555 Timer Astable Circuit Diagram (LED Flasher)

Inside the 555 are two comparators, a flip-flop and a discharge transistor. THR and TRIG both watch the timing capacitor C1. When C1 is below one third of the supply, the trigger comparator sets the flip-flop: OUT goes high and the discharge transistor on DIS turns off.

C1 then charges through R1 and R2 in series. When it reaches two thirds of the supply, the threshold comparator resets the flip-flop: OUT goes low and DIS pulls the junction of R1 and R2 to ground.

C1 now discharges through R2 alone into DIS. When it falls back to one third of the supply, the cycle starts again. On 9 V the capacitor swings between 3 V and 6 V, and because both thresholds scale with the supply, the timing does not depend on the supply voltage.

Charging runs through R1 + R2 and discharging through R2 only, so the high time is always a little longer than the low time: with 10 kΩ, 68 kΩ and 10 µF the LED is on for about 0.54 s and off for about 0.47 s, roughly 1 Hz. RST is tied to the supply because pulling it low stops the oscillator, and C2 on CTRL keeps noise off the comparators' reference.

• Expecting precise timing from an electrolytic C1. Electrolytics are often ±20 %, so the blink rate can be off by as much; for accurate timing use a film capacitor, a smaller value and larger resistors.
• Leaving RST floating. It is active low, and a floating reset can stop the oscillator at random.
• Making R1 very small. When DIS turns on, R1 connects the supply straight to the discharge transistor, and a tiny R1 means a large current through it.
• Trying to get a 50 % duty cycle from this circuit. It always stays above 50 %; a diode across R2 lets C1 charge through R1 alone, which makes a square wave possible.

Build notes

• The duty cycle is (R1 + R2) / (R1 + 2·R2) ≈ 53 %; it can never reach 50 % or below with this circuit.
• Use the CMOS TLC555 or LMC555 for lower supply current; the timing formulas are the same.