# Designing logic gates without transistors

#### Sebastian Aste B

Joined Apr 10, 2019
2
Hi, this question is for a design course im taking, any help is aprecciated !!

My homework is about designing in a circuit the logical function : (A OR B) AND (C OR D) (being A,B,C,D our inputs; 10V or 0V )

From the boylestad theory circuit book; i took the OR and AND circuit (attached files)
We simulated the logical function adding leds to know what is happening in the circuits

Our big problem is that; ignoring the voltage that leds requires, our output voltage when has to be 10V its always like 5-6V (we know its because of R1 and R2 , But we dont know how to fix that)

We also noticed that our LED 5 and LED 6 are being powered when they are mean to not (when the inputs are 0 in example)

i think we have a problem connecting the AND with the two separate OR circuits, because the OR/AND gates works fine separately

i hope someone can clarify my questions, we are really rookie in circuits

PD: we changed the 1k ohm (from boylestad OR and AND circuits) from R1 and R2 because that way the Vo in R3+V5 was closer to 10, and also 220 was meaningful enough to dont mess up with the leds voltage

#### WBahn

Joined Mar 31, 2012
26,398
What's out to the right of the circuit? Another LED and resistor as the final output?

One of the problems you have is that when the inputs are all low, you are relying on R1 and R2 to pull the inputs to the final AND gate low. But the very presence of the LEDs means that they can't go any lower than what is needed to turn on an LED and then the diodes D5 and D6 make the voltage at the top of R3 more than enough to turn on that final LED off to the right.

Connecting these types of primitive logic circuits is rather tricky -- there's a reason we don't use them except in special and isolated circumstances.

#### Sebastian Aste B

Joined Apr 10, 2019
2
What's out to the right of the circuit? Another LED and resistor as the final output?
OH SORRY thats just a voltmeter , thats supposed to be our "Vo" being "10 or 0"

#### RBR1317

Joined Nov 13, 2010
595
Suggest encapsulating the diode-logic circuits within appropriate gate symbols. Then construct the required logic function using the gate symbols. Unless LED indicators are required for the assignment, recommend using voltmeters to monitor circuit function instead. Cascading diode logic is not simple because the logic levels can shift with each cascade.