Now it works like an and gate

AND gate if you watch the LED, but like a NAND gate if you look at the output voltage.

 

If you move R1, is there still a connection to +6V. If so, where does it go?

The collector of the first transistor is directly connected to the 9V.

 

I rebuild it and it stil works fine.

If you put R1 where it belongs, the circuit works; if you move it to somewhere else in the circuit, it does not work. OK. So, what is the question?

ak

 

The collector of the first transistor is directly connected to the 9V.

That will not work. If you think of the transistors as switches, and both inputs are high so both transistors are on, then they are trying to create a dead short across the battery. Depending on the condition of the battery, this may or may not destroy one or both of the transistors, but the circuit will not function correctly.

ak

 

AND gate if you watch the LED, but like a NAND gate if you look at the output voltage.

See circuit below.
If the swithes are off only the LED on the right lights up.
if push both switches the 2 LED's are on, only the LED on the right dims a little. How come that the LED on the right isn't going off completely?

 

Because the circuit with R4 and LED on the right is not a valid way to test the voltage on the collector. Use a volt meter instead.

Or use another NPN transistor as an LED driver feeding off the NAND gate collector.

 

hi g,
For the first condition
If the swithes are off only the LED on the right lights up.

Are you sure its not wired like this image.?

E

 

hi g,
For the first condition
If the swithes are off only the LED on the right lights up.

Are you sure its not wired like this image.?

E

the two leds light up.
and its not wired up like u said.

 

Because the circuit with R4 and LED on the right is not a valid way to test the voltage on the collector. Use a volt meter instead.

Or use another NPN transistor as an LED driver feeding off the NAND gate collector.

why is it not a valid way to test it? and how should the schematic look like if i use another transistor?

 

hi g,
For the first condition
If the swithes are off only the LED on the right lights up.

Are you sure its not wired like this image.?

E

Also, if i remove R4 the LED on the right goes off if i push the buttons.

 

You can do it this way:

 

hi gill,
I would suggest you don't get into the bad habit of leaving transistor Bases 'floating' ie: disconnected as shown in some of the posted diagrams.
A resistor from Base to 0V is recommended, say around 47k .

You should also consider what 'load' the Outputs are driving.
eg: if the NAND output is connected to another Gate, its possible that above D1 LED may be dimly lit due to the current thru R5 and Q2 Base to 0V.

E

 

Also, if i remove R4 the LED on the right goes off if i push the buttons.

I think TS is confused between AND gate output voltage and LED being turned on when the output voltage is low.

The LED on the collector side of the output transistor indicates the opposite of the output voltage because the output sinks current when the output voltage is low.

 

why is it not a valid way to test it? and how should the schematic look like if i use another transistor?

Analyze the currents in the LEDs in the two situations above.

In A, current flows in R1 and not in R4. Only the LED fed by R1 is lit.
In B, a lower current flows in both R1 and R4. Both LEDs will be dim.