555 Timer Chip Pinouts

The 555 timer chip is one of the most ubiquitous chips seen in hobby and college electronics over about the last 45 years or so. Certainly before digital electronics and microcontrollers made such an impact.

First invented in 1972, this chip is probably a contender for the most manufactured chip in the history of integrated circuits.

The text below explains the packaging of a 555 timer in the classic eight pin DIL chip package, and the table shows the pinouts.

DIL = ‘Dual in line’.

Two rows of four pins sprout from each of two opposite and parallel sides of the body.

There is a very obvious notch, easy to find with a fingernail, on the top of one of the sides with no legs. If you hold the chip with this notch at the top, away from you, pin 1 is to the left of this notch, and the pins are numbered in an anti-clockwise direction round the chip. So pin 1 is at the top left, pin 4 at bottom left, pin 5 at bottom right, and pin 8 at top right.

I have seen some DIL chips with a small recessed dot next to pin 1 instead of the usual notch. I cannot remember whether this was the case with any examples of the 555 timer, but be conscious of it. If you cannot find a notch, feel around with the tip of a non-conductive but sharp object to see if you can find this little recessed dot.

This table shows the function of each pin:

Pin Function
2 Trigger
3 Out
4 Reset
5 Control
6 Threshold
7 Discharge
8 Vcc

The 555 has three principle applications. Below, each is described.

  • Monostable

The name monostable suggests the circuit has one stable state. When it is triggered, the output will change state, and then return to its original state after a period of time controlled by components placed in the rest of the circuit.

Common use of a monostable is to de-bounce the contacts of a push button.

  • bistable

As the name suggests, the bistable multivibrator has two stable states. For this reason it is also known as a flip-flop. Because each trigger event changes the state from one to the other.

The more correct name for a bistable is the SR (set/reset) latch.

  • astable

The astable multivibrator does not have a stable state. It oscillates between two states, triggered and not triggered. The output pin will oscillate between high and low.