LED Torch with adjustable brightness

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.

LED torch with adjustable brightness
LED torch with adjustable brightness
Part List
1. IC – LM3914
2. LED1-10 – 10 white led
3. R1 – 620 ohm
4. R2 – 100 ohm
5. VR1 – 100 k pot
6. C 1 – 10 μf 16 v
7. Power – 3-4.5 volt
8. Suitable cabinet and a general purpose PCB.

How it works?

The LM3914 is set to operate in bar graph mode so that the LEDs light progressively as its input signal increases.This signal comes from VR1, which provides a variable voltage between 0V and 4.5 v(supply voltage) to pin 5 of the LM3914. The internal resistor ladder network of the LM3914 has its low end (pin 4) connected to ground and the high end (pin 6) connected to the supply voltage via R2. Resistor R1 (620Ω) on pin 7 of IC1 sets the current through each LED to about 20mA. Set value of R1 as per input voltage. As VR1 is rotated from the 0V position (all LEDs off) to the supply voltage position (all LEDs on), the LEDs will progressively light. When all LEDs are off, the circuit will draw about 5mA. Whereas when all LEDs are illuminated, it will draw about 205mA and dissipate 307mW with a 4.5V supply.
Reference voltage at pin5 is given by- Vout=1.25(1+R2/R1)
The device dissipation will depend entirely on the input voltage and LED forward voltage. A resistor (R3) is inserted in series with the positive supply, chosen so that the LM3914’s dissipation is limited to about 500mW.

Mains Powered LED Lights

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

When 50 to 80 white LEDs are connected in series, a resistor can be used. For 50 white LEDs, use a 4k7 2watt resistor to provide 10mA average current.For 100 white LEDs, use a 2k2 1watt resistor to provide 10mA average current.
mains-powered-Led -multiple-led
The above circuit can be modified with a bridge rectifier to provide DC input to LED. This will prevent from flickering or strobing.

Mains light with 10 LED

The following circuit illuminates a column of 10 white LEDs. The 10u and 100 R prevents flicker.
 *Caution: All the circuits involve high current. There is a significant risk of death through electrocution if mains voltage electricity is allowed to pass through the body. There can also be a risk of fire and explosion if electricity is not cabled and fused correctly. Therefore precautions must be taken when using mains electricity

LED Flasher using NE555

LED Flasher circuit using NE555 timer IC

LED flasher


  • 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

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.


USB lamp
  1. c1,c2 – 100 mF 25 v
  2. Zener diode – 4.7v 400m
  3. R1- 220 r
  4. R2- 100 r
  5. T1 – SL100
  6. White LED – 5 nos.
  7. USB Cable

10 Stage LED Sequencer

10 Stage LED Sequencer

LED Chaser



  • 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.

LED Basics

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.

Example 2:
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