How to build AND Gate using transistors?

Question

I am very new to electronics field. Especially to the digital electronics.

I am trying to make an AND gate circuit using transistors, switches and LED. I have tried the below circuit:

^{simulate this circuit – Schematic created using CircuitLab}

But there is a problem when simulating this circuit.

SW1     |     SW2     |     LED
OFF     |     OFF     |     OFF
OFF     |     ON      |     80%
ON      |     OFF     |     20%
ON      |     ON      |     100%

As you can see that above mentioned truth table is not AND Gate but it looks more like OR Gate. So, what is my mistake??

Answer 1

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.

Answer 2

The Forward biasing of Q2's BE junction when SW2 pressed, is causing the LED to glow.

You can try below,

1. Relocate the diode D1 between +ve terminal of the power supply & collector of Q1 (i.e D1's Anode is connected to +9V & Cathode connected to Q1's collector).

Answer 3

As colin__s already stated in the comments you can best move the led to the collector of the NPN transistor. When using an NPN transistor as a switch, you can best use it to pull-down current (connect the emitter to the lowest potential and the switched element to the collector).

Futhermore an LED usually can handle a maximum of 20mA, so I would increase the series resistor to 470 (so you get a lower current, (9V-1.6V)/470 = 15mA)). With the 300 ohm resistor your current would be >20mA.

^{simulate this circuit – Schematic created using CircuitLab}

In this way the current flowing trough the base-emitter junction doesn't light the LED when only one of the switches is on.