Multivibrator Flasher Circuit Diagram

The basic 2 LED astable multivibrator flasher built by Mary. She chose to use 2 different colored LEDs and the red LED is clear when unlit. It is quite bright when lit compared to the yellow LED despite the fact that it only draws 0.5 mA more. The bread boarded test circuit was powered by a new 9 volt battery and was regulated by a L78L05 (in a TO-92 package as shown in the schematic). The 5 volt regulator was used to avoid exceeding the reverse breakdown voltage of the 2N3904. This topic will be discussed a little later on.

470 ohm current dropping resistors were chosen to keep the collector current draw less than 10 mA. The LEDS were bright enough to see well in dim lighting. You may change this resistor "R" value (lower R = brighter), but do not exceed the maximum current rating for the LED or transistor (this is more applicable to higher voltage multivibrators). You may also place 2 or more LEDs in series on each half of a multivibrator, however, the current dropping resistor may need to be reduced to maintain brightness. Consider using a power supply as opposed to battery power for your flashers.

To change the pulse (oscillation) frequency, you can change the base resistor or the timing capacitor values. For example, increasing the capacitor or the base resistor values will increase the time OFF per cycle and thus reduce the oscillation frequency. The oscillation frequency is 60 divided by the sum of the time OFF for each half of the multivibrator.  Do not feel you have to use the same timing capacitor for each 1/2 of the multivibrator. Multivibrators with different timing components on each 1/2 are termed asymmetrical.

Over time, some builders sent me emails that they could not get their multivibrator to run. I problem-solved with them and discovered many problems including bad parts, bread boarding errors, the oscillation frequency was too fast to observe, transistors were not saturated during their ON time and failure of the transistors due to excessive current or perhaps even reverse emitter-base breakdown.