Simple Strip LED Lamp

Strip LEDs are available in different colours powered by direct current (DC) source. These LEDs  are available as surface mount devices with current limiting resistors. Usually there are 300 LEDs in a 5-metre strip. The strip can be cut into pieces so that  the bits having three or four LEDs can  be used with 12V DC source. The circuit given here uses the strip LEDs to  make an automatic white LED lighting  source.

Simple Strip LED Lamp Circuit diagram:

The circuit is powered by a capacitor power supply connected to AC mains. Capacitor C1 drops the 230V  AC, which is further rectified by the bridge rectifier module and is made ripple-free by C2. Zener diode (ZD1) provides 12V DC to the comparator circuit. Resistor R1 is important in the  power supply as it provides discharge path to the voltage stored in capacitor C1 after the circuit is unplugged from  mains.

The automatic working of the circuit is based on the light-sensing property of the light-dependent resistor (LDR). Operational amplifier CA3140 (IC1) is used as a comparator with two potential dividers in its inverting and non-inverting inputs. LDR1 and  resistor R3 form one potential divider  that provides a variable voltage at the  inverting input pin 2 of IC1. Second  potential divider comprises resistors  R4 and R5, which provide half of the  supply voltage (6V) to the non-inverting pin 3 of IC1. The output of IC1 depends on voltage level at inverting  input pin 2 of IC1 as explained below.

In daylight, LDR1 has low resistance and the voltage at inverting input (pin 2) of IC1 is more than that of non-inverting input (pin 3). This makes IC1 output low, which drives transistor T1 into cut-off condition and strip LEDs do not glow. However, at night the light incident on LDR1 is low and its resistance is high. The voltage at inverting input of the comparator decreases, making it lower than the voltage at non-inverting input. This makes IC1 output high. Transistor T1 goes into saturation,  thus connecting cathodes of LEDs to  ground. All the LEDs in the strip turn  on and remain that way till morning.

Assemble the circuit on a general-purpose PCB and enclose it in a suit-able shock-proof case. Strip LEDs are available in ribbon-shaped form. Use 5cm bits (two bits) having three  LEDs each. The strip can be cut at supply-contact points. Strip LEDs are arranged on a flexible belt with  double-sided adhesive on the back  side, so it can be glued to any surface.  Connect the LED strip in the circuit  with correct polarity.

EFY note. Since the circuit uses 230V AC, there is a risk of electrical shock. Do not touch or troubleshoot when the circuit is plugged in.Before connecting the circuit to the power supply section, test it using 12V DC from a battery or DC power supply.

Source:  http://www.ecircuitslab.com/2012/05/simple-strip-led-lamp.html

Read More

White LED Lamp

Nowadays you can buy white LEDs, which emit quite a bit of light. They are so bright that you shouldn’t look directly at them. They are still expensive, but that is bound to change. You can make a very good solid-state pocket torch using a few of these white LEDs. The simplest approach is naturally to use a separate series resistor for each LED, which has an operating voltage of around 3.5 V at 20 mA. Depending on the value of the supply voltage, quite a bit of power will be lost in the resistors. The converter shown here generates a voltage that is high enough to allow ten LEDs to be connected in series. In addition, this converter supplies a constant current instead of a constant voltage.


A resistor in series with the LEDs produces a voltage drop that depends on the current through the LEDs. This voltage is compared inside the IC to a 1.25-V reference value, and the current is held constant at 18.4 mA (1.25 V ÷ 68 Ω). The IC used here is one of a series of National Semiconductor ‘simple switchers’. The value of the inductor is not critical; it can vary by plus or minus 50 percent. The black Newport coil, 220 µH at 3.5 A (1422435), is a good choice. Almost any type of Schottky diode can also be used, as long as it can handle at least 1A at 50V. The zener diodes are not actually necessary, but they are added to protect the IC. If the LED chain is opened during experiments, the voltage can rise to a value that the IC will not appreciate.

Resistors:
R1 = 1kΩ2
R2 = 68Ω
Capacitors:
C1 = 100µF 16V radial
C2 = 680nF
C3 = 100µF 63V radial
Inductors:
L1 = 200µH 1A
Semiconductors:
D1 = Schottky diode type PBYR745 or equivalent
D2-D5 = zener diode 10V, 0.4W
D6-D15 = white LED
IC1 = LM2585T-ADJ (National Semiconductor)

Read More