The timing components are Rl, R2, and C,. Cl is a bypass capacitor used to reduce the effects of noise. At start-up, the voltage across C, is less than the trigger level voltage (1/3 Vnn). causing the timer to be triggered via pin 2. The output of the timer at pin 3 increases, turning LEDl off, LED2 on, the discharge transistor at pin 7 off, and allowing C, to charge through resistors Rl and R2. When capacitor C, charges to the upper threshold voltage (2 /3 Vvv). the flip-flop is reset and the output at pin 3 decreases. LEDl is turned on, LED2 is turned off, and capacitor C, discharges through resistor R2 and the discharge transistor.
Alternating-led-flasher - schematic

When the voltage at pin 2 reaches 1{:, V00, the lower threshold or trigger level, the timer triggers again and the cycle is repeated. The totem-pole output at pin 3 is a square wave with a duty cycle of about 50%. The output alternately turns on each LED at slightly less than one blink per second. If the unit is battery operated, the TLC555 uses minimum current to produce this function. With a 9-V battery, the circuit draws 5 mA (no load) and 15 mA when turning on an LED. Most of the on current is for the LED.

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