# Traffic Light Circuit Help!

Discussion in 'Homework Help' started by typingpal, Dec 20, 2010.

1. ### typingpal Thread Starter New Member

Dec 18, 2010
5
0
Hi I need some help on building a traffic light circuit. Below I have drawn a schematic diagram and I am unsure if it's correct or not. I have builded this on a breadboard using three AND gates (74LS08), 1 OR GATE (74LS32P), a INVERTER (74LS14N) 555 timer chip and a counter chip (74LS193N). This is runned by volts, by having a regulator already setup and working. The 555 timer chip is included with two 470K resistor and one 100 uF capictor. The objective of this circuit is the opposite of the traffic lights will turn on, for example if red north south is on then east west green will turn on etc. The A2, B3, C6 and D7 on the diagram is the counter chip. Please let me know if I had drawn this diagram correct! Thank you!

http://img576.imageshack.us/img576/7469/trafficlighty.jpg

Dec 10, 2010
44
3
3. ### typingpal Thread Starter New Member

Dec 18, 2010
5
0
Thanks for the reply, just one though

4. ### RiJoRI Well-Known Member

Aug 15, 2007
536
26
Remember, typingpal, almost everyone on here is doing this voluntarily. There is no staff to answer questions!

Your diagram is quite complex; it would help if you put in the truth table you used.

One thing to remember is that Rns = Gew+Yew, and Rew = Gns+Yns. Because you can derive the Red signals from the Green and Yellow signals, this will simplify the circuitry as well as the logic.

--Rich

Last edited: Dec 21, 2010
5. ### typingpal Thread Starter New Member

Dec 18, 2010
5
0
Thank you for the reply RiJoRi. I have got the circuit to work but I am unsure if its correct or not. All LEDS are flashing really quickly and it is difficult to see if I'm correct or not.

Here is the truth table. http://img535.imageshack.us/img535/1803/hpqscan0001un.jpg

6. ### zgozvrm Member

Oct 24, 2009
115
2
Another thing you might consider in the interest of safety...

Being a purely solid state circuit, there is the chance that one of the green "light" output gates could fail in such a way that it is always on. Therefore, I would suggest running the green light outputs through relays to prevent this (see attached).

Of course, you'll probably need to amplify the signal in order to drive the relay coils.

File size:
21.5 KB
Views:
60
7. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,838
961
In the real world, the failure modes that could cause green in two conflicting directions is checked with a seperate logic circuit which sends the intersection to flashing Red all around should there be a conflict in the light controller which violates safety rules.

Dec 18, 2010
5
0
9. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
I take it you test the circuit under a simulation software. Why don't you replace the clock with a push-button connected to Vcc? Usually the software will transmit 0 voltage from an open switch. That way you can check your circuit step by step.

Otherwise you can replace the 555 with a very slow pulse generator or actually design a slow 555 clock.

10. ### typingpal Thread Starter New Member

Dec 18, 2010
5
0
Thanks for the reply and help! But I am not allowed to change these parts because this is a assignment.

11. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
Sometimes a simulation package has its own eccentricities and tricks. Use a slow clock or push button to test wheather your gate circuit is correct, and if it is, search deeper into the software and the waveforms.

I 'm not saying to change the circuit permanently, just for testing purposes.

It is always a good method to break the circuit down to smaller ones and test them separately.

May 30, 2012
2
0