Surely one of the most famous and well-loved IC’s of all time must be the 555 timers, invented by Hans Camenzind of Signetics in 1968. It’s because 555 timers are very stable and easy to use. It accomplishes its task with minimum components—comes in an 8-pin pack—provided you wire it up correctly.

Shown below is a simplified block diagram of the 555 and how the pins are connected internally. Fortunately, it is not necessary to understand the inside workings to use it successfully. You can see two comparators set for 2/3 and 1/3 Vcc, followed by a reset-able flip flop, followed by an output level driver. It got its name from the three 5k resistors in the threshold divider chain from pin 8 to pin 1.

It can do many things, including being a Schmitt trigger and even amplifying. But the main uses are as a monostable, a bistable, and an astable oscillator. In this article, we will look at each of those in detail and the single 555, the 556, which has two 555’s in a single package, and CMOS versions.

What’s inside

Internal block diagram
1GndNegative supply pin, ground
2TrigTrigger. When this voltage falls below 1/3Vcc, the output goes high, and timing starts. As long as this is low, the output will remain high.
3OutOutput. A push-pull output either high or low capable of driving up to 200mA
4ResetReset. Resets the timing interval if grounded. If not used, it should be connected to Vcc.
5ContControl. By applying a V the 2/3 Vcc, timing can be over-ridden. In astable mode, it can modulate the frequency at the output. If it is not used, it should be connected to GND with a 10nF cap.
6ThreshThreshold. When this voltage on the pin is >2/3Vcc, then Out high time ends, sending the output low.
7DischAn open collector can be used to discharge a connected cap; in phase with the output, in bistable and Schmitt mode, it is unused and can act as a second output.
8VccPositive supply pin 4.5 to 16V

Monostable Mode

A monostable is a circuit that likes to stay in one state but can be forced to switch to the opposite state for a period controlled by RC circuits R1, C3.

Shown below is the 555 version of a monostable. Pin 2 is kept HIGH by R2, but the instant it goes LOW, the timing period starts, and the output on pin 3 goes HIGH for a period determined by R1, C3 where t = 1.1*R1*C3. When this period ends, pin 3 goes LOW again. This is only true for input pulses longer than t. As pin 3 goes HIGH, the instant pin 2 goes LOW, and if pin 2 stays LOW for longer than t, pin 3 will stay HIGH until pin 2 goes HIGH again.

Monostable is very useful for cleaning up a pulse of unreliable length to force it to be a known length, such as a pulse stretcher in a burglar alarm vibration sensor or a momentary push-button circuit.


Bistable Mode

A bistable is a circuit that can exist in one of two states—either on or off. Examples include memory cells, flip flops and switch debouncers.

Shown below is the 555 version of a bistable. There are no timing capacitors as it is only on or off. S1 will send a LOW to either pin 2 or 4, causing the output to change state. It is worth noting that pin 3 will be HIGH on boot up if pin 2 is LOW.

Bistable with change-over switch trigger

We can replace the toggle switch with two separate push buttons below to achieve the same result

Bistable with push button trigger

Astable Mode

In astable mode, the 555 acts as a square wave oscillator. It can be controlled over a wide range (more than 10:1) of frequencies with a single cap and a variable resistor. Not only that, the mark-space ratio or duty cycle can be controlled.

In the circuit below, R1, R2, and C3 control the on and off periods’ timing. And the two periods together set the frequency of the output square wave.

In the example, t1 is 8mS and t2 is 13mS so T is 8+13 = 21ms. Hence, the frequency is 1/T = 47.6Hz.

In the astable, we can set each of the widths independently. his will affect the frequency as well as the duty cycle.

T= 0.7(R1+2R2)C3 and freq = 1/T.

Here’s an interesting circuit: one 555 set to oscillate at 1Hz and connect the output (pin 3) of this one to a second 555 via pin5 (remove the cap) set to run at a few hundred Hz. This will produce a police siren or burglar alarm (dee dah).


Overall, the 555 will be around for a long time because of its great performance, ease of use, and robustness. It is the kind of IC you should keep a drawer full in your stock.

You can read more about 555s here.