Hello! I've been working on an XOR gate made from transistors on a breadboard using the circuit design in this video and have been having some issues:
and have made several other logic gates and they have all worked fine. I measured with my voltmeter to make sure all the wiring is right, and I am pretty sure it is(the reason why I am not attaching a photo is due to the fact my camera is very low res and you can't see anything). The problem I am having however is that the LED does not activate when only Button B is depressed, that being said, the NAND gate works and when both buttons are depressed the LED turns off. Also, the LED turns on if Button A is depressed
, so that part of the circuit works. My proposed solution would be to wire up another AND gate for Button B, that being said, I would have to order more to do that( I only have five right now). If anyone has an idea as to why this is behaving in such a manner, I would greatly appreciate it.
Note: I'm using all 1K resistors and a 6V (4AA battery) power source.
If only-A turns the LED on, then if it's wired correctly, and both transistors are good, how could only-B not work? Have you tried switching the transistors?
When B is high and A is low the left B transistor may "steal" current from the right B transistor base. It depends how the buttons are wired. I hope you have a current limiting resistor between button output and transistor base. Each transistor usually needs its own resistor.
I have a resistor from the +5V line to the button input, the input then goes directly to the transistors, I assume that's fine? Also would my solution work(adding an AND gate from the emitter of the NAND gate and the output of the OR gate?
The circuit fails when button A is open (no connection between 5V and the transistors) and button B is closed (5V goes to bases of the B transistors)? If so try to add a 1k resistor between the button and each base it controls. I think it could help.
I did that, connecting the output of the buttons to unused spaces on my breadboard, then connecting resistors straight to each base it controled(2 per button), and got the exact same output.
The basic trick for debugging a circuit like this, which is symmetric, is to swap components between
the two parts of the circuit. If the problem moves with the component, its damaged, if it doesn't those
components aren't the issue.
Of course here you'd have had to swap the connections to the resistors rather than the resistors themselves.
And of course the main approach to debugging is measuring voltages to check they are what they should
be, which means first understanding the circuit, and secondly being methodical and going through all
the nodes in the circuit checking.
