On the 555 we have pin 6,2,3 with opamps but no feedback other than ground so I wonder how this can be connected at work OK because I see on Wikipedia it has 25 transistors but only 8 pins so maybe there are hidden connections to be made? Also only 3 modes are shown but no timer mode so maybe this is correct and something else must be done maybe some varying voltage on those pins do some function that is not in the documentation? Its amazing the chip has been around a long time but no information is provided.
closed as unclear what you're asking by PeterJ, Leon Heller, Olin Lathrop, Dave Tweed♦, Nick Alexeev♦ Jul 6 '13 at 16:19
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Because they are comparator circuits not 'op amps'.
There are THREE 5K (555) resistors in form a potential divider inside the chip. This provides the 2/3 supply and 1/3 supply references to the two comparators.
Pin 7 is a discharge transistor for the capacitor.
Pin 5 is connected between two of the 5k resistors and allows for an external change or modulation of the trigger values.
All of this information has been around since the chips were first designed.
Consider the astable circuit.
Pin 8 is connected to the positive supply and Pin 1 is connected to 0V.
Pin 4 - Reset- is also connected to the positive supply to enable the 555 to function.
The capacitor, C1, initially has no charge (0V). On powering up it is charged through R1 and R2. As it is already below the trigger level the output of this comparator doesn't change as the voltage rises.
The output, pin 3, will be HIGH.
When the voltage across the capacitor reaches 2/3 supply the threshold comparator switches the internal latch to 'reset',
The output (pin 3) switches to LOW.
It also turns ON the discharge transistor (pin 7).
The capacitor now starts to discharge through R2. The voltage falls across the capacitor until it reaches 1/3 supply when the trigger comparator 'sets' the output (pin 3) to high and the discharge transistor is turned OFF.
The capacitor is now charged through R1 and R2 until it reaches 2/3 supply and the whole cycle is repeated over and over.
A small capacitor (usually about 0.1uF) is connected to pin 5 (control) to smooth out any spikes in voltage on the internal divider. (the 555 produces spikes of current when it switches so decoupling with capacitors is important for reliable operation.)