Hi Guys,
I am not heavily into electronics, and are somewhat confused, and hope one of you clever guys can put me right.
I run an old layout convnverted to DCC, but with traditional 12V peco point motors with a switch attached which sends power to LED way indicators on the control panel. There is one pair of points wired in tandem, and the indicator LEDs got very dull, and I replaced them.
The query is that one pair of LED's light correctly when the point is thrown, but throwing the switch the other way and the two LED's will not light, but touch the input wire with meter. screwdriver. or finger makes them light, and stay on until the switch is thrown. Throw the switch again and will not light until touched. I admit to being at a loss.
Regards to all
Norman
I would suspect a dry joint in your soldering. Poking It makes the connection, try resoldering
it.
Andy
Thanks Andy, I will try that
While I fully agree with Andy, the 'tandem' raises my suspicion. Anything wired in 'tandem', i.e. one behind the other (what the experts call 'series'), splits the voltage as the resistance of all these items add up. If you have several point motors you want to switch with one switch, they ought to be wired in parallel, not in tandem. If your indicator LEDs (and the voltage-drop resistors wired in SERIES) are just one set for the pair of point motors, you wire those as you do for any other point motor, i.e. as if the 2 were just one.
I suggest you check your wiring, unless you're sure this was just a typo and your point motors are wired in parallel.
Just a thought
Waldschrat
Thanks for the input I will check solder joints and series/parrallel
If all your resistors are the same value, you will need one resister for every LED.
If you connect two LED
Thanks for that Brian, I do have one resistor per LED, but why would a red LED require a different resistor to a green one, arnt LED's the same but with different coloured covers?
Regards Norman
No. Red LEDS produce red light and green LEDs produce green light etc. You can get them in clear plastic housings. It is down to the physics. Blue light is more "energetic" than green which is in turn more energetic than red and blue needs more voltage than green and so on (the electrons have to "jump higher" and need a bigger kick - or rather fall further after receiving a kick). The more voltage the LED drops the less there is to drop across the resistor for the same current so the resistor value changes.
White LEDs are different in that they are blue or near UV and have a fluorescent coating that absorbs the light and re-emits it as white (a bit like a fluorescent tube). That is except for the Red-Green-Blue LEDs that can produces white or any other colour by mixing RGB.
Not sure, if it is me poor English, or some details are presented in a confusing way.
Why different resistors for different colour LEDs?
LEDs are Light Emitting Diodes, i.e. semiconductors that work like a diode, but where the choice of material combination triggers light to be emitted from the contact surface of these materials by the current passing through.
Note 1: diode is the operative word, i.e. LED conduct only in one direction like any other diode.
Note 2: the combination of materials produces a specific colour, i.e. the light is monochrome, typically with a quite narrow wave length.
Note 3: the contact surfaces of the materials have a certain resistance SPECIFIC TO THAT COMBINATION, as a result of which the resistance of different colour LEDs is different, as a result of which they have different operating voltages.
Note 4: the colour yellow is produced by 'mixing' red and green, i.e. these LEDs technically are a combination of a red and green LED.
Comment to note 1:
In the early days (gosh, was I excited when the first LEDs were available ;D) we were told you need to protect your LED with an anti-parallel diode, if reverse voltage is applied. I have ignored that and my LEDs have survived 12V for 30 years (occasional use). Nevertheless a sensible precaution if you can't be sure the voltage is applied correctly all the time. Yes, if voltage is applied the wrong way round the LED's resistance is almost infinitely high, i.e. the full voltage is applied regardless of resistor (and if high enough, can destroy the diode).
Such a diode may be a LED, i.e. with different colour LEDs antiparallel you can 'change colour' by reversing polarity, e.g. headlights switching to tail lights.
Comment to note 2:
The implicit of that is that any coloured 'glass' in front of the LED will only dim it, but not change the colour, e.g. a red LED will not produce blue or orange light by placing a coloured sheet in front of it. If the colouring is strong enough the LED will simply not shine through it! As there is no colour spectrum there is no choice of colours (=wave length) that can be filtered out.
'White' is a mix of all colours, i.e. on those LEDs 'colouring' with a tinted 'glass' will work, but how well, depends on the LED design (GreaneD detailed that well in the previous post); - practically all white LED do note have the full and uniform spectrum of the sun or an incandescent bulb, so the effects can be interesting.
Comment to note 3:
This is the reason why different colour LEDs require different values of resistors, if operated from the same supply voltage. However, the human eye perceives colour intensity different for different colours, e.g. a red, a green, and a blue LED shinning with the same brightness appear to us to be different bright! That is another reason different resistors are required; we see one colour brighter than the other colour, if they are operated with the same brightness.
However, it does not mean LEDs can't be operated with a common resistor. In most cases (on 12 to 16V supply voltage) the internal resistance in operating direction of an LED is minimal compared to the resistance of the resistor, and having up to about 5 small LEDs parallel behind a resistor does not produce a noticeable difference in brightness (the difference in resistance between 1 and 5 LED in parallel is less than the typical manufacturing tolerance of the resistor). Different, when wired in series; in that case the resistance across the LED changes markedly enough to be visible, i.e. a different resistor is necessary.
For all these reasons I recommend to test you planned setup (I use an adjustable resistor to determine the value I need, before fitting the fixed value resistor, checking with an Ohm-meter that the resistor actually has the right value).
Another recommendation is to dim the LEDs quite low; not only prolongs it LED-life, but at exhibitions you can see that practically all lighting on model railways is way to bright; - making the layout look like a funfair rather than the countryside or city-scape that is modelled. Even today locomotives and signals are NOT fitted with anti-aircraft search lights, even though the old 'Wild-West' engines with their arc - or acetylene lights came fairly close.
Comments to note 4:
That, of course, means that you can get either green or red with respective tinted glass from a yellow LED. But why the trouble as you can get dual-colour LEDs (essential 2 LEDs in one housing with 3 wires) where you can 'switch' and 'mix' the colour as you fancy, i.e. they produce all three colours depending on which of the 2 LEDs are on. And that's not even correct, as, depending on the resistor combination the yellow can be anything between orange and pale green.
Footnote: Most cheap resistors have substantial tolerances (10 to 20% region; 'cheap' mass production is the operative word), big enough to make even same colour LEDs shine at different brightness. You might have to try a couple to find 'matching' pairs. Thus, while Brian is correct with the pure physics, a nominal 360 Ohm resistor at PLUS 20% (=432 Ohm) and a 560 Ohm at MINUS 20% (=448 Ohm) are de facto the same; call it luck of the draw. :laugh:
Therefore, IF you need or want uniform brightness from several LEDs you need to either use more expensive resistors with a tighter tolerance (about 5% tolerance is below the visual perceptive level), or you use one common resistor with either LEDs in parallel or series.
Tip: To check if your infrared LED works (TV remote, track occupation detection etc.), switch your mobile to taking pictures (or use your digital camera), and look at the infrared diode on the screen; you will see it light up, even so you don't see that looking directly at the infrared diode.
I hope, I have now maximised the confusion. ;D
Have fun
Waldschrat