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I asked on a forum some time ago about why my logical AND operation doesn't work and I received this diagram:
I don't use resistors, I use potentiometers to lower the voltage and I tried to adapt this diagram for potentiometers(wired with only 2 legs). Would this work in real life?:
EDIT: I'm asking if the second diagram would work in real life, an AND operation made with potentiometers in place of standard resistors.
The problem is that the original forum post you received does NOT work as an AND gate for the LED.
Since there are no base current limiting resistors, SW2 can turn on the LED with all the LED current flowing through the BE junction. SW1 does nothing.
If you want an AND function use something like this:
simulate this circuit – Schematic created using CircuitLab
R4 and R5 are optional, their only purpose is to handle the CB leakage.
DO NOT use potentiometers in the manner you show in your schematic, turning the pot to one end results in very low resistance and very high currents. Buy some resistors.
Not very well. Even if just S2 is closed, current can flow through the lower transistor's base emitter junction and light the LED. If the transistor isn't designed to handle the resulting base current, it could blow the transistor
I'm asking if the second diagram would work in real life, an AND operation made with potentiometers in place of standard resistors.
The potentiometers on the transistor base-emitter junctions are doing nothing.
NAND gates are very simple to make and since they have only one stage they have a short propagation delay (the delay between input changing state and the output changing state).
simulate this circuit – Schematic created using CircuitLab
Figure 1. An AND gate built with discrete transistors.
Table 1. Truth table.
A B NAND Inverted NAND
0 0 1 0
1 0 1 0
0 1 1 0
1 1 0 1
Figure 3. A CMOS AND gate. Why are NAND gates used to make AND gates in computers?
The AND gate of Figure 1 is not very good as the pull-ups are done using resistors and these would be slow in charging up the gate capacitance of the inputs it was feeding. That would limit the maximum speed of the logic.
The design of Figure 3 uses transistors to pull low and to pull high. These will have much lower resistance than the resistors of Figure 1 and so will facilitate high speed logic.
