Light From Flat Batteries

 

Button or coin cells that appear to be flat in their normal function may yet be discharged further. This is because in many cases, for instance, a quartz watch stops to function correctly when the battery voltage drops to 1.2 V, although it can be discharged to 0.8 V. Normally, however, not much can be done with a single cell. In the present circuit, a super-bright LED is made to work from voltages between 1 V and 1.2 V. This may be used for map-reading lights, a keyhole light, or warning light when jogging in the dark. When a yellow, superbright LED is used with a fresh battery, it may be used as an emergency reading light or to read a front door nameplate in the dark or to find an non-illuminated doorbell.
Circuit diagram:

Light From Flat Batteries Circuit Diagram

Normally, LEDs light at voltages under 1.5 V (red) or 1.6–2.2 V (other colours) only dimly or not at all. The present circuit uses a multivibrator of discrete design that oscillates at about 14 kHz. The collector resistor of one of the transistors has been replaced by a fixed inductor, which is shunted by the LED. Because of the self-inductance, the voltage across the LED is raised, so that the diode lights dimly at voltages as low as 0.6 V and becomes bright at voltages from about 0.8 V up. The circuit requires a supply voltage of 0.6–3 V and draws a current of about 18 mA at 1 V.

High-Intensity, Energy-Efficient LED Light

 

Here is a rechargeable LED lamp that gives you bright light for a long duration of time as it consumes little power. The circuit presented here is compact, automatic, reliable, low-cost and easy to assemble. The circuit comprises power supply, battery charging and switching sections. The power supply section takes power from 230V AC mains supply without using a transformer. Capacitor C1 is used as an AC voltage dropper a well-known transformer-less solution. This helps to make the circuit compact without generating heat, as capacitor C1 dissipates negligible power. Capacitor C1 also protects against fluctuations in mains.

Current required for the battery charging circuit is provided by capacitor C1. Capacitor C1 discharges through resistor R1 when the circuit is disconnected from the mains voltage. This helps to prevent a fatal shock due to any voltage remaining in the input terminals. Capacitor C1 must be rated at least 440V AC, with mains application class X2. The AC mains voltage after capacitor C1 is given to bridge rectifier diodes D1 through D4 to convert alternating current into direct current and filtered by capacitor C2. The voltage from point B+ is given to positive terminal of the battery (BATT), anodes of LEDs (LED2 through LED21) and transistor base-bias resistor R3 through slide switch S1.

The circuit is operated in three modes (AC/charge, off and batt) by using three-position switch S1. When switch S1 is in middle position, the circuit is off. When S1 is towards right, white LEDs glow by drawing power from 4V battery. When S1 is towards left, the circuit connects to AC mains and battery starts charging. The presence of AC mains voltage and battery charging is indicated by LED1. White LEDs remain off if AC mains supply is available and glow in the absence of AC mains. When switch S1 is towards left position and AC mains is available, the battery charges through diode D6 and the white LEDs don’t glow. The negative DC path through diode D5 makes the transistor cut-off, preventing the battery current from LEDs to the negative terminal through the transistor.

Thus the white LEDs don’t glow. On the other hand, if AC mains is not available, charging stops and the base of transistor SS8050 gets positive voltage from the battery through slide switch S1 and resistor R3. The transistor conducts and the current flows from the battery’s positive terminal to the negative terminal of the battery through the LEDs (LED2 through LED21), collector to emitter of transistor T1 and switch S1. Thus the white LEDs glow. When the switch is in ‘batt’ position, the white LEDs (LED2 through LED21) get the supply directly from 4V battery through switch S1 and therefore all the white LEDs glow. Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Fix the mains power cord on the back of the cabinet and slide switch and LEDs on the front side.

Fine Control Super Bright LED Pulser

 

This circuit, designed on request for a Halloween prop, allows fine control of a pulsing Super Bright white LED. The four potentiometers or trimmers will set precisely: on, off, ramp up and ramp down time-delays respectively. Ramp up and ramp down time-delays can be set roughly in the 1 - 15 seconds range, whereas on and off time-delays can range from a few seconds to about one minute. A 12V battery or regulated power supply is required, provided it is reasonably stable. Total current drawing is about 25 - 30mA when the LED reaches maximum brightness.
Parts:
R1,R5,R12,R13___10K 1/4W Resistors
R2,R5___________10K 1/2W Trimmers or Lin. Potentiometers
R3______________47K 1/4W Resistor
R4______________22K 1/4W Resistor
R6_______________1K 1/4W Resistor
R7,R8,R9,R14___100K 1/4W Resistors
R10,R11__________2M2 1/2W Trimmers or Lin. Potentiometers
R15____________220R 1/4W Resistor
C1,C2__________100nF 63V Polyester or ceramic Capacitors
C3,C4___________22µF 25V Electrolytic Capacitors
C5_____________220µF 25V Electrolytic Capacitor
D1,D2________1N4148 75V 150mA Diodes
D3______________LED Super Bright white (e.g. RL5-UV2030)
Q1____________BC337 45V 800mA NPN Transistor
IC1___________LM324 Low Power Quad Op-amp IC
IC2____________4093 Quad 2 input Schmitt NAND Gate IC
Notes:

• Wanting to use two white LEDs, the second device must be wired across the Emitter of the transistor and negative ground with its own limiting resistor wired in series, like R15 and D3 in the circuit diagram.
• If common red, yellow or green LEDs are required, please wire two of them in series, in order to present roughly the same voltage drop of one white or blue LED.
• Please note that the unused sections in both ICs must have their inputs tied to negative ground whereas the outputs must be left open, as shown at the bottom of the diagram.
• All time-delays can be increased by changing the value of C3 and C4 to 47µF 25V or even higher. Please vary the value of these capacitors only, as the values of the resistors wired to the four control pots are rather critical and should not be changed.

Single FET Controls 2nd LED Array in Switch-Mode LED Backlight

 

White-LED backlights are gaining acceptance because they offer higher reliability and simpler drive circuitry than those based on CCFL and EL technology. As a result, the white-LED backlight is increasingly common in PDAs, cell-phones, digital cameras, and other portable devices.
A design in which the display is backlit (or frontlit) for extended periods needs an efficient circuit that drives the LEDs with a controlled current, and eliminates the wasted power associated with current-limiting resistors. A switch-mode boost design that regulates current instead of voltage accomplishes this purpose (Figure 1).

Figure 1. When this circuit turns off the backlight LEDs, the keypad LEDs remain on with no change in intensity.

Because all LEDs are connected in series, they all receive the same current without need for ballasting resistors. Identical currents help achieve uniform intensity. And because the output current is small (20mA in this case), the output filter capacitance (C2) can be smaller than for a load consisting of parallel-connected LEDs. The Figure 1 circuit's conversion efficiency (90%) provides a distinct power-saving advantage over resistor-limited and linearly regulated designs.
It might appear that a series-LED connection is not suitable for applications in which some (but not all) LEDs must be turned off. That capability is sometimes needed in a cell phone for which the display is off but the keypad remains lit, or in a handheld PDA that needs to play a sound file while maintaining illumination in the buttons but not the display. Actually, switching off individual LEDs or groups of LEDs is not a problem, even when all are driven in series.
Applying a logic-high level to the gate of a simple MOSFET switch (Q2) turns off a subset of LEDs (backlight LEDs in this case) by shunting their current. The remaining (keypad) LEDs remain on, and their intensity remains constant because their current is regulated by IC1, which senses the voltage across R2 (300mV at full brightness). When turning the LEDs on and off, an RC network at the gate of Q1 (R2,C4) slows the load changes sufficiently to prevent transient changes in the LED drive current. Other features include adjustable intensity via the ADJ pin, and full shutdown via the SHDN-bar logic input.
A similar version of this article appeared in the April 12, 2001 issue of EDN magazine.

White LED Array Lamp

C. V. Niras/VU3CNS

When I decided to make a battery operated LED lamp, I thought it may better to use a cluster of low power discrete LED array rather than a single high power LED. The light produced will be scattered and It considerably reduces irritation or damages to a naked eye, if accidentally looked on it. Also the light spreads nicely so it is good for room lighting and no heat sink required for the LEDs!

The circuit is designed to operate 3W, 5 x 7 white LED array powered from four NiMH cells. The design is straight forward DC to DC boost circuit using a LM3410 which includes a 170 mΩ NMOS switch. The circuit is completes with few external components. The switching frequency is internally set to either 525 kHz for LM3410-Y devices or 1.60 MHz for LM3410-X, allowing the use of extremely small inductors and capacitors. The 525 kHz switching frequency is selected due to the availability of suitable ferrite having lower core losses. The inductor value needed for the circuit depends upon the voltage and current output needed. A detailed calculation is described in the datasheet of the LM3410. A higher efficiency of approximately 88% is achieved. The IC is featured with an external shutdown and the standby current of only 80 nA.

The operating current for a single branch the LED array is ~25mA, and since there are 7 parallel paths a total output current of 190mA is drawn from the DC to DC convertors. This current is set by the 1O resistor R4 (I = 0.19/R4). An optional output over voltage protection is provided using a 22V zener diode D23 and a resistor R3. This will protect the IC if the LED load becomes an open circuit.

Figure 1: circuit diagram of the LED drive circuit.

Since the absolute maximum input voltage to the IC is 5.5 V, a voltage regulator using R1 and zener diode D31 is required if the circuit is operated from a voltage source more than 5.5 V. The DIM input pin of LM3410 can be used for either on/off or brightness control of LED array. A PWM dimming signal whose duty cycle from 0 to 100% with a frequency of 200 ~ 1 kHz is best suited for this brightness control, although a PWM with maximum of 25 KHz can be used. In this application DIM pin is tied to V_IN for maximum brightness.