Discrete Version Of The LM3909 Oscillator IC

A De-Integrated Circuit

  This page features 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.

  The LM3909 can still found but is fairly expensive, one possible source is Futurlec. The LM3909 is also available through some surplus electronics sources but usually only in quantities of hundreds or thousands of pieces.

  Naturally the circuit will not be as compact as the integrated circuit but it does offer the ability to adjust individual component values and can be less expensive than the integrated circuit itself.

LM3909 Data sheet - National Semiconductor (.pdf)


Datasheet Circuit Of The LM3909 IC


Redrawn Internal Circuit Of The LM3909 IC


Discrete Component Replacement Circuit For The LM3909 IC

  The following schematic is of the discrete component replacement for the LM3909.

Discrete Component LM3909 Circuit Notes

  NOTE: This circuit has not been tested in any other mode other than that shown in the above schematic. There is no apparent reason that it shouldn't work in the other circuits shown on the LM3909 data sheet if the zener diode D1 is replaced.


A Slightly Different Version Of The Discrete Circuit

  In this circuit, the current mirror formed by Q2A and Q2B has been replaced by one transistor and a resistor.

  The value of resistor R10 was based on measurments taken from the current mirror portion of the circuit operating at 1.5 volts. The 330 ohm value allowed capacitor C1 to discharge fully.

  The only savings with this circuit is the cost of a transistor versus that of a resistor.


LM3909 Working Principle

  The following diagram illustrates the basic working principle of the LM3909 oscillator.

  The circuit acts as a 'voltage doubler'. Capacitor C1 from the battery and is then put in series with the battery to boost the voltage.

  This allows a 1.5 volt source to provide more than the 2 volts required by the LED.


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Please Read Before Using These Circuit Ideas

  The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.

  If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.

  Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.

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  08 March, 2008