USB POWERED PC LAMP

It is a simple circuit to see the keyboard of  PC or laptop at night without using external power supply.

The power for glowing the LED is taken from the USB socket.

—The main part of the circuit is LDR which can act as a switch(light-dependent) for the ON & OFF of LED.

This is a simple circuit that is used to see the keyboard of PC or laptop at night without disturbing other's sleep. These circuit does not take power externally. The power for glowing the LED is taken from the USB itself. It can work in darkness as well as the ambient light is poor during daytime. The power available from the USB socket of PC is 5V regulated DC.

            The main part of the circuit is light dependent resistor (LDR) which can act as a switch for the ON and OFF of the LED. The other components used for the circuit are, two transistors T1,T2 ,certain resistors and LEDs. In the working of the circuit, if T1 is on then T2  is off and vice versa. The conduction of the transistor T1 is indicated by a standby LED. the conduction of T2 produces the soft light to see the keyboard.

            LDR acts as a light dependent switch to turn on the lamp, when the ambient light in the room drops below the preset level. The lamp consist of six LEDs (LED2 - LED7). The base of the transistor T1 is connected to the voltage divider comprising LDR1 and preset VR1.

            When light in the room is sufficient, the resistance of LDR is low. Then the transistor T1 goes to saturation. When the transistor T1 conducts, transistor T2 goes to cut off. This shows the lamp is in off condition.

            When the ambient light in the room is poor or at night, the transistor T1 goes to cut off and T2 conducts. All the white LEDs are glow with sufficient brightness. All of the LEDs are connected to the power supply through series dropper resistor R2 through R7. These resistors are used to limit the current through white LEDs to a safe level.

            White LEDs are arranged in particular as each white LED requires a minimum of 2V. Preset VR2 is used to set the base voltage of transistor T2. Adjust preset VR2 until the white LEDs turn off in the preset light intensity level during the daytime.

           Assemble the circuit on general-purpose PCB and enclose in a suitable case. Power to the circuit is obtained from the USB socket using a USB cable for the purpose. Cut the ends of the USB cable to get the red and black wires for positive and negative supplies of the circuit. Cut the green and white wires of the USB cable and solder the red and black wires to the PCB. Use the USB plug at the end to draw power from the USB socket. Fix the unit near the keyboard so you can see the key buttons easily.

Block Diagram

BLOCK DIAGRAM  DESCRIPTION

Light Dependent Resistor [LDR] :

            LDR is the main component of the circuit. This resistor is sensitive towards light. When the light in the room is sufficient, the resistance of LDR is low and the light in the room is lesser than the predetermined level, the resistance of LDR is high.

Transistor T1 ON :

            During the heavy light, the resistance of the LDR is low. This results in a high voltage at the base of T1, driving it into saturation. When the transistor T1 conducts, transistor T2 is cut off. This disconnects the power supply to all white LEDs. LED1 glows as it is forward biased indicating the standby mode.

Transistor T2 ON :

            During the poor light, the resistance of the LDR is high. Then the transistor T1 is cut off and T2 conducts. All the white LEDs are glow with sufficient brightness.

LED OFF :

            When the light in the room is sufficient, it disconnects the power supply to all the white LEDs (LED2 through LED7).

LED ON :

            When it is dark, or the ambient light in the room is less than the predetermined level set by VR1, then T2 conducts. All the white LEDs glow with the sufficient brightness as these are connected to the power supply through series dropper resistor R2 through R7.

USB:

            Power to the circuit is obtained from the USB socket using a USB cable. For this purpose cut the ends of the USB cable to get red and black wires for positive and negative supplies of the circuit. The circuit is powered by regulated 5V DC available from the USB socket of the PC.

 Circuit Diagram

CIRCUIT DESCRIPTION

       This is a simple circuit. It consist of two transistors (T1 & T2), seven resistors (R1 to R7), seven LEDs (LED1 - LED7).

            The base of transistor T1 is connected to the voltage divider comprising LDR1 and preset VR1. The base of the transistor T2 is controlled by preset VR2. When transistor T1 conducts T2 is cut off and vice versa. All the white LEDs are with series dropper resistors R2 through R7 is connected to the emitter of transistor  T2. The end of the circuit is connected to the red and black wires of USB.

Working

    This is a simple circuit which is used to see the keyboard of PC or laptop at night without disturbing other's sleep. This circuit does not take power externally. The power for glowing the LED is taken from the USB itself. It can work in darkness as well as the ambient light is poor during daytime. The power available from the USB socket of PC is 5V regulated DC. The main part of the circuit is light dependent resistor (LDR) which can act as a switch for the ON and OFF of the LED. The other components used for the circuit are, two transistors T1 & T2, certain resistors and LEDs. In the working of the circuit, if T1 is on then T2 is off and vice versa. The conduction of the transistor T1 is indicated by a standby LED. The conduction of T2 produces the soft light to see the keyboard. LDR acts as a light dependent switch to turn on the lamp, when the ambient light in the room drops below the preset level. The lamp consist of six LEDs (LED2 - LED7). The base of transistor T1 is connected to the voltage divider comprising LDR1 and preset VR1.

      When light in the room is sufficient, the resistance of LDR is low. Then the transistor T1 goes to saturation. When the transistor T1 conducts, transistor T2 goes cut off. This shows the lamp is in off condition. When the ambient light in the room is poor or at night, the transistor T1 goes to cut off and T2 conducts. All the white LEDs are glow with sufficient brightness. All of the LEDs are connected to the power supply through series dropper resistor R2 through R7. These resistors are used to limit the current through white LEDs to a safe level. White LEDs are arranged in particular as each white LED requires a minimum of 2V. Preset VR2 is used to set the base voltage of transistor T2. Adjust preset VR2 until the white LEDs turn off in the preset light intensity level during the daytime. 

DETAILED COMPONENTS DESCRIPTION

Transistors [BC547]

    General Description

            The transistor used for the circuit are BC547. It has three region of working. They are active region, cut off region, saturation region. In this circuit we use two regions cut off region and saturation region. In cut off region transistor is in offline mode and in saturation region transistor is in online mode.

  Features

                        Voltage: BC547 ;  = 45V

                                           = 50V

                                           = 6V

                        Low Noise Figure

                        Complement to BC556/557/558/559/560

  Applications

            General Purpose Applications

            Switching Applications

LDR        

  General Description

            Two cadmium sulphide (CdS) photoconductive cells with spectral responses similar to that of the human eye. The cell resistance falls with increasing light intensity. Applications include smoke detection, automatic lighting control, batch counting and burglar alarm systems.                                 

  Features

            Minimum Operating Temperature = C

            Maximum Operating Temperature = C

            Maximum Power Dissipation = 100 mW

            Maximum Current = 75 mA

            Maximum Voltage AC/DC Peak = 320 V

            Typical Dark Resistance = 1 MΩ

            Typical Dark Capacitance = 3.5 pF

   The light dependent resistor is used as switch. This resistor is sensitive to light. When there is light then the resistance is low and there is dark then resistance is high.

Guide to source illuminations

Light source Illuminations	LUX
Moonlight	0.1
60W Bulb at 1m	50
1W MES Bulb at 0.1m	100
Fluorescent Lighting	500
Bright Sunlight	30,000

Table 4.1 Source Illuminations

  Applications

            Photoconductive cells are used in many different types of circuits and applications.           

           Analog Applications

                        Camera Exposure Control

                        Auto Slide Focus - dual cell

                        Photocopy Machines - density of toner

                        Colorimetric Test Equipment

                        Densitometer

                        Electronic Scales - dual cell

                        Automatic Gain Control - modulated light source

                        Automated Rear View Mirror

            Digital Applications

                        Automatic Headlight Dimmer

                        Night Light Control

                        Oil Burner Flame Out

                        Street Light Control

                        Absence / Presence (beam breaker)

                        Position Sensor

LEDs [Light Emitting Diodes] 

 General Description    

LEDs are used for emitting light. In the circuit we use two types of LEDs. They are green and white. Green LED is used for indicating the on condition of T1 and white LEDs are used to see the keyboard at night. Each white LED requires a minimum of 2V.

 Features

            Forward Voltage ( ) drop across LED

                        Diodes are current driven!

            Wavelength variations

                        Crystal and junction growth defects

            Brightness variations

                        Crystal defects resulting formation of phonons and non radiation energy transfer

            Temperature

                        Junction temperature of the device affects each of the parameters above 

 Applications

            Old days

                        Signal Indicators

                        Numeric and Alpha-numeric displays

            Nowadays

                        Automotive

                        Backlights

                        Flashlights for portable devices

                        Projector light sources

                        Signage

                        Torch Lights

                        Traffic Lights

Resistors

  General Description

            The resistors of 220Ω are used in this circuit. White LEDs are connected to the power supply through series dropper resistor R2 through R7. These resistors are used to limit the current through white LEDs to safe level.

  Applications

            In general, a resistor is used to create a known voltage-to-current ratio in an electric circuit. If the current in a circuit is known, then a resistor can be used to create a known potential difference proportional to that current. Conversely, if the potential difference between two points in a circuit is known, a resistor can be used to create a known current proportional to that difference.

Current-limiting. By placing a resistor in series with another component, such as a light-emitting diode, the current through that component is reduced to a known safe value.

An attenuator is a network of two or more resistors (a voltage divider) used to reduce the voltage of a signal.

All resistors dissipate heat. This is the principle behind electric heaters.

            A resistor has a maximum working voltage and current above which the resistance may change (drastically, in some cases) or the resistor may be physically damaged (overheat or burn up, for instance). Most resistors  are rated with a maximum power which is determined by the physical size. Common power ratings for carbon composition and metal-film resistors are 1/8 watt, 1/4 watt, and 1/2 watt. Metal-film and carbon film resistors are more stable than carbon resistors against temperature changes and age. Larger resistors are able to dissipate more heat because of their larger surface area. Wire-wound and resistors embedded in sand (ceramic) are used when a high power rating is required.

Presets

 General Description

                The variable resistor is a resistor whose value can be adjusted by turning a shaft or sliding a control. These are also called potentiometers or rheostats and allow the resistance of the device to be altered by hand. Variable resistors can be inexpensive single-turn types or multi-turn types with a helical element. Some variable resistors can be fitted with a mechanical display to count the turns. The presets are used for control the base of the transistors.