# Troubleshooting DTL NOT gate circuit design

My guide says that the following diagram is a DTL NOT gate. Its output is a LED.

For learning purposes, I tested it via a simulator. The measured current is not really significant to power on the LED. Opening and closing the switch does not bring enough difference to perform negation as the reading nearly stays at 0.974nA. My understanding of negation is that when I open the switch, the current must be higher. I first thought to have a junction between the switch and resistor connect to ground, but the expected result is that opening the switch will result in 0 A.

• WHAT switch? There is no switch on the schematic, so we can't tell how (or even if) the switch is connected.
– user16324
Mar 19, 2021 at 12:32

Opening and closing the switch

There is no switch in the original circuit, and this kind of logic does not work by opening and closing a switch.

Rather, the circuit works by bringing the voltage at the input pin "high", or bringing the voltage at the input pin "low".

When the switch is opened in your circuit, the input at D1 is "floating". In such a case, it will remain "high". The voltage from V2 will "pass through it".

[Or, more accurately, because there is no current through it, by Ohm's law, the voltage on each side will be the same. Now, you may say, "wait a minute, a diode is not a resistor, and no current flows through a diode when it is reverse biased". That is not quite true. A small "leakage current" does flow through a diode that is reverse biased, and a diode does behave like a resistor to the extent that, if there is really no current flowing through it, the voltage drop across it is 0.]

If you want to use a switch to provide an input for such circuits, use something like this:

simulate this circuit – Schematic created using CircuitLab

R1 is called a pull-up resistor, and R2 is called a pull-down resistor. The range of values that are acceptable depends upon the logic family that this circuit is supposed to be an input for. In this particular case, the values of the resistors can be 0. That is, you can omit them, and use wire instead. However, in practice, resistors are used to limit current and power consumption.