digital-logictransistors
(source: electronics-tutorials.ws)
I recently constructed a NOR gate using the following schematic
but when one of the transistors is set to HIGH an led I connected to the output of the gate glows brighter, but the other one works just fine. What's causing this?
This is because of the base current. The current comes from the signal source.
Also, note that the demonstrated schematic is simply wrong. The proper AND gate has to amplify the signal, so with similar technique you can only build NAND element. If you need AND - simply make one NAND and then invert the output signal once more.
simulate this circuit – Schematic created using CircuitLab
No you cannot (in general) use just one resistor and short the inputs together, assuming that's what you are doing.
If you connect (short) multiple RTL outputs together you'll have a so-called wired-OR circuit (actually it will be an AND circuit) and that will appear to work (at least until all those output resistors in parallel become too low), and the inverter will give you a NAND output.
However, by connecting the outputs together they no longer have individual logic states and cannot be connected anywhere else and appear as the correct logic state. In other words the wired-OR (AND) function is occurring at the OUTPUTS of the gates that are connected together rather than inside the gate.
You think you are seeing a NOR function because open-circuit is actually appearing as a high (1).
Best Answer
It looks like you have a NOR gate now, albeit non-standard RTL input levels. TTL is two diode drops and HCMOS is Vcc/2 threshold. The saturated gain of a transistor is only 10 to 20. So choose R values for a gain of 10 for reliable current switching.
CMOS NOR Gate
This is a simplified CMOS NOR gate with fancy triangular inputs running at 100 & 110 Hz to give a 10 Hz beat frequency on the output. Purpose is to show the linear input threshold. This uses n and p Mosfets to give low impedance for both output states, although the high side usually has a resistor to current limit it.