The Led torch with adjustable brightness circuit uses LM3914 as the basis of a 10 step variable brightness current-regulated white LED . This circuit has only four components in the control and regulation circuit: R1,R2, VR1 and LM3914.
All mains powered LED circuits work with 240 volt AC current.
Mains powered LED – Single LED
The power diode in the first diagram is designed to discharge the 220n during one half of the cycle so that the capacitor will charge during the other half-cycle and deliver energy to the LED. The 1k resistor limits the peak in-rush current when the circuit is first turned on and the main happens to be at a peak. A single LED can be illuminated by using a 100n or 220n capacitor with a rating of 400v.
- Two LEDs can be driven from the same circuit as one LED will be illuminated during the first half cycle and the other LED will be driven during the second half of the cycle.
Mains Powered LED – Multiple LED’s
Mains light with 10 LED
LED Flasher circuit using NE555 timer IC
- R1, R2, C1 and the supply voltage determine the flash rate. For a variable flash rate, replace R1 with a 1 MΩ pot in series with a 22k resistor.
- The purpose of R3 and R4 is to limit current through the LEDs to the maximum they can handle (usually 20 milliamps). 470 ohms works well with a supply voltage of 9-12 volts.
- The duty cycle of the circuit (the percentage of the time LED 1 is on to the time it is off during each cycle) is deterimed by the ratio of R1 to R2. If the value of R1 is low in relationship to R2, the duty cycle will be near 50 percent.
- The NE555 timer chip can be damaged by reverse polarity voltage being applied to it. You can make the circuit fool proof by placing a diode in series with the supply leads.
USB reading lamp is powered using USB port. The USB port provide 5 v and 100mA which is sufficient for this circuit. Cut the one end of the USB cable and use RED(pin1) and Black(pin4) for 5v positive and negative respectively.
- c1,c2 – 100 mF 25 v
- Zener diode – 4.7v 400m
- R1- 220 r
- R2- 100 r
- T1 – SL100
- White LED – 5 nos.
- USB Cable
10 Stage LED Sequencer
- IC1- CD4017
- IC2- NE555
- C1 – 1μ
- C2- 0.01 μ
- R1 – 470 Ω
- R2 – 100 KΩ
- R3- 100 Ω
- LED1-10 – RED LED
- 9volt DC power supply.
For power supply you can use 9 volt battery or can design separate power supply using step down transformer and 1N4007 diodes.
Today LED has become an integral part of consumer electronics.
LED TV, LED Display, LED Lights and so on. These are becoming very popular because of there low power consumption.
What is LED?
LED stands for Light emitting diode.
A light emitting diode is essentially a PN junction semiconductor diode that emits a monochromatic(single) colour light when operated in a forward biased direction.
For detail in technical evolution refer the following url
Early LEDs were only bright enough to be used as indicators, or in the displays of early calculators and digital watches. More recently they have been starting to appear in higher brightness applications.
LED Basics – Characteristics voltage drop
When a LED is connected around the correct way in a circuit it develops a voltage across
it called the CHARACTERISTIC VOLTAGE DROP. A LED must be supplied with a voltage that is higher than its “CHARACTERISTIC VOLTAGE” via a resistor – called a VOLTAGE DROPPING RESISTOR or CURRENT LIMITING RESISTOR
How LED works?
LED and resistor are placed in series and connected to a voltage.As the voltage rises from 0v, nothing happens until the voltage reaches about 1.7v. At this voltage a red LED just starts to glow. As the voltage increases, the voltage across the LED remains at 1.7v but the current through the LED increases and it gets brighter. As the current increases to 5mA, 10mA, 15mA, 20mA the brightness will increase and at 25mA, it will be a maximum.
This is just a simple example as each LED has a different CHARACTERISTIC VOLTAGE DROP and a different maximum current.
In the diagram below we see a LED on a 3v supply, 9v supply and 12v supply. The current-limiting resistors are different and the first circuit takes 6mA, the second takes 15mA and the third takes 31mA. But the voltage across the red LED is the same in all cases.
LED Basics – Head Voltage
As the supply-voltage increases, the voltage across the LED will be constant at 1.7v (for a red LED) and the excess voltage will be dropped across the resistor. The supply can be any voltage from 2v to 12 or more. The resistor will drop 0.3v to 10.3v. This is called HEAD VOLTAGE.
The voltage dropped across this resistor, combined with the current, constitutes wasted energy and should be kept to a minimum.
Most supplies are derived from batteries and the voltage will drop as the cells are used.
Here is an example of a problem:
Supply voltage: 12v
7 red LEDs in series = 11.9v
Dropper resistor = 0.1v
As soon as the supply drops to 11.8v, no LEDs will be illuminated.
Supply voltage 12v
5 green LEDs in series @ 2.1v = 10.5v
Dropper resistor = 1.5v
The battery voltage can drop to 10.5v
Suppose the current @ 12v = 25mA.
As the voltage drops, the current will drop.
At 11.5v, the current will be 17mA
At 11v, the current will be 9mA
At 10.5v, the current will be zero
Many batteries drop 1v and still have over 80% of their energy remaining. That’s why you should design your circuit to have a large HEAD VOLTAGE.
Some Basic circuits using LED
1. Polarity Tester
2. Continuity Tester
3. USB Reading Lamp