555 Timer Astable LED Flasher

This circuit uses a 555 Timer in its classic astable oscillator configuration. RA, RB, and CT set the charge and discharge timing, while the output drives a current-limited green LED.

Suggested Applications

• Blinking indicators and simple status lights.
• Low-frequency clocks for counters or simple logic.
• Learning how the 555 discharge pin controls an RC timing capacitor.

Featured Component

The 555 Timer is an 8-pin timing IC with internal threshold comparators, a latch, an output stage, and a discharge transistor. In this demo it repeatedly charges and discharges CT to create a square-wave output.

Design Context

RA is 10 kOhm from VCC to DISCH, RB is 47 kOhm from DISCH to the timing node, and CT is 100 nF from the timing node to ground. CCTRL bypasses the control-voltage pin. The LED load lets the output state be inspected visually and electrically.

The Vout probe shows the output waveform and the Vtim probe shows the timing capacitor ramp between the internal threshold levels.

Expected Behavior

Vout should alternate between low and high states while Vtim ramps up and down. Increasing RA, RB, or CT slows the flash rate; reducing those values speeds it up.

Suggested Experiments

Change RB or CT to adjust the blink rate, compare Vout and Vtim on the chart, or increase the LED resistor to reduce output load current.