Radio & Electronic Projects: An LED flasher

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The LM3909 is an integrated circuit (IC) which will flash a light-emitting diode (LED). Using only two extra components and a battery, the circuit is cheap and has a very low current drain from a 1.5 V cell. The circuit can be used as a novelty flasher, an indicator for a dummy alarm bell box, or it could be attached to a torch so that it could be found easily in the dark! The simple circuit is shown in Figure 1.

Figure 1 Circuit diagram of the LED Flasher. Pins 1, 3 and 7 of the IC are not used


The circuit can be built on a small piece of Veroboard (the piece shown in Figure 2 measures 15 holes by 10 strips). Using such a board, follow these instructions.

1. Depending on how far away you want the LED from the circuit board, solder a length of insulated wire to each lead of the LED. Use different colors of insulation - say, red and black, connecting the red lead to the anode (a) lead (the longer one) of the LED, and the black one to the cathode (k). Figure 2 shows these leads.

2. Cut the copper tracks as shown in Figure 2, using a 3mm (1/8 inch) diameter drill, rotated between thumb and forefinger, or use the proper tool. Make absolutely sure that the tracks are completely broken!

3. Fit the IC holder in the correct position, using the cut tracks as guides, and make sure the small notch is facing towards the top of the board.

Solder the pins to the copper tracks.

4. Mount the capacitor, positive end to the left, so that the positive lead is soldered to track F, which connects it to pin 2 of the IC; the negative lead is soldered to the right-hand side of track E, this being connected to pin 8 of the IC.

5. Solder on the battery leads, positive to the right, and the extended LED leads, positive downwards.

6. Check the circuit, and hold up the board to a bright light and look carefully for solder bridges between the tracks and pieces of copper swarf which may have escaped your inspection in 2 above! Remove whatever you find.

7. When all seems well, put the IC into the socket, ensuring that the notch or dot on the upper surface of the IC lines up with the notch on the holder. Line up each pin on the IC with the hole below it before pressing gently on the IC with the board supported on a firm surface.

8. Connect the battery; the LED should start to flash. The circuit is complete and working!

Figure 2 Board layout viewed from the component side. The tracks are cut under the board where shown

If you prefer, the whole circuit (battery included) can be mounted in a small plastic box, with the LED mounted on a clip and protruding through the panel. There are many other possibilities, and it is up to you to find an application for your own use.


Parts list

  • Maplin code
  • LM3909 Integrated circuit WQ39N
  • IC socket 8-pin DIL BL17T
  • LED 5mm diameter WL27E
  • 100 microfarad (uF) Electrolytic capacitor (10V) FB48C
  • Battery holder For AA-size cell YR59P
  • Battery 1.5V AA cell
  • Small piece of Veroboard (15 holes by 10 strips)
  • Small plastic box (if required)
  • LED clip (if required)
  • Two lengths of colored, insulated wire for LED (as required)
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