0
\$\begingroup\$

I am a newbie in electronics. I was trying to replicate this traffic control diagram:

my basis

I recreated the circuit in Multisim using 2 clocks and an AND gate, the 3 LEDs light up:

multisim circuit with and gate

If I replace the AND gate with a 7408 IC, the green LED does not light up anymore and I am not sure why.

I put the 2 inputs and 1 output wire according to its pin diagram and I connected the VCC and ground.

enter image description here

I can't figure out why the green LED won't light up.

\$\endgroup\$
4
  • 1
    \$\begingroup\$ Welcome to the EE Stack Exchange. As soon as you start using net labels in EDA software instead of routing wires, your confusion will immediately reduce by 50%. \$\endgroup\$
    – Syed
    Dec 14, 2021 at 9:22
  • 1
    \$\begingroup\$ The above comment is not generally true IMHO \$\endgroup\$
    – Andy aka
    Dec 14, 2021 at 11:28
  • \$\begingroup\$ Which IC are you using for the D-type flip flops? \$\endgroup\$
    – Finbarr
    Dec 14, 2021 at 11:54
  • \$\begingroup\$ Thank you all for your answers, it also now worked grounding set and reset \$\endgroup\$
    – elsa anna
    Dec 14, 2021 at 12:32

3 Answers 3

1
\$\begingroup\$

Forget about the 7408 and...

Ask yourself what the reset pin on the D type flip flop does when it is set high: -

enter image description here

Fix the basic circuit first.

\$\endgroup\$
1
\$\begingroup\$

Contrary to your assumption, the 74LS08 supply is not connected to the 5V VCC supply.

\$\endgroup\$
0
\$\begingroup\$

TTL logic is not a good choice for driving LEDs, particularly green, blue, or white LEDs.

TTL logic families have a very weak ability to source current and their output voltages for a logic 1 are significantly less than 3V. I would expect that your red LED would glow weakly but green would not be illuminated at all.

You also need to remember that trying to drive an LED with a TTL logic gate is likely to cause the gate's output voltage to be an invalid logic level. If you use the output of a gate to drive an LED then don't use that output as an input to any other logic gates.

The best idea is to use a completely separate driver for LEDs. Add an open-collector TTL chip like the 7406 or 7407 or use discrete transistors to drive each LED.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.