How to Design an LED/IR Lamp Flasher Circuit

You can easily build an LED flasher circuit with a 555 timer chip

(Reference 1) You can easily put an LED flasher together with a 555 integrated circuit (IC) timer and a few outboard parts. You can choose any color LED, including the infrared (IR) varieties. The 555 is a versatile timing pulse generator, producing variable-width pulses. On its own, it provides up to 600 milliwatts of drive power---perfect for small-to-medium LED flasher projects. The 555's low cost and simple connections means you can build this project inexpensively in under an hour.

Things You'll Need

• 555 timer integrated circuit
• 555 timer data sheet
• Light emitting diode
• Resistor assortment
• Capacitor assortment
• Prototyping breadboard
• Short pieces of 22 gauge solid wire
• Calculator
• 9 volt battery
• 9 volt battery clip

Instructions

1. Examine the 555 timer's data sheet, especially the application notes for astable operation. A timing network of 2 resistors and a capacitor determine the 555's frequency and duty cycle. The duty cycle is the percentage of time the timer spends in the "on" state at each cycle. This isn't usually a critical value for a flasher project, though you'll probably avoid very small or large percentages.

2. Determine a flash speed for your project. Most LED flashers work at about 1 cycle per second, but the 555 can run anywhere from minutes per cycle to several thousands of cycles per second. Select a capacitor 10 microfarads, for example. Calculate resistances for the two resistors, Ra and Rb using the following formula:
   F = 1.44 / (C x (Ra + 2Rb))
   Where F is frequency in hertz, C is the capacitor's value in farads, and Ra and Rb are the ohm values for two resistors. If you want a frequency of 1 Hz, and using 10 microfarads for C, gives you a total of 144,000 for Ra + 2Rb. You can satisfy this by setting Ra = 100K ohms and Rb = 22K ohms.

3. Insert the 555 timer into the prototype breadboard. Work the chip in slowly and make sure the pins go straight into the holes. With the battery disconnected from the battery clip, wire the 9-volt battery clip into the breadboard so that the positive (red) lead goes to pin 8 of the 555 IC and the negative (black) lead goes to pin 1.

4. Insert the capacitor into the breadboard. If it's an electrolytic capacitor, insert the negative lead so it connects to pin 1 of the IC. The positive lead goes to pin 6. Insert a jumper wire to connect pins 4 and 8 together. Insert another jumper wire to connect pins 2 and 6. Insert the 100K ohm resistor Ra so that one lead connects to pin 8 and the other lead goes to pin 7. Insert the 22K ohm resistor Rb so that one lead connects to pin 7 of the 555 and the other lead connects to pin 6. Insert the LED into the breadboard so that its anode connects to pin 3 of the 555 and the cathode connects to one lead of a 150 ohm resistor. Connect the other resistor lead to pin 1 of the 555.

5. Double-check your connections, then connect the 9-volt battery to the clip. The LED should start flashing immediately.

Tips & Warnings

• Experiment with different values of Ra, Rb, and the capacitor to alter the flasher's speed and duty cycle.

• If you want to drive a string of LEDs, or use high-brightness LEDs, you may need to add a transistor to the 555's output at pin 3. The transistor's higher current rating will handle the increased load.

References

• National Semiconductor: 555 Timer Data Sheet
• Electronics Club: 555 Timer Circuits