Interface, do u mean the FET match the impedance or interfaces with the circuit and 555 timers output together better?
No, just do a search for "LOGIC FETS". The Vgs turn-on voltage is compatable with TTL and they require little current to tun on. Also some CMOS IC's require levels very close to Vss and Vdd. Adding a diode drop is too much.
the OP said:
How would u do the DC ground better?
How do you tell what's an AC ground and what's an DC ground? the ground symbols look the same
Mostly I would see a parallel cap to ground , that's when I know it's an AC ground
Chassis and Earth have two different symbols. Generally, in most systems, Earth and protective ground and some internal grounds are tied together at one place. There are times, however where one might want a clean low-current ground. Usually inductors and a resistor are used.
What MIGHT be happening in your case, is that any AC transients are coupled through the capacitor. The resistor makes the other side a cleaner ground.
Ground is a very mysterious concept to many people. In a hospital and other sensitive facilities, you may have a green for ground and also a green with yellow stripe for ground. There are ORANGE receptacles that are used for devices that need an "isolated ground".
This is analogous to signal ground and Protective ground (Earth).
It really is the right way to do things, but imagine every outlet in your house having a separate breaker, just to have the proper ground. What happens now, if a parallel circuit of outlets gets hit, it may take out devices in that circuit. If every outlet had their own ground, the devices would be safer.
the OP said:
Vz means the zeners voltage Yes, say the zener is 12 V
Zeners voltage + 0.6 volts = what? [so 12 V + 0.6 V from the OTHER diode]
There bidirectional clamps , protect the op amp from what? if the feedback resistor opens or shorts the bidirectional clamps can't do much right? so what are they there for?
OK, this might be harder to explain. Lets go simpler. The unity gain OP amp. The input is applied to (+) and there is a wire from the output to (-). Maybe simple to understand. Maybe not. In any event, the OP amp tries to make the (-) input the same as the (+) input because of feedback. Easy enough.
Now let's only apply a (+) signal, but put a reversed biased (4V Zener diode) in the loop. So, instead of a wire from out to (-) we have this diode. When the output gets bigger than 4V, the zener will start to conduct. Any voltage below that will effectively pass through. So, the Op amp could see a +12 V signal and clamp it at 4V to be compatible with say the input to TTL logic.
The Back to Back zener diodes are clamps. Almost like a bidirectional TVS diode. Because the diodes are in series, there is an extra diode drop.