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Monday, April 29, 2019

Battery Brightness and a Blinking LED

Many many thanks for this quick reply again.

We are also experimenting with stobing the LEDs very fast using digital out of the MCU's I/O to kind of PWM the LED to calibrate it properly for our application .

I plan to use 3 x AA batteries. If I add a 4th battery, The solution has to be really cheap since there will probably be 4 constant current circuits -

The 74HC14 is cost effective, so it's a possible solution if the circuit works well. Yes - I need to be 100% sure that the LED brightness is the same for full battery to say 80% battery

Mail from AB

You could look at this TPS61043: Constant Current LED Driver

The TPS61043 is a high-frequency boost converter with constant current output that drives white LEDs or similar. The LED current is set with the external sense resistor (RS) and is directly regulated by the feedback pin (FB) that regulates the voltage across the sense resistor RS to 252 mV (typical).

The 74HC14 can be used to drive-light 4-6 LEDs. It can be directly connected to MCU.

The LEDs should be flashed at around 5 Khz. Then a constant current source is not required.

Why do you wish to use constant current. Is it for a steady light.

If the problem is dropping voltage. Another battery or voltage doubler is required

An voltage doubler may not be cost effective. Another battery may be a simpler solution.

By using a 74HC14 you can avoid the PNP transistors. This is an inverter, you can use a buffer chip too.

A series resistor with LED or 10Kz flash is to limit current.

The 74HC14 just replaces your PNP transistor saving 1/2 a Volt. The Microcontroller PWM can be used to control brightness. Then you can skip even the series resistor. You can even use a 555 chip for pwm.

Flashing must be above 1 KHz. The persistence of Vision is 20mS. Very high rate will make it dim, something optimum.

A better Idea is to use LM317 to get 3.3V regulated supply from 5-6V of batteries. Use this regulated supply to drive LEDs. This also needs to be tried practically. As voltage is constant the current in leds will be constant. for 20% drop.


Sunday, April 28, 2019

The Return of Bright LED

From my Thunderbird archives. it may be interesting to read, maybe you may learn a thing or two.

Many thanks for your quick reply - much appreciated. The FET solution is new to me, but I have tried most of the others.

I think the problems are:

1) As voltage for these LEDs drops from 4.65v, fairly quickly, the constant current starts to move.

2) The 'constant' I can acheive would be only 4% while the current is stable.

Do you think my problem can be solved? Many thanks again,

Mail from AB 2006

NSSBC : the status indicator

On the right we see the status indicator drivers. One 74HC14 gate buffers (and inverts) the signal. The other gate re-inverts the signal. The trick now, is to mount the LED and resistor in parallel to the second buffer gate.

The Return of Bright LED

The example above may be the solution you need...

The LED you use needs 20mA at say 3.3V as per datasheet you sent. 3 AA batteries are 4.5V right, and say 20% drop is 3.8V ok.

Of the 3.8V 3.3 is needed by LED we have only 3.8-3.3=0.5 left With 0.5V dropped across Control Transistor PNP, you have nothing left.

A simple idea is to use a cmos chip 74HC14 to drive LED. Instead of constant current you flash LED at High speed. This way Battery and LED will last longer and work at lower voltage.

1.5 Volt LED Flashers

The LED flasher circuits below operate on a single 1.5 volt battery. The circuit on the upper right uses the popular LM3909 LED flasher IC and requires only a timing capacitor and LED.

1.5 volt dual LED flasher

This 1.5 volt led fasher runs more than a year on a single 'd" cell and alternately flashes 2 LEDs at about a 1 second rate. The circuit employs a 74HC14 CMOS hex inverter that will operate at very low voltages (less than 1 volt).

delabs - 2006

Saturday, April 27, 2019

Driving a LED from MCU

I just read your blog and found it very helpful - thank you.

I am working on a new toy project and have a problem I think you can help with.

I am turning an LED (see attached) on and off from a small taiwanese MCU (like a PIC chip). I am using a PNP transistor for this.

My application needs constant current to the LED from 3xAA batteries down to about 3.6v-3.8v. Since it's a toy, the circuit has to be very very cheap! So far I have only managed 'constant' to be about a 4% drop, but I need to do better.

I need a circuit which includes the PNP from the I/O pin of th MCU. Help!

Mail from AB USA in 2006

I am trying to understand your need. All you need is a PNP transistor driving a LED. A series FET constant current element has to be added.

Current Source for LED

The circuits below can show parts of the circuits. If you need something special, i can design for you.

 Constant Current Source LED Drive

A current source based on a JFET

In Lecture 1 we introduced the idea of a current source, and mentioned that these were slightly more difficult to implement than the perhaps more familiar voltage source.

White LED Stroboscope

With Constant Duty Cycle and, Constant Current Drive. The circuit is composed of an oscillator, a pulse slicer, and a gated constant current source. All of the circuits except the anode of the LED receives power from an LM78L05 5 volt regulator.

delabs - 2006