1.5V LED Flasher Oscillator

Mimics the now discontinued LM3909 IC

Can be adapted to a wide variety of applications

Design description:

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.


Typical LED flasher

Warning Flasher High Voltage Powered

Incandescent Bulb Variable Flasher

12 Volt Flasher (2 Wire)

Basic-Block circuit diagram:

1.5V LED Flasher Oscillator


R1,R2__________390R  1/4W Resistors
R3,R5___________22K  1/4W Resistors
R4______________10K  1/4W Resistor
R6_____________100R  1/4W Resistor

Q1____________BC337   45V 800mA NPN Transistor
Q2,Q3_________BC550C  45V 100mA Low noise High gain NPN Transistors
Q4____________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

Size CellType
AA3 months6 months
C7 months15 months
D1.3 years2.6 years
(Continuous 1.5V Flasher Operation)

Warning Flasher High Voltage Powered circuit diagram:

1.5V LED Flasher High Voltage Oscillator

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:

Bulb Variable Flasher

Parts to be added to the Basic-Block:

P________________4K7 Linear potentiometer or trimmer

R_______________68R  1/4W Resistor
Ra_______________2K2 1/4W Resistor
Rb_______________3K3 1/4W Resistor
Rc_____________100R  1/4W Resistor

C______________100µF to 330µF 6V or higher

Q_____________BD681  100V 4A NPN Darlington Transistor

LP____________Incandescent bulb (Any 2.5V to 6V type, 1A max.)

V+ Range_________3V - 6V

12 Volt Flasher (2 Wire) circuit diagram:

12 Volt Flasher (2 Wire)

Parts to be added to the Basic-Block:

R_______________33R  1/4W Resistor
Ra_____________220R  1/4W Resistor
Rb_______________1K  1/4W Resistor
Rc_______________3K3 1/4W Resistor
Rd_____________100R  1/4W Resistor
Re_____________470R  1/4W Resistor

C_____________2200µF to 3300µF 6V or higher

Q_____________BD681  100V 4A NPN Darlington Transistor

LP____________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.


2 Wire Flasher Usage:

2 Wire Flasher Usage