Electronics > Beginners
Traffic light circuit
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NivagSwerdna:

--- Quote from: sokoloff on June 10, 2018, 07:18:06 pm ---
--- Quote from: Benta on June 10, 2018, 04:35:00 pm ---Don't know about Australia, but in Europe the traffic light sequencing is a bit more involved:

Green -> Amber -> Red -> Red/Amber -> Green
--- End quote ---
In Austria last week, I noticed an additional light phase:
Green -> Blinking Green -> Amber -> Red -> Red/Amber -> Green

--- End quote ---
Who thought up 'Blinking Green' in that context?  Isn't that is what Amber is for?  Maybe it was a bug in the controller s/w?  ;)
(Doesn't blinking green have an entirely different meaning in other places?, e.g. USA)
David Chamberlain:
Yes those posts above are technically relevant but as OP mentioned she is 7 years old!

When I was a kid of that age my dad (and it was mostly him) and I made traffic light project for a school fair. Ours was made of cardboard, dry cell batteries, wire, 12V incandescent bulbs, coloured cellophane, paperclips for switches and thumb tacks to fasten everything together. It worked well enough, everyone was impressed and because it was something I could understand - at the time - it kick started what has become a lifelong (so far) interest in electronics.

Coincidentally I've helped design actual traffic light controllers in a professional capacity so I look back on that memory with fondness. 

What I'm trying to say is please do not stifle a young mind with your own ego.
wraper:

--- Quote from: Rerouter on June 10, 2018, 06:46:58 am ---Flying, you missed where he said he didn't want to go to any large dip chips, specifically saying against the 4017

--- End quote ---
Then why do you want to use 555 at all? I don't see much difference between 8 pin and 16 pin DIP. It's not like it will save significant number of components anyway. You could use following circuit + single BJT invertor for each led as 2 out of 3 LEDs are lit each time. Or just build this running light as is. Which probably is better as don't show unnecessary complication to the beginner.



Zero999:

--- Quote from: wraper on June 11, 2018, 09:32:49 am ---
--- Quote from: Rerouter on June 10, 2018, 06:46:58 am ---Flying, you missed where he said he didn't want to go to any large dip chips, specifically saying against the 4017

--- End quote ---
Then why do you want to use 555 at all? I don't see much difference between 8 pin and 16 pin DIP. It's not like it will save significant number of components anyway. You could use following circuit + single BJT invertor for each led as 2 out of 3 LEDs are lit each time. Or just build this running light as is. Which probably is better as don't show unnecessary complication to the beginner.





--- End quote ---
That's also a good idea. Connecting the LEDs across the transistors should also work but is inefficient.


Another alternative is to add emitter followers, which also has the advantage of allowing smaller capacitors to be used, but it's more parts.
Ian.M:
Unfortunately the Jaycar XC3720 LED Traffic Light Module is common cathode.

The only 8 pin DIP and no MCUs constraints are making this project far far harder than it needs to be.   I strongly recommend getting a small breadboard and using normal DIP ICS of up to 16 pins, and normal wire ended components.   If breadboard + Snap-Circuits integration is required, get their snap to male pin jumper pack: http://cs-sales.net/snsetsc.html

Here's a 555 + 4017 traffic light circuit that will work with the Jaycar XC3720 LEDs.   It implements the full red, red+amber, green, amber and back to red sequence, with realistic timings for each phase.  LTspice sim attached.

It could easily be extended for two directions of lights on a full cross-roads junction with a red in both directions phase for the junction to safely clear.   It could also be extended so that some phases hold the 555 in reset so a button or sensor pulse on the other 4017 clock pin is required to proceed to the next phase.

I recommend using a CMOS 555 and a 74HC4017 as they are more forgiving of low supply voltage, and the Jaycar LED module is intended for 5V operation, with integrated resistors to suit, so the 6V max supply voltage for 74HC logic isn't a disadvantage.   If you were driving higher voltage bulbs, the classic CD4017 would be more suitable.

N.B. if using a CD4017 at a supply voltage above 6V, due to the transistors' reverse Vbe breakdown limits, the paralleled NPN emitter follower OR gating that drives the Red and Amber LEDs should be replaced with diode OR gating driving a single NPN emitter follower transistor for each LED.
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