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Versatile LED/Lamp Flasher

The LM3909 was a legendary IC, which the designers (National Semiconductor) modestly described as a "LED Flasher/Oscillator". Its popularity was surely due both to its great simplicity and versatility. It could flash a LED off a wide range of voltages, at a wide range of frequencies. It could also flash LEDs in parallel, could produce a tone in a loudspeaker, trigger a Triac or pulse an incandescent bulb - among other things. Sadly, though, the LM3909 has been discontinued and is now very difficult to find.

By Thomas Scarborough

The module shown here is designed to do just about everything that the original LM3909 did – and more!

Click for larger image
Two of the many possible versions of the LM3909 Replacement Module - on the left, the alternate LED flasher with LINK2, RX at 4.7MW, RB at 1kW, no R2 and RB (hidden behind capacitor at top) at 47kW. At right is the LED/bulb flasher, with LINK3, RX at 2.2MW, R2 at 470kW (in series with its diode), RB at 1kW and no RY. Note the MOSFET lamp driver is also in place on this PC board. For most of the time, LINK1 stays in place (the circuits won’t work without it).

There are a couple of differences – the most obvious one is that the module is quite a bit larger than the DIP-sized LM3909.

Supply voltage is much more usable 3V-18V, compared to the LM3909’s 1.15V to 6V. Current consumption may be as low as 100µA, rather than the LM3909’s typical 0.55mA. Pulse width may be controlled more easily than it could with the LM3909.

And not least, this circuit can pulse two LEDs alternately. It will also serve, among other things, as a quartz clock driver and continuity tester. Rather than present a host of similar circuits, however, a single module is shown here, along with a table showing how the module can be used in a variety of ways.

Circuit description

IC1a is a Schmitt RC oscillator or "clock generator". Only the capacitor, C1, can be regarded as a fixed part of the oscillator; R1, R2, RX, RY and D2 are all components which can be changed to allow the module to perform in different ways.

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