555 timer modes

The 555 timer has 3 main modes of operation.

These three 555 timer modes are astable, monostable and bistable. You can achieve each of the modes by connecting up components in different ways. Each mode has its own uses and furthermore goes to show just how useful and versatile this integrated circuit is. It was also the first integrated circuit I ever bought!

Astable mode

I find this the most useful mode of operation and have used it in many circuits.

One can use it to produce sound, flash lights or control other more complex circuits.

This circuit oscillates and the output it produces is a square wave. I frequently use this circuit to produce clocks for other circuits in addition to audio circuits.

I’ve also used it to produce very high-quality synthesiser circuits. Thomas Henry used the 555 timer to produce a VCO or voltage-controlled oscillator. I’ve built modified versions of this circuit as the basis of my modular synthesiser project.

The image below shows the basic circuit for the astable mode. With the component values shown, the output is around 5Hz.

R1 10K

R2 10K

R3 680R

C1 10uF

C2 .1uF

L1 LED

The exact frequencies that the 555 operates over can vary from each 555 and the leakage of the timing capacitor.

You should be able to get at least 500kHz and some will go up to 2MHZ. Especially the newer ones. The lowest I’ve needed one to work was .1hz (1 cycle every ten seconds.) This was for a clock controlling an analogue sequencer. You can probably get them lower if you need to.

If you want more information on this subject, check out my guide on 555 astable frequencies and component values. It contains a table and a calculator for you to download.

The image below shows the astable circuit built on a breadboard.

I’ve connected an LED to show the output of the 555 timer.

This is also just a scruffy build, but as it’s so simple I didn’t want to take up too much time.

If you are building more complex circuits I’d advise you to follow my breadboard building tips for a neater, less problems build. A 5-volt to 15-volt supply can power the 555 timer.

Monostable mode

Another mode I’ve used lots of times in my circuits is the monostable mode.

With this mode, you can produce an output that stays high for a predetermined time. This time can be varied from a few milliseconds to minutes. This is perfect for lights that stay in for a time or delay switching in circuits. I’ve also used it to debounce a push button switch in a circuit I built to interface a drum machine to a sequencer project.

The picture below shows the circuit diagram.

With the component values shown the output stays high for about a second.

R1 10K

R2 10K

R3 680R

C1 100uF

C2 .1uF

L1 LED

S1 Push button (Push to make)

Pin 2 is held high by R2, the pull-up resistor. The push button switch S1 pulls this lower when pressed. This starts the monostable. Once again this circuit can be powered by anything between 5 volts and 15 volts.

Also, the input doesn’t have to be a switch. It can be another circuit that produces high and low outputs.

Bistable mode

Finally, we have the bistable mode.

For me, this is the least used of the 3 modes. I have, however, still used it several times in my circuits.

Shown below is the circuit diagram.

R1 10K

R2 10K

R3 680R

C2 .1uF

S1 Push button (Push to make)

S2 Push button (Push to make)

Initially, Pin 2 and pin 4 are held high by R2 and R1. These act as pull-up resistors. Pressing S2 or S1 lowers the inputs to the “Trigger” and “Reset” pins of the 555 timer.

This has the effect of switching the output “high” and “low” as indicated by the LED. Also, like the monostable circuit, these inputs can be from the outputs of other circuits, not just simple switches.

The picture below shows the circuit built on a breadboard. It’s powered by a 9-volt supply however, like the two previous circuits, it can be powered by anything between 5 volts and 15 volts.

555 timer modes – Final thoughts

The 555 timer is still used today. This just goes to show its versatility.

You can even build a 555 timer yourself using transistors and a kit like the one below.

I’ve lost count of the amount of circuits I’ve used it in. Although it is usually used in basic circuits, it can be the heart of some excellent performing circuits. If you are looking for a versatile and stable voltage-controlled oscillator, I’d definitely advise you to look at the Thomas Henry 555 VCO.

As I’ve mentioned with the breadboard builds, these are just put together quickly. If you intend to build more complex circuits, then check out my breadboard construction tips. In addition to the tips to speed up construction, these will help you to build error-free circuits.

Finally, If you can’t get a breadboard circuit working, check out my guide to the most common breadboard errors, by working through this you’ll give yourself the best chance to get it working properly.