I don't want to get into all of the ins and outs of how the 555 timer works, so if you know, great! If you don't know here is a good tutorial with lots of theory and operation information. This basic operation allows the 555 timer to operate in various ways with various configurations. When the threshold swings higher than 2/3 Vcc (its reference voltage) the comparator Resets the flip flop, which pulls the output low and turns the discharge on. When the trigger voltage goes below 1/3 Vcc (its reference voltage) the comparator Sets the flip flop, which pulls the output high and turns off the discharge. From the flip flop there is the output stage which leads to the output pin and the transistor that controls the discharge.These are the basic parts of the 555 timer. Although it doesn't say on the image, the flip flop happens to be a SR flip flop. The outputs of the comparators lead to a flip flop. One input of the Threshold comparator is at 2/3 Vcc and one input of the Trigger comparator is at 1/3 Vcc.
We can also see that they are connected to a voltage divider.
In the first picture we can see that there are two comparators, one on the trigger pin and one on the threshold pin. So what the heck is going on inside the 555 timer? Well here are a couple of schematics from the National Semiconductor datasheet to help explain it. Now that I've hopefully convinced you somewhere within that intro, lets get started! You will break the black plastic barrier that stands between you and your integrated circuits and see the circuit in all its glory (well almost, making your own transistor could be difficult)! If you like working with electronics, especially breadboarded electronics, this should be a fun little project. Now, you can read about it or even see a simulation, but nothing it quite as good as actually making it yourself. It is used very often so it is important to understand how the thing works. That's why it is the number 1 most produced chip. The 555 timer is a very versatile and useful chip. The 555 timer combines both digital and analog circuitry and while digital circuitry is taking over, analog is still important. Learning may be a scary thought to some people, but this project has taught me a lot about comparators and analog circuits as well as a lot of the basics of the 555 timer. This instructable will show you how to build your own 555 timer using only transistors and resistors, no ICs! Why build this? It is the most popular IC of all time having been around since 1971 and now selling over 1 billion annually. A chip that can act as an oscillator, a schmitt trigger, PWM driver, a siren/alarm, a light or dark detector, and much much more. A chip so versatile that it has been used in everything from toys to spacecraft.
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