Question - The Specification of Traffic Lights/Signals

[TerryJC]

Peter,

During our telephone conversation a week or two back we discussed the possibility of adding traffic lights / signals to the system to replace the white LEDs that were originally required.

I am about to finalise the design of the system and need to know the specification of the lights (or what they will be if they haven't been bought yet).

Can you provide this, or if not, an example of the type of lights that the Railway Team might buy?

[peter]

I can setup the coloured lights to run in two ways. The LEDs are 3v DC.



1. Either, a two wire system reversing the polarity.



2. Or, a three wire system with a common negative and the other two switching positive.




HOPE THIS MAKES SENSE

[TerryJC]

I can setup the coloured lights to run in two ways. The LEDs are 3v DC.

1. Either, a two wire system reversing the polarity.

2. Or, a three wire system with a common negative and the other two switching positive.

HOPE THIS MAKES SENSE

Well, not quite. Are you saying that for the two wire system, there is a circuit inside to turn on the red for one polarity and green for the other? If so that adds to the complexity in the Pi side of things.

Assuming the above, the easiest for me is the second choice. Do you know what current the LEDs take?

[TerryJC]

Peter,

With reference to our discussion at WMT this morning, could you please ask Jeff the following questions:

1. How he will wire the LEDs in the three wire system?
2. What current the LEDs operate at?
3. Do the LRDs have inbuilt current limiting resistors? (If not, I can add them at the Pi Interface (see [url]viewtopic.php?f=21&t=118#p1758[/url.)]

With reference to Question 2 and to clarify Question 3, I have designed the Interfacing circuit to cater for 4 LEDs (two Reds and two Greens), see Sheet 1 of the circuit (this may be how Jeff's three wire scheme works anyway). My assumption at the time was that the LEDs did not have limiting resistors built in, so the 110 Ohm resistors would provide up to 30 mA from the 3.3 V provided by the outputs of the 74HC4050 buffer IC. Clearly, if the LEDs are 20 mA types (as I suspect they might be), then I would have to increase the value of the limiting resistors to something like 178 Ohms, which is the nearest preferred value to the theoretical value of 165 Ohms.

On the other hand, if the LEDs have got resistors built in, then for 3 V devices the internal resistor is probably 150 Ohms. To protect the LEDs from the 3.3 V from the buffers, I would have to add 15 Ohms at the interface.

Does this make sense?

[peter]

according to Geoff the LedS are ones that we had already they dont have builtin resistors and he is going to test them but thinks they pull less than the 20ma

He is trying to get registered on the site so if you are asked he is Geoffrey Derry

regards
Peter

[TerryJC]

Peter,

Ok. I'll await his posting.

[TerryJC]

My assumption at the time was that the LEDs did not have limiting resistors built in, so the 110 Ohm resistors would provide up to 30 mA from the 3.3 V provided by the outputs of the 74HC4050 buffer IC. Clearly, if the LEDs are 20 mA types (as I suspect they might be), then I would have to increase the value of the limiting resistors to something like 178 Ohms, which is the nearest preferred value to the theoretical value of 165 Ohms.

On the other hand, if the LEDs have got resistors built in, then for 3 V devices the internal resistor is probably 150 Ohms. To protect the LEDs from the 3.3 V from the buffers, I would have to add 15 Ohms at the interface.

I'm afraid that I got my sums wrong when I calculated this. The thing is that I forgot to allow for the forward volt-drop across the LEDs. For a red LED that would be about 1.8 V and for a green one it would be about 3 V. That would mean that the resistors would have to be about 75 Ohms for a red LED and 15 Ohms for a green one.

All assuming that there is no limiting resistor in the devices of course.