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ME: Having used the LM/NE555 in hundreds of circuits (spanning 4 decades and reading every data book/app note, etc.) it is my professional assertion of 2 facts: 1: You do not HAVE to connect pin 4 to Vcc, nor to Pin 8, ever. Connecting it is a complete waste of breadboard time, and PCB "real estate". Leave pin 4 Not connected --> unless you are USING Pin 4, in which case you won't be connecting it to Vcc anyway, rite? Note above that IMSAI guy IS using Pin 4, and by connecting it through a switch to Ground it RESETS the 555. (The 555 has an internal pull-up resistor (R10 - 82K) to Vcc.) Otherwise: Leave Pin 4 alone. 2: You do not HAVE to connect Pin 5 to a capacitor going to Ground(Pin 1). It is a complete waste of a component, breadboard time, and PCB "real estate". Leave pin 5 Not connected --> unless you are USING pin 5, in which case you won't be decoupling Pin 5 to Ground through a capacitor anyway, rite? Otherwise: Leave pin 5 alone. I stand by the above advise because how Pin 4 and Pin 5 are portrayed actually DO in schematics has fallen into the realm of copy-pasting in data books for decades: "That's the way we have always done it" ...and that pair of Pins are completely unnecessary extranoius connections to the FINE 555 chip design! (Unless you are running your 555 circuit inside of a Tesla coil winding...)
555 Breadboard Prototyping Tip: Cut Pin 4 & Pin 5 OFF off of a couple of 555's so that you gain 2 columns of breadboard space for your variable pots on 5&7, and the LED on pin 3. The 555 is really just a 6 pin part in 90% of applications!
You are right. The original document from Signetics in an app note says 'It is strongly suggested to bypass pin 5 with a 0.01uf capacitor to reduce false triggering in noisy environments". I've never seen it happen and always leave it off too. There is no pull up on pin 5 it is a floating base of a PNP transistor. Since no current in, it should not need to be tied to Vcc. It may be a problem with the CMOS version.
