digital-logiclogic-gatesnandschematics
Im using 123d circuits by Autodesk and I am trying to make a NAND logic gate.
When i simulate my circuit (one in picture) it does not work and the LED does not turn on. I am using 2 NPN transistors, 2 switches and 3 1k resistors. The battery is 9v. Can someone help me find whats wrong with it?
CMOS parts like the 4011 have a very high input impedance and will have a random value if left unconnected.
If you are connecting your switches between an input pin and the positive supply, you need a pull-down resistor (10K or so will do) to ensure that the input will be low when the switch is open. If your switches are between the input pin and ground, use a pull-up to the positive supply to make the inputs High when the switch is open.
Also, any unused inputs must be connected to either the positive supply or ground to ensure they don't sit at a "maybe" state and cause the input circuit of the gate to draw excessive current.
You don't need any resistors between the gates.
As one comment says, it is good practice to have 0.1 uF bypass capacitors between the power supply and ground at each IC - more important with flip-flops and other clocked logic than with simple gates, but it is good to get in the habit...
simulate this circuit – Schematic created using CircuitLab
So, what is my mistake??
The mistake you have made is that current passing through SW2 will pass through the base and out of the emitter and then through the LED. A BJT has a base-emitter junction that is a forward biased diode.
If you used MOSFETs instead you wouldn't have this problem because the gate is insulated from the source.
Best Answer
Your circuit is a strange mix of upside-down-ness. Figure 1 shows a more-likely-to-work configuration.
simulate this circuit – Schematic created using CircuitLab
Figure 1. The standard solution to this problem.
A few tips: