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Old 13th Nov 2018, 4:49 pm   #1
Studio263
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Default Traffic lights - ideal Christmas present!

Every little boy likes to play with cars, but how much more fun that would be with a proper set of traffic lights!

Having seen some coloured LEDs on the workbench, my son asked me to make him a traffic light. I wondered how this could be done, to the proper sequence (see the Highway Code), using as few components as possible. Of course it would be easy to write a programme for a PIC, but how much does one learn from that? This circuit is also easy to replicate even if you don't have programming facilities.

Having drawn up the sequence in full, I then used the standard simplification techniques to reduce the number of gates. The circuit was then re-drawn to use only one kind of gate, reducing the number of IC packages required. While doing this, I realised that a second set of lights (as for a road junction) could be added very easily, since they mostly do the opposite of the first one. The NAND gate is the universal workhorse of all basic logic projects so I used those, initially in two 4011s (quad two input). This required two packages to perform all the functions, but one 4023 (triple three input) can nearly do it in one. A 2N7000 MOSFET (Q3) is then needed to serve as an inverter to make this work though. The 4020 provides a binary count, it is clocked by a simple astable circuit with two BC548s (Q1 and Q1).

All these things can be easily found in the typical junk box, if not they cost only pence to buy. The circuit runs from 4 'AA' batteries, alkaline ones from the pound shop are just fine (these will last much longer than the PP3 alternative). I've set the rate to 'scale' and to accommodate the attention spans of small children(!), it can of course be varied by changing the value of R3 or R4. I made my prototype up on a Veroboard offcut, also from the junk box (see photo). This then goes into a plastic project box, some of which have the battery holder already built in.

Each LED goes between its respective output and the positive supply (+6V), that's why the outputs are shown as inverted (/R1 for example). This makes the circuit simpler and avoids the need for a buffer - these ICs are better at sinking current than they are at sourcing it. /R1 and /G1 are red and green for the first light, /R2 and /G2 are red and green for the second. Both amber LEDs (/A) are in series as they come on at the same time, this saves a little bit of battery power. Wooden blocks and dowels painted black serve to easily make the bodies of the lights, 5 pin DIN connectors on the ends of the cables make them easy to pack away neatly come bedtime.

Older children may enjoy making up the circuit too, and it could also be of use to railway modellers. Is there a way of doing the same thing even more simply?
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Old 13th Nov 2018, 4:56 pm   #2
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Default Re: Traffic lights - ideal Christmas present!

Clever stuff as usual!

I can remember my physics teacher at school doing this with ex-GPO relays which the technician had mounted on little wooden pattresses with spring terminals. The whole affair covered most of the bench and was very unreliable, but it was a real talking point for us. We must have been about 10 at the time.

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Old 13th Nov 2018, 5:12 pm   #3
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Default Re: Traffic lights - ideal Christmas present!

A related problem/puzzle :

There are clearly 4 states to a single traffic light. Given the 3 light emitters (bulbs or LEDs, it doesn't matter which) and a couple of SPDT relays (or switches if you don't mind flicking them by hand) and of course a suitable power supply for the light emitters) can you come up with a way to wire them to the relays _with no other components_ so each of the 4 possible states of the relays gives a different (valid) traffic light state.
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Old 13th Nov 2018, 6:03 pm   #4
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Default Re: Traffic lights - ideal Christmas present!

What about the (short) delay period in a real life junction traffic light set where every light is red to mitigate against orange light jumpers meeting mid junction? Or the much longer one needed for roadworks alternate single lane control?

Then there's the crossroads with four way control where only one direction is allowed to proceed at a time.....

Lots of scope for ever more complex logic!
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Old 13th Nov 2018, 6:33 pm   #5
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Default Re: Traffic lights - ideal Christmas present!

Quote:
Originally Posted by TonyDuell View Post
A related problem/puzzle :

There are clearly 4 states to a single traffic light. Given the 3 light emitters (bulbs or LEDs, it doesn't matter which) and a couple of SPDT relays (or switches if you don't mind flicking them by hand) and of course a suitable power supply for the light emitters) can you come up with a way to wire them to the relays _with no other components_ so each of the 4 possible states of the relays gives a different (valid) traffic light state.
Yes.

Connect S1 common to line or + and S2 common to neutral or -.
Connect red from S1A to neutral/-.
Connect orange from S2B to line/+.
Connect green from S1B to S2A

1A 2A > red
1A 2B > red orange
1B 2A > green
1B 2B > orange


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Old 13th Nov 2018, 6:42 pm   #6
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Default Re: Traffic lights - ideal Christmas present!

Yes, that's the solution I found. I first thought of it when I got a couple of HP59306 relay actuators (HPIB-controlled boxes containing 6 SPDT relays).

It doesn't seem to be that widely known.
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Old 13th Nov 2018, 8:12 pm   #7
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Default Re: Traffic lights - ideal Christmas present!

I remember doing something with traffic lights connected to an Economatics smart box at school, we had an old demo thing made up that (I believe) one of the old control technology teachers had made. Of course, this used simple flowchart logic on the computer and no actual circuitry so a lot simpler in that respect.
I do remember having to reverse all the LED's as the product it was originally designed to work with had the opposite polarity and one side of the LED's were connected together with one side of the smart box output being un-switched and thus you could only switch on/off all the LED's rather than controlling each one individually.

If anyone was wondering the Economatics smart box (which I believe is still a current product) is basically an output box for the PC/Archimedes possibly BBC Micro originally), with basic inputs and outputs, sensor inputs and a few motor outputs all controlled with a piece of software that used flowcharts as the main programming method. Not cheap but good for learning basic programming and with primarily banana jacks for I/o easy to connect stuff to.


As for simpler, a 3 way switch could work, with the green on one light connected to the red on the other combined with the amber lights on each side wired to the middle contact, or you could use a standard ON-OFF-ON with a NOR gate wired to the red on each side for turning on the amber lights when the switch is in the off position.

This would give position 1 as Green on one side and red on the other, position 2 as amber on both and position 3 as the reverse of position 1. Adding amber and red on one light together could also be possible with a 6 position 2 way switch (which is which can return to position 1 directly from position 6).

Apologies for crudity of drawings.

See attached diagram for more info. The ambers in both scenarios cane be connected together as they are only ever on together.
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Old 13th Nov 2018, 8:30 pm   #8
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Default Re: Traffic lights - ideal Christmas present!

Oh dear not only are the ambers not on together red amber is 2 seconds and amber leaving is 3 seconds.

In the main the period between greens is 5 seconds 3+2 or more, usually more.

Some sites have an intergreen of 4 seconds which means the amber overlap by 1 second but this is not common.

I know only a few sites like that mostly roundabouts.

Cheers

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Old 13th Nov 2018, 8:36 pm   #9
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Default Re: Traffic lights - ideal Christmas present!

I remember, as an apprentice, making up a set of model traffic lights as an excersise. We used a power transistor, pulsing a relay every second, which drove a uni-selector, wired to give the correct sequence for the lights.

How times have changed!

All the best.

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Old 13th Nov 2018, 8:53 pm   #10
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Default Re: Traffic lights - ideal Christmas present!

Actually they have not changed as much as you might think.

The basic timing structure and safety arrangements are the same in a 1937 controller as they are today.

The difference is that a 1937 controller designed for 2 to 4 traffic movements today it's 32.

I am currently restoring a 1937 controller with 3 traffic movements ex Newcastle for the Poole museum.

Oh and by the way the 1937 controller was vehicle actuated.

It had min, max, intergreen just like a 2018 controller.

Cheers

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Old 14th Nov 2018, 1:30 am   #11
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Default Re: Traffic lights - ideal Christmas present!

Old traffic lights used to have a rubber tube to sense vehicles but modern ones have "slug trails" instead.
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Old 14th Nov 2018, 9:37 am   #12
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Default Re: Traffic lights - ideal Christmas present!

When I was young there was a waggonway that connected two pits (coal mines) locally. This crossed the road about 200 yards from where we lived. There was a set of traffic lights - just red and green - that was triggered by an approaching line of tubs.

No barrier, of course. No orange.

Different times. Fewer vehicles.

Those lights went by 1960 after closure of the Stargate and Ryton pits.

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Old 14th Nov 2018, 10:08 am   #13
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Default Re: Traffic lights - ideal Christmas present!

The pneumatic tubes take me back

By the time I started in traffic in 1978 the only ones still in use in my area were on the Tamar bridge the rest had all been replaced by inductive loops, the slug trails as ref calls them are the bitumen backfill of the slots the loops are installed in.

Changing those tubes was a right pain they were in a cast iron frame and they were a tight fit .

Despite many other technologies including video and radar as well as magnetometer the humble loop is still the mainstay of the industry. Reliable and accurate unaffected by wind snow rain and pedestrians.

It's weak point is roads breaking up and people digging up the road.

Cheers

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Old 14th Nov 2018, 12:06 pm   #14
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Default Re: Traffic lights - ideal Christmas present!

Hi,
I made one of these in 1990 for my son, attached is my log book entry for 12Nov90 so I guess it must have been a christmas present. I recall it was a big thing about 2 feet tall but I cant recall if the sequence was accurate!!

Just a couple of diagrams , I think it drove VMOS switches to lamps not shown.
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Old 14th Nov 2018, 12:22 pm   #15
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Default Re: Traffic lights - ideal Christmas present!

In the 1950's I had a toy traffic light that was a replica of the temporary lights used at road works, with a pyramidal base painted with black and white horizontal stripes that held a 4,5V flat battery. Two bubs in the head, with windows facing opposite directions and a switch on the base to select either. One bulb had a red lens pointing one way and a green lens pointing the other way, the other had the colours reversed. Simple, and worked OK, but of course the colours were the wrong way round on one side.
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Old 14th Nov 2018, 3:06 pm   #16
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Default Re: Traffic lights - ideal Christmas present!

Quote:
Originally Posted by Cobaltblue View Post
The pneumatic tubes take me back

Despite many other technologies including video and radar as well as magnetometer the humble loop is still the mainstay of the industry. Reliable and accurate unaffected by wind snow rain and pedestrians.

It's weak point is roads breaking up and people digging up the road.

Cheers

Mike T
Those loops are also used to switch on the brakes on supermarket trolleys.
If they are laid in new tarmac they become at very high risk of the new surface being dug up if the smell of the new tar gets mistaken for a gas leak.
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Old 16th Nov 2018, 5:07 am   #17
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Default Re: Traffic lights - ideal Christmas present!

I can’t help thinking that one or two people are taking this a little too seriously - it is only toy for a young boy, after all!

When I was Tim’s son’s age, I used to make model record players out of Lego, Meccano, cardboard boxes and anything else suitable. I will confess here and now that the tracking angles and playing weights may not have been 100% accurate!
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Old 16th Nov 2018, 7:47 am   #18
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Default Re: Traffic lights - ideal Christmas present!

Traffic signal toys have probably been around as long as the signals themselves.

This is one in my collection.

It's probably from the 1950's

It promoted itself as an educational toy.

Cheers

Mike T
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Old 16th Nov 2018, 1:01 pm   #19
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Default Re: Traffic lights - ideal Christmas present!

Then there is the ones for boats that are red for ages while the swing bridge is use by road traffic.
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Old 18th Nov 2018, 4:33 pm   #20
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Default Re: Traffic lights - ideal Christmas present!

I remember this being a standard exercise for the microprocessor programming part of the course I did a number of years ago. It would be a good exercise for anyone now wanting to learn basic functions with an Arduino (without cheating lol).
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