I'm pretty sure it was all "relay logic" which is basically 1 or more banks of relays, timers, and contactors, etc.. Nowadays, it's all "programmable logic" which utilizes a plc (programmable logic controller) which as you say is computerized. Using a processor and internal relays, timers, etc. that can just be reprogrammed instead of having to be rewired to make any changes. Lot less space involved and alot more easier to troubleshoot.

The really old controllers that are still common in some places are a synchronous motor that drives a set of cams. The cams can be broken off in segments to set green, yellow and red times. Micro switches are operated by the cams, microswitches drive relays. Total cycle time is determined by gear ratios.

The "next newer" generation of controllers uses various types of electronic timers, both analog and digital. The outputs drive a (usually) manufacturer specific electronic relay that consists of SCRs wired in opposition (older) or triacs (newer). The older ones use pots to set timing, the newer ones use thumbwheel switches.

Next newer is all digital timing, TTL and CMOS stuff. The outputs are through an industry standard "loadswitch", which is a standard size housing that contains 3 solid state relay modules. The loadswitches have a standard connector that is still used in currently produced equipment.

Practically everything since the 80s is some form of microprocessor based controller, more often than not a 6800 or 68000 type.

The cabinets contain a loadswitch per "phase". A phase is a group of lights that come on together. A simple intersection where there are 2 streets that cross can be handled with 2 phases. Left turns will require a phase each, and require the opposing phase to be wired separately. So, a 4 way intersection with 1 left turn is 4 phases, 2 left turns would be 6 phases. Ped movements require a phase each. A "full blown" 4 way with 4 left turns and full peds would have 8 vehicle phases and 4 ped phases.

The loadwitches are mounted in rack.

If the intersection doesn't use fixed timing it will have some sort of vehicle detection devices. The most common is a "loop detector" which is a device that compares 2 tuned circuits, one of which uses a wire loop in the road. The metal in a vehicle detunes the circuit and the comparator sends a detection call to the controller. This kind of circuit has "aged" well and everything from old transistor units to microprocessor types are in use. Detection is also accomplished with ultrasonic, radar, and video detectors.

Tha cabinet also contains a monitor that will put the intersection in flash if it senses opposing movements both show a green or yellow. It will also trip if it senses there is no red indication on a movement. The controller also puts out a pulse that indicates it is still timing. If the pulse disapears the intersection goes into flash. More sofisticated monitors check other things like line voltage.

The cabinet may also contain a "co-ordination controller" that will hold a certain direction green for a fixed time. The newer controllers have this function built in.

Many signals talk to each other and a central control and the range of equipment used for this varies from hard wired relay to twisted pair/phone modem to spread spectrum radio and fibre optic interconnects.

Many signals are equipped with a battery backup. These vary from something that will run the intersection in flash for a few hours to one that will operate the signal normally for extended periods (the indications are usually all LEDs in this case)

If the signal uses incandescent lamps the 12 inch sized heads typically use 150 watt lamps, 8 inch are around 70 watts, and ped heads are about 115 watts(the LEDs that replace them are about 10 watts for green and red and 20 watts for yellow). Loadswitches are usually good for around 20 Amps with one of the 3 relays turned on.

Thats what I was going to say.

Anyone got any graphics? Diagrams, photos, etc?

Relay logic fascinates me :)

itsunclebill's response will keep me busy digesting for about 45 minutes, at least. :)

LOL, unless Uncle Bill used to work with traffic light controllers, uncle bill has way too much time on his hands. I am, however, deeply impressed :) I have always found relays fascinating and useful and have used them in a variety of ways over the years...

... in a previous life (see the thread entitled 'Criminal Background Check'), I would have had plenty of time to analyze the contents of that box, and the public works guy probably wouldn't have been anywhere around to be seen...

... but I need to stay out of that realm, hence my asking the question on here instead. :)

Hmm ok thinking about where you might also find that sort of collection of relays etc, maybe you shouldn't even be asking here ;)

Hope everything else is going well for you :)

There are probably thousands of elevator systems using nothing but relay based logic installed all over the country.
There are still a lot of relay based functions scattered throughout pieces of equipment.
The hard start kit for an A/C condenser unit is a timer relay
with an extra capacitor to allow smoother starting of the compressor.
Gas dryers with a glow plug igniter often use the drop in resistance of the igniter to trip a relay that allows the gas valve to open.
Cars have increased the use of relays to allow the microprocessor to control the loads more easily.
Relay logic used to be covered in an electrical engineering class, but has been removed from just about every school I have seen in the past 20 years.
Throw in some diodes and you can do all sorts of logic functions with relays.
The electronic tube was viewed as a huge advance sine no moving parts were required to switch a circuit on and off.
Tubes have now been replaced with transistors that perform the same function.
One signal is used to control another circuits on/off mode.

I too have always found relay-logic to be incredibly fascinating. It's amazing what engineers of the time could accomplish without the use of electronics. I always say they were creating functional works of art. A few years back a friend of mine gave me a tour of the building he works in (1960s era high-rise). I could have spent hours in the elevator machine room just staring at the massive bank of relays. MG's humming, relays clicking... it was music to my ears! (Another fascinating piece of old technology is the mechanical "selector" used in pre-electronic elevator systems... but I think we're getting a little off topic for a DIY electrical board :-)

Here is a link that might be useful: Elevator selector pictures