Getting to Grips with 555 Timer BasicsFollow article
Learning about the operating principles and applications of a 555 timer.
Recently on my HND course I have been using 555 timers to build simple circuits like a traffic light system. However, we haven’t really spent much time looking at exactly what a 555 timer is and consists of. I decided I would do some of my own reading and get down to the nitty-gritty of what one is and how it works.
What is a 555 timer then?
A 555 timer is possibly one of the most popular ICs to ever be made! It is used in a wide variety of timer, pulse generator and oscillator applications and can be used to provide time delays both as an oscillator and a flip flop element.
The 555 has 8 pins and is a dual in line package (DIP).
The general schematic layout for a 555 timer is as follows.
Supply is at the top, GND is at the bottom, the inputs are to the left and the output is on the right.
The 555 timer has three key functions: monostable, bistable and astable.
Monostable functions as 'one shot'. Applications of this type include: timers, bounce switches, touch switches, frequency divider, capacitance measurement, pulse width modulation and missing pulse detection.
Bistable mode, or also known as the Schmitt trigger, can also operate as a flip-flop (changing between two stable states) as long as the DIS pin is not connected and there is no capacitor used. This mode can also be used to make multivibrator circuits.
Astable In this mode the 555 timer can act as an oscillator. There are many uses for this, which include: pulse generation, tone generation, pulse position modulation, logic clocks, LED and lamp flashers, as well as many more. This operation of a 555 timer can also be used as a simple analogue to digital converter. There are many more applications for the use of an astable 555 timer, however, this list keeps going on!
I found a really good book which is free to download online called Engineers Mini Notebook – 555 timer IC circuits by Forrest M Mimm. This book runs through many different examples of uses and applications, providing the schematics.
To get a better understanding of 555 timer circuits I decided to have a go at building the monostable touch activated switch application.
Image source: Engineers Mini-Notebook – 555 Timer IC Circuits
Above is an extract from the Engineers Mini-Notebook – 555 Timer IC Circuits, showing the hand drawn schematic for how to wire up a touch activated switch using a 555 timer, where it also shows the effect that touch has to the output pulse.
I assembled the touch activated switch on breadboard and connected an LED with the cathode to Pin 3 (output) and anode to pin 6 (VCC). Following this the LED should blink when the contacts are touched.
The circuit was connected the GND and VCC rows on the breadboard, with 5V being supplied from the bench top power supply. When the circuit was first powered up the LED illuminated and stayed this way until pins two and five were touched, which caused the LED to blink.
Next I wanted to try a circuit which was a little more fun and decided to construct the Toy Organ project. This uses seven push buttons each with a capacitor in series and connected up to the 555 timer. There is also an 8 Ohm speaker connected to the output. Using different values of capacitor changes the frequency of the tone generated.
Image source: Engineer's mini-Notebook – 555 Timer IC Circuits
For the values of the capacitors I chose to pick seven of every other one ranging from 0.15uF to 0.0015uF in the table in the bottom right hand corner.
Instead of using a 100K potentiometer between pin 8 and 7 I decided to use a 100K fixed resistor. I also chose not to include a 1K potentiometer between C8 and the speaker for volume control. When I first wired up the circuit on breadboard I had made a few wrong connections to the switches, which meant when I pushed a button the tone was constant and did not change.
To fix this I altered the connections to the switches and reconnected the power supply, following which the circuit worked as it should!
The circuit looked a little messy due to the way I had wired the switches, but I managed to tidy this up when I altered the connections to the switches. The circuit worked and I had a miniature electronic organ using a 555 timer!
I think if I were to repeat this project I would alter the values of the capacitors used. The values I selected produced a very uneven tone between the buttons and also some of the tones were almost ear piercing!
555 timers are pretty cool, I can see how they are included in so many applications and why they are so popular. This will almost certainly not be the last time I use 555 timers!