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1.5V LED Flasher Oscillator
Mimics the now discontinued LM3909 IC
The 8-lead plastic mini-DIP LM3909 IC was developed by National Semiconductor in the mid ‘seventies of the past century. It was a monolithic oscillator specifically designed to flash Light Emitting Diodes. By using the timing capacitor for voltage boost, it delivered pulses of 2 or more volts to the LED while operating on a supply of 1.5V or less. The circuit was inherently self-starting, and required addition of only a battery and capacitor to function as an LED flasher.
Unfortunately, since 1998, the manufacturer discontinued the production of this chip. For this reason, and on request of some correspondents, I tried to emulate this IC operation using common discrete components, obtaining unexpected satisfactory results.
Obviously, this circuit requires more room on the PCB than the original IC, but it is perhaps a bit cheaper, yet performing in a very similar manner.
Basic-Block circuit diagram:
R1,R2__________390R 1/4W ResistorsR3,R5___________22K 1/4W ResistorsR4______________10K 1/4W ResistorR6_____________100R 1/4W ResistorQ1____________BC337 45V 800mA NPN TransistorQ2,Q3_________BC550C 45V 100mA Low noise High gain NPN TransistorsQ4____________BC327 45V 800mA PNP Transistor
Basic-Block circuit used as typical LED flasher
The following table shows the features of several LED flasher circuits obtained by changing C or R parts (drawn in red color in the diagram) and voltage supply. The LED (D) should be preferably of the red type.
Circuit typeC valueR valueBattery voltageNominal flash rateAverage current drain
1.5V Flasher330µF 6V3K3 1/4W1.5V1 Hz0.64mA
Low power 1.5V100µF 6V8K2 1/4W1.5V1.2 Hz0.32mA
Fast Blinker330µF 6V1K 1/4W1.5V2.6 Hz1.2mA
3V Flasher330µF 6V8K2 1/4W3V1 Hz0.77mA
6V Flasher330µF 6V10K 1/4W6V1.5 Hz0.7mA
Note: The flashing rate can be different from nominal values shown above: it should be remembered that some electrolytics have very broad capacitance tolerances, for example -20% to +100%.
Typical 1.5V Flasher
Estimated Battery Life
AA3 months6 months
C7 months15 months
D1.3 years2.6 years
(Continuous 1.5V Flasher Operation)
Warning Flasher High Voltage Powered circuit diagram:
Typical Operating Conditions
V+ RangeNominal Flash HzC valueR valueRa valueRb value
5V-25V2470µF 6V3K3 1/4W1K 1/4W1K5 1/4W
13V-50V2220µF 6V3K3 1/4W3K9 1/4W1K 1/4W
85V-200V1.7220µF 6V3K3 1/4W47K 1W1K 1/4W
Incandescent Bulb Variable Flasher circuit diagram:
Parts to be added to the Basic-Block:
P________________4K7 Linear potentiometer or trimmerR_______________68R 1/4W ResistorRa_______________2K2 1/4W ResistorRb_______________3K3 1/4W ResistorRc_____________100R 1/4W ResistorC______________100µF to 330µF 6V or higherQ_____________BD681 100V 4A NPN Darlington TransistorLP____________Incandescent bulb (Any 2.5V to 6V type, 1A max.)V+ Range_________3V – 6V
12 Volt Flasher (2 Wire) circuit diagram:
Parts to be added to the Basic-Block:
R_______________33R 1/4W ResistorRa_____________220R 1/4W ResistorRb_______________1K 1/4W ResistorRc_______________3K3 1/4W ResistorRd_____________100R 1/4W ResistorRe_____________470R 1/4W ResistorC_____________2200µF to 3300µF 6V or higherQ_____________BD681 100V 4A NPN Darlington TransistorLP____________Incandescent bulb 12V 1A max.V+ Range________12V – 14V
The above diagram shows a higher power application such as would use an automotive storage battery for power. It provides about a 1 Hz flash rate and powers a lamp drawing a nominal 600 mA.
A particular advantage of this circuit is that it has only 2 external wires and thus may be hooked up in either of the two ways shown in the diagram below. Further, no circuit failure can cause a battery drain greater than that of the bulb itself, continuously lit.
- The circuit should operate safely in the 6V-14V range, provided the bulb used has the same nominal voltage of the supply used.
- Decreasing voltage supply, C capacitance value should also be decreased to keep the 1 Hz flash rate.
- If flasher case insulated, it will operate in positive or negative ground systems.
2 Wire Flasher Usage:
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