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Car Alarm?

by B Sandle

I have found a use for the LM3909 LED flasher as a warning device in my car. By adding a capacitor, an LED and a 1.5V cell, this IC will cause the LED to flash at a rate determined by the value of the capacitor. I fitted the LED in the dashboard, and when switched on it looks as if I have a sophisticated car alarm. It produces a very bright flash which is visible from a distance - just the thing for the dark evenings. A standard AA type 1.5V cell lasts about 7 months (the IC is said to be "optimized for low power drain - 0.5 to 0.75 mA - and Operation from weak batteries.)
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The Circuit (Fig. 1) could hardly be simpler; Fig. 2 shows the internal circuitry of the IC. Because of the small number of external components, the flasher can easily be incorporated in a pocket torch, using the torch battery. The LED is mounted so that it is visible externally, and so provides a means of finding the torch in the dark. As the current drain is so small, the IC can be left in circuit at all times, though an on-off switch could be fitted if desired. In any application a high intensity LED makes for greater visibility.

The National Semiconductor data sheet shows a number of other applications, one of which uses the IC as an audio oscillator (Fig. 3). The output is roughly a square wave, and this can form the basis for a simple continuity tester (Fig.4).

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Although the maximum permissible supply voltage for this IC is 6V, higher voltages can be applied through a resistor of suitable value. A bypass capacitor of 100F must be connected between pins 4 and 5 to smooth out voltage variations and ensure that the voltage applied to the IC stays within acceptable levels (Fig. 5).

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The LM3909 is available from most suppliers at a price between 1 and l.50.