There are plenty of online 555 tutorials available. It is not my intent to produce "the best" one. I am writing this for my own benefit. Making the effort to explain how the 555 works is probably one of the best ways for me to really learn how it works. The teacher learns more than the students.
Two background comments deserve to be mentioned. One is that the 555 was invented in 1971 by Hans R. Camenzind at Signetics. The name 555 comes from the three 5K resistors in the voltage divider. At the time of this writing (2026) this chip is 55 years old!
The following tutorial seems as good as any and provides the diagram I include below.
The actual IC contains 25 transistors, 2 diodes, and 16 resistors. The actual schematic can probably be found somewhere, but it is more useful to think in terms of the following basic elements:
An RS flip-flop Two comparators An NPN transistor 3 resistors An output driver
The output driver allows the chip to source or sink up to 200 mA of current. Apart from that useful function we can pretty much ignore it.
The 3 resistors are arranged as a voltage divider, providing reference voltages of 1/3 and 2/3 of VCC -- routed internally to the two compatators. Note that the 1/3 VCC goes to the + of one (comparator 1), while the 2/3 Vcc goes to the - of the other (comparator 2)
Comparators do exactly what their name implies -- they compare voltages on their two inputs. When "trigger" is less than 1/3 Vcc, comparator 1 turns on, driving the S input of the flip flop. When "threshold" is greater than 2/3 Vcc, comparator 2 turns on, driving the R input of the flip flop.
When both R and S are low, the flip flop just holds the current state. What happens when and if both R and S are high? It is the same as just S going high.
Now notice the transistor. The base is driven by the Q* output of the RS flip flop. So when the flip flop is set (Q is 1) then Q* is low (the inverse of Q) and the transistor is off. When the flip flop gets reset (Q is 0) then Q* is high, and the transistor will be turned on, connecting "discharge" to ground.
Only one thing remains, and that is the "reset" signal (from pin 4). This is an active low input, and is another way to reset the flip flop other than the R input from comparator 2. This is most often just connected to Vcc to prevent it from doing anything.
And there you have it. The question now is how to make use of this.
"What about pin 5?" I hear you asking. This is labeled "control voltage" and here is unconnected. It is probably "better" to connect this to a 10 nF capacitor if you don't intend to use this, but this is rarely done.
Note that pin 4 (reset) is not used, and is connected to Vcc.
A 220 ohm current limiting resistor is provided for the LED (since the 555 would be happy to source 200 mA which would destroy the LED.
Understanding the circuit all boils down to understanding what goes on with trigger, threshold, and discharge.
Without the NPN transistor inside the 555 pulling "discharge" to ground, we have the capacitor being charged by resistors R1 and R2 in series. The capacitor will start off at 0 volts, which will turn on comparator 1 and will set the flip flop. With the flip flop set, the transistor will not be turned on and voltage will rise.
At some point the voltage on the capacitor will be greater than 1/3 Vcc and the S input to the flip flop will go low. The R input will remain low until the capacitor voltage reaches 2/3 Vcc, then the flip flop gets reset, the transistor turns on, and the capacitor discharges through R1 only. Discharge will continue until the voltage drops below 1/3 Vcc when we set the flip flop and start all over.
Ignoring startup when the capacitor starts charging from 0 volts, the circuit alternates between charging through R1 and R2 and discharging through R1. The voltage on the capacitor changes from 1/3 to 2/3 to 1/3 to 2/3 over and over.
Note that the output driver just buffers the Q* signal and is either fully on or off.
Charge time - 0.693 * (r1+r2) * C = 76 ms Discharge time = 0.693 * (r1) * C = 69 msThe period will be 145 ms -- so this circuit blinks at 6.89 Hz (pretty fast).
Tom's Electronics pages / tom@mmto.org