Step 1: Gather the Parts
Here is what you will need:
1 - Timer 555 8 pin IC
1 - 2N3905 PNP general switching transistor
1 - 2N3053 NPN general purpose amplifier (I dont know if this is a switching or amplifer but I used MPSA2222A instead and it worked fine, I also tried 2N3904 and it worked, but a littler worse than the one I used)
20 - red LEDs (although you can use any color choice you want)
20 - blue LEDs (although you can use any color choice you want)
1 - LED (this is used to verify if your circuit works, can be any color)
1 - 1uF Electrolytic capacitor
1 - .1uF disc capacitor
1 - 150k resistor
1 - 4.7k resistor
1 - 160 ohm resistor
1 - 220 ohm resistor (although I used a 160 ohm)
20 - 100 ohm resistors (I didnt have 20 so I used 10 of 100 ohm and 10 of 120 ohm)
1 - 6V source
Step 2: Build the Circuit
The notes were taken from the page mentioned later.
"Two sets of 20 LEDs will alternately flash at approximately 4.7 cycles per second using RC values shown (4.7K for R1, 150K for R2 and a 1uF capacitor). Time intervals for the two lamps are about 107 milliseconds (T1, upper LEDs) and 104 milliseconds (T2 lower LEDs). Two transistors are used to provide additional current beyond the 200 mA limit of the 555 timer. A single LED is placed in series with the base of the PNP transistor so that the lower 20 LEDs turn off when the 555 output goes high during the T1 time interval. The high output level of the 555 timer is 1.7 volts less than the supply voltage. Adding the LED increases the forward voltage required for the PNP transistor to about 2.7 volts so that the 1.7 volt difference from supply to the output is insufficient to turn on the transistor. Each LED is supplied with about 20 mA of current for a total of 220 mA. The circuit should work with additional LEDs up to about 40 for each group, or 81 total. The circuit will also work with fewer LEDs so it could be assembled and tested with just 5 LEDs (two groups of two plus one) before adding the others."
Step 3: Modify the Final Project
Okay, now editing the flashing rate is easy. You only need to change 1 or more of the 3 values of R1, R2, and C.
Use these equations to do so.
Positive Time Interval (T1) = 0.693 * (R1+R2) * C (time first set is flashing)
Negative Time Interval (T2) = 0.693 * R2 * C (time other set is flashing)
Frequency = 1.44 / ( (R1+R2+R2) * C) (flashes per second)
Step 4: View the Final Product
This project was found at http://ourworld.compuserve.com/homepages/Bill_Bowden in a beginners 555 timer circuit page.
I bought the LEDs from http://www.abctronics.com