Something happend while building ttl gates

[Dhruvramani]

I am studying in 10th grade and was building a binary counter with an XOR gate and an AND gate.. The half adder (XOR) was working properly but the and wasn't working properly.. I've build many and gates before but this was the first time.. Then I took the transistors out to check them and build a separate and gate not connected to XOR gate.. It was responding to base input of only 2nd transistor while the base input of 1st was 0.. The led light up when input of only 2nd transistor was 1 while the base input of 1st was 0.. Then I removed the first transistor and I saw the led was still emitting light.. Then I removed the collector wire and +9v was to the base and led to emitter going to ground.. But the led was lit..

Can you tell me how this is happening?! Picture attached below!

 

[(*steve*)]

It's kinda hard to figure what you're doing there, but I suspect that you're powering the LED via the resistor and either a base-emitter or a base-collector junction (assuming that's a bipolar transistor).

You need to draw us a schematic of what you're trying to do.

 

[Dhruvramani]

Okay.. Well I was just building an AND GATE with an n
NPN bc 548 transistors.. But it was responding to input of the transistor connected to led (turned on when the input of this transistor went high and both went high)
.i learned that a transistor has to be connected to collector base and emitter to work but here only the emitter and base are connected and its working..
My question is how is this possible..

 

[(*steve*)]

It's kinda hard to figure what you're doing there, but I suspect that you're powering the LED via the resistor and the base-emitter junction.

You need to draw us a schematic of what you're trying to do.

 

[Dhruvramani]

Here it is.. This is what I've done..

 

[(*steve*)]

Yes, the current flows through the base-emitter junction (it's just like a diode), then the resistor, then the LED to ground.

If you want help with the XOR and AND gates, show us the schematic for them and we'll try to tell you if you have it right or not.

 

[Harald Kapp]

YOu're operating the transistor as a diode using the base-emitter diode.
I can't see any sign of an AND gate here.

 

[Dhruvramani]

This is what I wanted to build but that and gate was working like a diode can you tell me what to do..

 

[(*steve*)]

The 2 input AND gate needs at least 2 transistors, the 2 input XOR gate, many more.

The construction on this page is simpler, but beware that the logic levels deteriorate.

 

[Dhruvramani]

Hey thanks for that!!

 

[(*steve*)]

This may be more useful because it uses gates that feature gain rather than loss!