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
Assembly
Assembly
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
colours 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 3 mm (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!
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.
Led Flasher with the LM3909 IC Diagram
Parts list
LM3909 Integrated circuit
IC socket 8-pin DIL BL17T
LED 5 mm diameter WL27E
100 microfarad (#F) Electrolytic capacitor (10 V) FB48C
Battery holder For AA-size cell YR59P
Battery 1.5 V AA cell
Small piece of Veroboard (15 holes by 10 strips)
Small plastic box (if required)
LED clip (if required)
Two lengths of coloured, insulated wire for LED (as required)
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