Yes, for me, it’s nostalgic to think about my favorite LED Driver LM3909 from NSC! This ubiquitous chip is very smart in that it stores up energy in a timing capacitor and then switches it in series with the single cell powering it to boost the voltage for a short pulse of energy. Wonder thing about it was it could flash an LED off a single cell for over 12 months!
Most linear integrated circuits are designed to operate with power supplies of 4.5 to 40V. Practically no battery/portable equipment is provided with indicator lights due to unacceptable power drain. Even LEDs (solid state lamps) won’t light from a 1.5V battery, and drain the common 9V radio battery in a few hours. The LM3909 changes all this. Obtaining long life from a single 1.5V cell, it opens a whole new area of applications for linear integrated circuits. Sufficient voltage for flashing a light emitting diode is generated with cell voltage down to 1.1V. In such low duty cycle applications batteries will last for months to years of continuous operation. Such flasher circuits then become practical for marking location of flashlights, emergency equipment, and boat mooring floats in the dark.
The LM3909 is simple in design, easy to use, and includes extra resistors to minimize external circuitry and the size of the completed flasher or oscillator. The LM3909, although designed as a LED flasher, is ideal for other applications such as high current, trigger pulse for SCRs and Triacs. The frequency of oscillation adjusts from under 1 Hz to hundreds of KHz (waveshape can be set from pulses a few ms wide to approximately a square wave). Thus the LM3909 can perform as a sound effects generator, an audible alarm, or audible continuity checker.
Finally it can be a radio (detector/amplifier), low power one-way intercom, two-way telegraph set, or part of a “mini-strobe” light flashing up to 7 times per second. In the LED flasher, the LM3909 uses the single capacitor for both timing and voltage boosting. The simplicity of LED flasher is shown here.
Unfortunately the LM3909 is now extinct though I have only one in my junk box. Recently, I looked for good alternatives of LM3909 which was no longer available, and some interesting things found both sides of the street!
As the title says, first one is a replacement circuit for the LM3909 LED Flasher/Oscillator using discrete components. The circuit is functionally the same as the integrated LM3909 but has a minor variations in values of the components used. Needless to say, this circuit for 1.5V to 3V only operation will not be as compact as the LM3909 but it does offer the ability to adjust individual component values (there is plenty of room to experiment with this circuit) and can be less expensive than the integrated circuit itself.
The NTE876 (from NTE Electronics Inc.) is a monolithic oscillator integrated circuit, packaged in a plastic 8−Lead mini−DIP, specifically designed to flash Light Emitting Diodes. By using the timing capacitor for voltage boost, it delivers pulses of 2 or more volts to the LED while operating on a supply of 1.5V or less. The circuit is inherently self−starting, and requires the addition of only a battery and capacitor to function as an LED flasher. Timing capacitors will generally be of the electrolyic type, and a small 3V rated part will be suitable for any LED flasher using a supply up to 6V.
When it comes to blinkers and beepers, the renowned tiny timer chip 555 is irresistible. Here is a powerful and efficient equivalent to the evergreen 555 chip, ZSCT1555-from Zetex Semiconductors. Tailored for low voltage/low power portable/battery powered systems, pin connections are fully comparable with 555 series timers, and 0.9V supply operating voltage is guaranteed by the manufacture. The circuit, based on ZSCT1555,shown here generates a 5 volt supply from a single battery cell. The circuit output voltage is maintained down to 0.9 volts input and power economy is optimised for extended battery life.
My Own White LED Flasher
Inspired by the Joule-Thief concept, again I fabricated another cheap and cheerful Joule-Thief circuit, which is nothing but a 5mm White LED flasher, running on an input of 0.9V to 1.5V DC. No doubt, so many JT LED Flasher circuits are now available, but I want to say that this is an improved design from me. The flashing is very powerful even with an input of 0.9V DC!
X1 used in myprototype consists of 20 + 20 turns -L1 + L2- of 0.18 mm enamelled copper wire on a toroid ferrite core having 10mm outer diameter x 6mm inner diameter x 4mm height.
This article contains borrowed materials from many online resources including: