How to Build a 4 LED Slow Blinking Circuit

You can build an LED flasher circuit with 555 timer ICs.

You can use the 555 timer integrated circuit (IC) to build a variety of light-emitting diode (LED) blinking circuits. The 555 can drive LEDs directly and needs only a few extra parts. One such circuit, sometimes called the "railroad flasher," alternately turns two LEDs on and off. Two of these circuits will give you a total of four slowly blinking LEDs. The components used are very inexpensive, and you can build these circuits in under an hour with little difficulty.

Things You'll Need

• (2) 555 timer ICs
• (4) 220 ohm, ¼-watt resistors
• (4) 5mm LEDs
• (2) 47K ohm, ¼-watt resistors
• (2) 10 microfarad capacitors
• Prototype breadboard
• 9-volt battery
• 9-volt battery clip
• Assortment of #22 gauge jumper wires

Instructions

1. Insert the two 555 ICs into the breadboard so they straddle the horizontal slot in the plastic board. Orient the ICs so the dot or key slot on one end faces to your left. Space the ICs about 2 inches apart. Take care to not bend the pins out or under the ICs.

2. Insert one 10-microfarad capacitor so its positive lead connects to the first IC's pin 2 and its other lead connects to pin 1. Insert a jumper wire to connect pin 2 to pin 6. Insert one lead of a 47K ohm resistor so it connects to pin 6 of the IC. Insert its other lead so it connects to pin 3. Insert a jumper wire to connect pin 8 and pin 4 together.

3. Examine the LEDs and note that the round case has a flat spot. The lead nearer to the flat spot is the LED's cathode. Insert the cathode lead of one LED so it connects to pin 1 of the IC. Insert the other lead, the anode, into an unused column on the breadboard. Insert one lead of a 220-ohm resistor into this column. Insert the resistor's other lead so it connects to the IC's pin 3. Insert another 220-ohm resistor so one lead connects to pin 3. Insert its other lead into an unused column. Insert the cathode of another LED into this column. Insert its anode lead so it connects to pin 4.

4. Repeat Steps 3 and 4 for the second 555 timer.

5. Insert the red (positive) lead coming from the 9-volt battery clip so it connects to pin 8 of the first IC. Insert a jumper wire so it connects pin 8 of both ICs together. Insert the black (negative) lead coming from the 9-volt battery clip so it connects to pin 1 of the second IC. Insert a jumper wire so it connects pin 1 of both ICs together.

6. Double-check all your connections. Clip a fresh 9-volt battery into the clip. The four LEDs should begin blinking.

Tips & Warnings

• By trying larger values for the capacitors or the 47K resistors, you can slow the blink rate much further.

References

• National Semiconductor: LM555 Timer IC Data Sheet
• 555 Timer Circuits: Flashing Railroad Lights Circuit