Problem with my logic gate

Here's an AND gate I built for practice and it's working with one quirk. Whenever I press the button closest to the LED by itself, the LED dimly lights up. I think there's enough voltage going through the transistor base/emitter to power the LED. How can I prevent that from happening? If it matters I have my calculations below. FYI I'm new to circuits so please talk to me like I barely know what all this means... Thanks in advance!



R1 Resistance

Vin = 9V
LED = 3V drop / .02A
Transistor 1 = .2V drop
Transistor 2 = .2V drop

R1 = (9V-3V-0.2V-0.2V)/.02A = 280 Ohms

Transistor Base Current

IC = .02A
HFE(MIN) = 200
Overdrive factor = 10

IB = .02A/200(10) = .001A

R2 and R3 Resistance

VBE(sat) = 0.7

R = (9V-0.7V)/.001A = 8300 Ohms

Move the resistor and LED to the collector.

Jeez that was way too easy. Here I am banging my head on the desk over my calculations. Thanks a ton!

You can also put a shunt resistor across the LED ... something like 2.2k or so. We see this a lot with the newer LEDs that are very sensitive to micro currents.

In your case the transistors BASE's are floating allowing them to behave like an antenna for anything attached to them. A resistor from the Base to the Emitter on each transistor will minimize this current as well. Anything from 10k to 100k should be fine.

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Beau Schwabe said:

You can also put a shunt resistor across the LED ... something like 2.2k or so. We see this a lot with the newer LEDs that are very sensitive to micro currents.

In your case the transistors BASE's are floating allowing them to behave like an antenna for anything attached to them. A resistor from the Base to the Emitter on each transistor will minimize this current as well. Anything from 10k to 100k should be fine.

Click to expand...

When you say shunt do you mean bridging the positive LED to ground with another resistor?

Bear in mind how a BJT transistor have an equivalent circuit as two opposite diodes:

That is you'll see a dim light from the LED - small current through R3, R1 and base-emitter of the rightmost transistor.

The 2.2k resistor shunt is directly across (in parallel) with the LED