# i need a help with a project about traffic light timer

Discussion in 'Homework Help' started by abuarab88, Apr 14, 2009.

1. ### abuarab88 Thread Starter Member

Nov 6, 2008
19
0
hi everybody
my Doctor asked my group 2 make
a project about traffic light timer
and i dont know what 2 do
i dont even have any idea about it
help me:
* the circut
* the tools
* how to work

and l"ll be gratefull
thanxxxxxxxxxxxxxxx

2. ### Darren Holdstock Active Member

Feb 10, 2009
262
11
The answer will depend on which country you live in abuarab88, as the sequence of lights varies. That aside, it's a popular mini-project to set students, and there should be many circuits scattered around the internet - go google. In my day there was a clock (invariably generated by a 555 timer) fed into a 4017 CMOS decade counter, the output of which fed red, amber and green LEDs via buffering NPN transistors. Diode ORing between the outputs can be used if red and amber (say) need to be turned on simultaneously during the sequence.

Over and above that you'll need to provide a more accurate spec; e.g. is there a requirement for the amber light to flash before turning to green, etc.?

3. ### PRS Well-Known Member

Aug 24, 2008
989
35
I've done this as a problem in sequential logic. You're going to have this light at an intersection and thus the need for red, green and yellow facing 4 directions, let's say 'Main Street' and 'A Street'. Consider just one of these streets. Their lights work together. That is, its two sets of lights do exactly the same thing. Same goes for A Street. So we really have 2, not 4 sides.

It's really the yellow light that makes for the complication of this problem. Here's what to think about:

Main Street ---------------A Street
Red ---------------------Green or yellow
Green or Yellow -----------Red

You need two clocks. One clock is for the Red and Green lights, the other is for the yellow lights. The first clock has a long duration between switching and is conditional upon the logic of the yellow clock, while the yellow clock has a short duration and is conditional upon the other clock.

This is sequencial logic and it is best to use a sequencer chip and NAND gates driven by the clocks. Make a Truth Table so that you can clearly define the situation, then the logic will come clear to you. It's a pretty neat project.