remains in OFF state. The base acts like a control valve to control the flow. It behaves as a electronic switch.

PNP Transistor:

PNP Transistor acts like a normally closed push button. Only when the base of the transistor is provided with the necessary voltage and current it turns OFF the transistor else it remains in ON state. Its just the opposite of the NPN transistor

Push Buttons

Push button is a piece of metal contact with two metal pieces. They provide a temporary path, so that charge can flow. They have a inbuilt spring which makes it return to the normal position when released.

There are Two types of Push buttons basically:

• Push Button Normally Open (PBNO)

• Push Button Normally Closed (PBNC)

 

Figure 1: Basic Push Button

Capacitor Discharging pattern using LED

Components Required:

1) Breadboard

2) 470 ohm resistor

3) 1000uf Capacitor

4) LED (Any Color)

5) 9Volt Battery 

6) Push Button Normally Open (PBNO)

7) Digital Multimeter 

Procedure:

• Connect the circuit as shown in the circuit diagram below.

• The PBNO switch is normally open and the circuit is not closed therefore the capacitor will not be charged and the LED will not glow.

• Once the push button is pressed the charge flows from the battery and charges the capacitors and glows the LED.

• Even after the push button is released the LED glows for a period of 470 ms based on the RC time constant calculated for 1000uf capacitor and 470 ohm resistor. 

• Resistance value multiplied by the capacitance value gives the RC time constant.

• This is because when the push button is pressed the capacitor is charged and even after when the push button is released the charge stored in the capacitor closes the circuit and gets discharged through the closed circuit.

• Charge stored in the capacitor discharges exponentially which can be visually noticed by the brightness of the LED which becomes dimmer when discharging.

• To observe discharging pattern clearly use large resistance values say 10Kilo ohm for the same capacitance value which results in larger RC time constant for nearly 10 sec.

Circuit Diagram:

Figure 1: Capacitor Discharging pattern using LED Circuit simulation made in Multisim

Figure 2:  Capacitor Discharging pattern using LED (Push button open)

Figure 3:  Capacitor Discharging pattern using LED (Push button closed)

Capacitors

A capacitor (originally known as a condenser) is a passive two-terminal electrical component used to store energy electrostatically in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors (plates) separated by a dielectric (i.e., insulator). The conductors can be thin films of metal, aluminum foil or disks, etc. The 'nonconducting' dielectric acts to increase the capacitor's charge capacity. A dielectric can be glass, ceramic, plastic film, air, paper, mica, etc. Capacitors are widely used as parts of electrical circuits in many common electrical devices. Unlike a resistor, a capacitor does not dissipate energy. Instead, a capacitor stores energy in the form of an electrostatic field between its plates.

The dielectric can be any non-conductive substance. However, for practical applications, specific materials are used that best suit the capacitor's function. Mica, ceramic, cellulose, porcelain, Mylar, Teflon and even air are some of the non-conductive materials used. The dielectric dictates what kind of capacitor it is and for what it is best suited. Depending on the size and type of dielectric, some capacitors are better for high frequency uses, while some are better for high voltage applications. Here are some of the various types of capacitors and how they are used.

• Air - Often used in radio tuning circuits
• Mylar - Most commonly used for timer circuits like clocks, alarms and counters
• Glass - Good for high voltage applications
• Ceramic - Used for high frequency purposes like antennas, X-ray and MRI machines
• Super capacitor - Powers electric and hybrid cars

  

Figure 1: Basic Capacitors

Capacitor Value Calculator: http://www.muzique.com/schem/caps.htm

LED Brightness Control using Light Dependent Resistors (LDR)

Components Required:

1) Breadboard

2) 1N4007 Diode

3) 470 ohm resistor

4) 15 Kilo ohm Light Dependent Resistor (LDR)

5) LED (Any Color)

6) 9Volt Battery

7) Digital Multimeter 

Procedure:

• Connect the 1N4007 diode and 470Ω resistor to one terminal of 15 Kilo ohm Light Dependent Resistor (LDR) in series. Connect the other terminal of LDR to the LED in series in a breadboard. 

• Connect the 9V battery's positive end to the anode of 1N4007 diode and negative end to the cathode of the LED as shown in the circuit diagram below.

• Measure the voltage across various points by altering the Light input on the LDR using a digital multimeter and note the change in brightness of the LED accordingly.

• Observe the change in LED brightness in a dark room and in a bright light. Here I have used a cap to cover the surface of the LDR to observe the change in LED brightness.

• Also observe the change in resistance value of the LDR using a digital multimeter which is the cause for the alteration in LED brightness.

• The resistance of LDR is low in bright light and high in darkness.

Circuit Diagram:

Figure 1: LED Brightness Control using Light Dependent Resistor (LDR) (LDR in Room Light)

 

Figure 2: LED Brightness Control using Light Dependent Resistor (LDR) (LDR covered with a cap)

LED Brightness Control using Potentiometer

Components Required:

1) Breadboard

2) 1N4007 Diode

3) 470 ohm resistor

4) 100 Kilo ohm Potentiometer

5) LED (Any Color)

6) 9Volt Battery

7) Digital Multimeter

Procedure:

• Connect the 1N4007 diode and 470Ω resistor to one fixed terminal of 100 Kilo ohm Potentiometer in series. Connect the variable terminal of potentiometer to the LED in series in a breadboard. 

• Connect the 9V battery's positive end to the anode of 1N4007 diode and negative end to the cathode of the LED as shown in the circuit diagram below.

• Measure the voltage across various points by altering the potentiometer value using a digital multimeter and note the change in brightness of the LED accordingly.

Circuit Diagram:

   

Figure 1: LED Brightness Control using Potentiometer Circuit simulation made in Multisim

 

Figure 2: LED Brightness Control using Potentiometer (100% Brighrness)

Figure 3: LED Brightness Control using Potentiometer (50% Brighrness)