394Views1Replies

Author Options:

Transistor NAND Gate does not completely work.? Answered

I have been trying out some logic gates using transistors and I am confused already. I have built a NAND gate using 3 2n3904 BJT's. The resistor values are aprox the values in the schematic. The Diodes are 1N4003.

Vcc is 5v (~4.9v)

The schematic can be found here https://www.instructables.com/id/Digital-Logic-Gates-Just-Using-Transistors/

I have everything hooked up like the schematic shows. The issue I am having is that it is not working as I would expect a nand gate to work.

The truth table shows that if A and B are both Low then C is High. 
If A is High and B is Low then C is High.
If A and B are both High then C is Low

Here is what really happens. 

A Low: B Low: C High
A Low: B High : C High
A High: B Low : C Low (but still ON just very very dim) <=Shouldn't this still be a High state?
A High: B High: C Low

Does any one have any ideas as to why it is behaving like this. I have rebuilt this circuit about a dozen times now making sure I have things correct. I am stumped.

Discussions