Build Instructions

Collapse

• 1

  Understanding the Basic building blocks of the 555 Timer

  Block diagram of the 555 timer.

  
  

  Below is the original Component level circuit of the 555 timer designed by sir Hans Camenzind

  ( not my circuit )

  I will modify the above circuit and optimize it for discrete components

  If we look at the above circuit which is the actual circuit of the 555. But when tried to reverse engineer it and try to understand it , first off all its very confusing, I mean why using two different types of comparators one NPN based and other PNP based.
  

  So I decided to ignore the 555's original circuit and learn the fundamental blocks of 555 form scratch and redesign it for discrete component form.

  The first and the most important circuit of the 555 is the two voltage comparators. Which brings me to step 2. understanding the differential pair.

• 2

  The Voltage comparators

  Reference - For designing these differential pairs to serve as a very basic voltage comparator i found a gem on YouTube for understanding the internal working of Op -amps and fundamentals of differential pair - w2aew YouTube channel video on long tailed pairs. ( Not my video/Channel  ) . He also has many fundamental videos on electronics for any beginner like me . Very very nice channel on YouTube ,his way of explaining things is just outstanding and very different.

  Firstly I grabbed some random NPN transistors form my parts bin and just tested the behavior of the simple long tailed pair - in this case used some S9013 NPN BJTs
  

  I used NPN because I am much more comfortable working with NPN designs

  I was surprised at the common mode input response. The output was quite balanced but not as good as a real op amp.  

  Even if the two transistors are not matched ,as per my experimentation on breadboard  it works fairly well as a voltage comparator and we only want to use this circuit to operate so that it slams the power rails 0V and the supply rails ( Digital output ) so matching here doesn't  make any significant difference. ( But for analog application it is a must to precisely match those transistors.)

  So if you plan to build a DIY voltage comparator out of discrete components it is not important to match the diff. pair transistors.

  But there are many problems in the above circuit -

  • Output doesn't remain stable if common mode level is changed. Also by varying Vcc the steering current changes.
  • The gain of the above long tailed pair is low
  • input impedance is very low
  • No single ended output 

  Lets solve them one by one-

• 3

  Adding a Constant current sink instead of tail resistor

  Here I used a classic BJT constant current sink instead of the tail resistor. in this case about 1.5 to 1.7 mA tail current  depends how the BJT is biased.

  After the constant current sink the input impedance got slightly higher that is what we want.

  Also by changing common mode level the output is slight more stable than before.

  And as a plus point by varying Vcc the steering current remains constant.

View all 13 instructions