This circuit consumes more power, but it's advantage is when you need a variable flash rate, like for strobe circuits. You can actually use this circuit as a remote control for strobes that have a remote input. Of course, it has many other applications besides strobes.
- R1, R2, C1 and the supply voltage determine the flash rate. Using a regulated power supply will do much to insure a stable flash rate. For a variable flash rate, replace R1 with a 1 megohm pot in series with a 22k resistor.
- The duty cycle of the circuit (the percentage of the time LED 1 is on to the time it is off during each cycle) is deterimed by the ratio of R1 to R2. If the value of R1 is low in relationship to R2, the duty cycle will be near 50 percent. If you use both LEDs, you will probably want a 50 percent duty cycle. On the other hand, if R2 is low compared to R1, the duty cycle will be less than 50 percent. This is useful to conserve battery life, or to produce a strobe type effect, when only LED1 is used.
- The NE555 timer chip can be damaged by reverse polarity voltage being applied to it. You can make the circuit goof proof by placing a diode in series with one of the supply leads.
- The purpose of R3 and R4 is to limit current through the LEDs to the maximum they can handle (usually 20 milliamps). You should select the value of these according to the supply voltage. 470 ohms works well with a supply voltage of 9-12 volts. You will need to reduce the value for lower supply voltages.
- Rainbow Kits offers several kits to build the above circuit. You can also order these kits from RadioShack.com. The Radio Shack catalog numbers (and web pages) are as follows: standard kit with two 5mm red LEDs, (990-0067), kit with two red, two green and two yellow 3mm LEDs, (990-0063), kit with jumbo green LEDs, (990-0048), kit with jumbo red LEDs, (990-0049). You can also buy all the parts to build the circuit at your local Radio Shack store, including a circuit board (276-159B).
I have built a miniature strobe circuit as follows. Use a 250k pot in series with a 4.7k resistor for R1. The 4.7k resistor sets the upper flash rate limit. Use 2.2k for R2. That sets a really short duty cycle. For this circuit, you don't use LED 2 or R4. For LED 1, I used a two Radio Shack white LEDs in series and no R-3. The circuit runs on a 9 v battery. link
