HOW TO MAKE A LED FLASHER USING LM3909 IC?
Introduction
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.
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.
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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