555 Astable with Dual Flashing LEDs

Here is a 555 circuit driving two LEDs in parallel through its output pin 3 with only a single voltage drop resistor R3. The simplest way to understand this astable circuit is to imagine the current flow from the positive voltage rail passing through R1 and R2 charging C1. When C1 has sufficient charge, the voltage across it activates the trigger pin 2, and then the circuit begins the discharge cycle where C1 discharges through R2 and pin 7. The capacitor will go through charge and discharge cycles in this manner and therefore the LEDs blink as shown in the animation.

The values of R1, R2, and C1, determine the charging time, and the values of R2 and C1 determine the discharging time. Since this circuit has fixed components, the LEDs will blink with one specific frequency determined by the component values.

Although adjusting the value of capacitor C1 can also change frequency, it is usually not practical to do so as variable capacitors tend to be in the pF range. For adjusting frequency, we usually modify the circuit by replacing one of the fixed resistors with a variable resistor.

By increasing the value of R1 (or R2), we reduce the current that charges C1, which results in longer time required to charge it, thereby reducing frequency.

Time High Calculation

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