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Electronic Project I (Part 1) – 555 Timers

Posted August 31, 2009 12:00 AM by Jaxy

Designed and invented by Hans R. Camenzind, the 555 timer chip is an integrated circuit (IC) chip that has the versatility to handle multivibrator and timer applications. It can include over 20 transistors, 15 resistors and 2 diodes in one 8-pin chip, depending on the company that manufactures it.

Pin Description of the 555 Timer

Pin 1: Ground – the ground for the chip

Pin 2: Triggers – triggers the input signal and starts the waveform.

Pin 3: Output – where signal is outputted

Pin 4: Reset terminal – forces the output to zero volts

Pin 5: Control voltage

Pin 6: Threshold – resets output waveform to zero volts

Pin 7: Discharge – discharges the capacitor charge to zero volts

Pin 8: Power – where the chip gets power

The Three Operating Modes

Monostable Mode

In this mode, the 555 timer becomes a one-time pulse generator. The pulse emitted depends on the value of a resistor in a capacitor. Changing these values can lengthen or shorten the pulse width, t, by using this formula: t = R*C*ln(3) = ~1.1RC The schematic of the 555 in monostable mode is presented at right. The image at left is an output time graph for the 555 timer chip during monostable operation.

Astable Mode

The output of the 555 timer in this mode becomes a steady stream of pulses. This stream of pulses can be considered an AC signal and has a specified frequency based upon the values of two resistors and a capacitor. The graph at left shows you different frequencies and allows you to choose values of the resistors and capacitor so you can achieve the desired free running frequency. Astable mode is going to be used in this project.

Bistable Mode

This is also known as a Schmitt trigger. This is where the 555 timer acts like a flip-flop which is just a term referring to an electronic circuit that has two stable states. When one of the inputs is triggered, it sets the output to a low state; triggering a different input changes the output to a high state. These are the two stable states.

Other Versions of the 555

There are different versions available for the 555, such as the 556. The 556 is a dual timer and features two unique 555 timers in one 14 pin chip. There is also the 558, which is the quad timer. Four 555 timers are in this 16 pin package and all modules share the same control voltage and reset lines. The discharge pin and threshold are internally wired together in the 558 timer.

Previous Blogs in Series:

Electronic Projects for Beginners – Components (Part I)

Electronic Projects for Beginners – More Components (Part II)

Electronic Projects for Beginners – Reading Components (Part III)

Electronic Projects for Beginners – Translating the Schematic onto a Breadboard (Part IV)

Electronic Projects for Beginners - Notes Concerning Chips (Part V)

Resources:

http://home.cogeco.ca/~rpaisley4/LM555.html

http://www.eleinmec.com/article.asp?1

http://en.wikipedia.org/wiki/555_timer_IC#Monostable_mode

http://en.wikipedia.org/wiki/Flip-flop_(electronics)

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Guru
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#1

Re: Electronic Project I (Part 1) – 555 Timers

08/31/2009 11:56 PM

"It can include over 20 transistors, 15 resistors and 2 diodes in one 8-pin chip". This raises a question in my mind. Obviously, not all 555's are created equal, and one would expect prices to vary depending on the internal configuration. As you note, there are a large number of vendors selling the basic 555 design, and my usual choice for my projects (being non-critical, non-life threatening experimental applications) is the cheapest version currently available. If, let us say, I build a project today that gives me the results I am looking for, then in a couple of years I try to reproduce the experiment, am I going to get different results if I switch to a different manufacturer's version of the 555?

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#2
In reply to #1

Re: Electronic Project I (Part 1) – 555 Timers

09/01/2009 7:57 AM

Provided you've gone for the "typical" values quoted in the datasheet for whichever manufacturer's offering you've designed around, you should be OK. There may be one or two % difference in the timing, input & output levels etc, but you could expect that between different chips from the same manufacturer, as there will be a spread of values of parameters - even within the same batch.

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