# TTL AND and OR logic gate

#### lex_ph

Joined Jan 15, 2006
5
Guys, my first post, I need help on my circuit. basically this circuit is the TTL AND & OR. On the circuit attached, you will find out that R4 and R8 are missing. I really don't know what resistance to use in order for the LED bulb (red) to light properly without being damage. Please help me guys I know this is easy, it's just that I'm really new to this and need more experience. Thanks!

PS. Nevermind the value of R8, I just place a random resistor there just to have a resistor, I still need to know what R8 should I use.

-Lex_ph

#### hgmjr

Joined Jan 28, 2005
9,027
lex_ph,

Try a standard 5% resistor values of 270 ohms, 330 ohms, or 360 ohms for R4 and R8. That will give you an LED current of around 10 milliamps assuming that the LEDs forward voltage drop is around 1.4 volts.

hgmjr

PS. You may need to reduce the 10K resistors in series with the base. 10K is somewhat on the high side for resistance in series with the base anyway. 1K should be sufficient.

#### lex_ph

Joined Jan 15, 2006
5
Ok, now I get it. The above circuit works perfectly on multisim, but I haven't tested in

Well actually, this circuit is just a part of a circuit i have to build. It is kinda
like a tool wherein one can use it to learn the basic concept of the basic logic gates
including the buffer. I attached the multisim file here so that you can understand it
better. (and some screenies too)

Now the problem is that the other basic logic gates, NAND, NOR, NOT, XNOR are
not working when i simulated it in multisim. I don't know why. One thing I
noticed is that when I measured the voltage drops across the outputs, i.e. accross the
LEDs, it seems that the values are LESS than the voltage drops of the outputs of the
AND & OR gates (which are already working when simulated).

Here are the screenies of the circuit that I made.

AND, OR, NOT, NAND, NOR, Buffer circuit:

XOR, XNOR:

And the circuit files for both of the circuits:

#### hgmjr

Joined Jan 28, 2005
9,027
I see the problem.

Take your NOT gate for example, You will notice that when Q9 is "off" the LED is getting its current through the 4.7K resistor and the 330 ohm resistor. This series combination does not provided enough current to lite the LED. Try making the value of R17 equal to 330 ohms then remove R18 and connect the anode of LED5 to the collector of Q9. By making these changes you will see that LED5 will lite when Q9 is "off" and when Q9 is "on" the transistor will short across the LED and turn it off. This change will need to be made on the other logic circuits that are driving the LED in the same configuration.

Good Luck,
hgmjr

#### lex_ph

Joined Jan 15, 2006
5
Thank you for the help. I'll try to check up on that and hope it will work.