It is often desirable to have two LEDs turn on and off alternately. Advertising signs frequently use this type of operation. Alternating LED action is easily accomplished using the circuit shown in Figure 7-28 with the TLC555 operating in the astable mode.
The timing components are R1, R2, and Ct. C1 is a bypass capacitor used to reduce the effects of noise. At startup, the voltage across Ct is less than the trigger level voltage (1/3 VDD), causing the timer to be triggered via pin 2. The output of the timer (pin 3) goes high, turning LED1 off, LED2 on, the discharge transistor (pin 7) off, and allowing Ct to charge through resistors R1 and R2.
When capacitor Ct charges to the upper threshold voltage (2/3 VDD), the flip-flop is reset and the output (pin 3) goes low. LED1 is turned on, LED2 is turned off, and capacitor Ct discharges through resistor R2 and the discharge transistor. When the voltage at pin 2 reaches 1/3 VDD, 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 LED current.