[Savage]

Guys, I want to power my Lightbridge from a USB lead on the PC.

I have the IR emitters rated at 1.1 to 1.4v, and will want a coloured led to indicate power to the lights.

My question is, if I wire them all in series is it just a case of making sure they add up to approx. 5v ?

Would the 2x IR be OK with a coloured LED rated at 1.9v or would I need a resistor to lower the voltage to 4.5v ?

 

[GregK]

Hi Tel,

The voltage ratings are an indication as to how many of the little fellers you can safely send through the emitters without releasing the magic smoke. They are not cumulative, they are used to calculate the size of a dropping resistor, eg circuit voltage = 5V, emitter = 2V, therefore you need to "drop" the voltage by 3V. Even if your calculations show that the final value might be 0, you must still add a 0 ohm resistor to protect the emitter/LED.

Here is a link to an automatic calculator .......... :thumbsup:

 

[slotcrazy]

You also need to know the max current rating for the LEDs.

As it won't hurt to treat the indicator LED as if it had a lower voltage rating (the same as the IR emitters), work on say 1.2 volts each (which is very common), check that they can all handle say 20ma current draw, and use say a 62 ohm resistor with those 3 LED in series.

If you need less current, you will need a higher value resistor, if they can all handle say 25ma, you could drop to a 51 ohm resistor. 1/8th watt would work, they are dirt cheap, I usually use 1/4 watts resistors.

 

[Savage]

Thanks for the replies guys.

So the IR max forward current says 30ma each.

The LED says 20ma.

Assuming voltage drop on the calculator is 1.2v, same as the rating ?

The calculator tells me nearest rated 10% is 82 ohm, does that sound right ?

so from +ve wire I need resistor>LED>IR>IR>-ve. correct ?

Does it matter which way round resistor goes ?

Sorry for sounding flummoxed, I am an electronics doofus :lmfao:

 

[GregK]

Sounds OK to me ........ 100 ohm is probably easier to acquire. The resistor attaches to the +ve (longest) leg.

Another use link full of helpful stuff ...... :xmas:

 

[Savage]

Many thanks guys

Found some 82 ohm on the evilbay, so should be good.

 

[RichD]

Regular LEDs will not light at all at 1.2 volts. The IR LEDs that I have are rated for 1.3 volts and 150 mA (0.15 amps). In order to size the dropping resistors properly you will have to be sure about the current rating because the drop across the resistor is proportional to the amps. Red LEDs run at 2 volts, so you would need to drop 3 volts across the resistor. If the LEDs draw 30 mA they need a 100 ohm dropping resistor with a 5 volt supply voltage or a 150 ohm resistor if they draw 20 mA. Running at a volt higher than the recommended value will burn out the LED immediately. Other colors of LEDs run at a higher voltage, usually that is 3.0 volts. If you wire LEDs in series the voltage drops across the LEDs themselves will be additive. Since they are in series the same current will have to go through all of them, which is why you can't have your indicator and IR LEDs in series. You could wire everything in parallel with a separate dropping resistor for each LED. If the IR LEDs used 150 mA they would need 25 ohm resistors. You could also have the IR LEDs in series with a dropping resistor with those in parallel with the indicator light and it's dropping resistor. For two IR LEDs in series you get a 2.4 volt drop, so you would need a 2.6 volt drop across the resistor which would be 18 ohms. If you had four IR LEDs in series you could skip the dropping resistor for those.

 

[scuderia_turini]

Yeah, wise words from RichD, having the LED in series withe IR Emitters will limit the IR output to less than their peak. I would play safe and run in parallel with separate dropper resistors for each unit.

Leo

 

[Savage]

Now my head starts spinning ! I can follow instructions (mostly ) and solder but beyond that electronics is a black art !

OK my IR are rated 1.1 to 1.4v and 30ma. So I should be able to wire 2 of those in series with a 82 ohm resistor right ?

I can then take another feed from the usb wire for my red led which is rated at 1.8 to 2.1v 20ma, can I use 2x 82 ohm resistors in series to get 164 ohm (calculator says 150 ohm minimum) ?

 

[Savage]

Kind of this is what I was thinking:

 

[slotcrazy]

Savage, no need to over complicate it, you will be fine, and yes, that parallel diagram you have shown is the normal way to do it - but you can just run one wire from the USB and "common" the negative line at the LEDs.

Regular LEDs will not light at all at 1.2 volts.

Now you are a pretty knowledgable guy Rich, but we are talking a voltage drop from 5V, and these things are pretty tolerant.

I have built an awful lot of light bridges and controller stations, and I use coloured LEDs when requested as pilots on my bridges, and I use pilot LEDs on my controller stations. - and in a pile in other applications.

For my controller stations I generally run resistor values determined as if the supply was cranked to 14V, and the pilots run perfectly well all the way down to 6V - just getting pretty dim by then.

Here is a 1.9v 20ma rated red LED with an 82 ohm resistor. I set the main supply at 5.1 volts - it's analogue and not that accurate, then wound down the rheostat for the individual output on my test bench. - The pilot says 4.3 volts, but these cheap panel meters are a bit (quite) inaccurate at very low current draws, and it is actually more like 3.2v across the line, and the draw wasn't really 40ma, ore like 15ma

The LED is still glowing nicely.

In the latter pics, a green pilot LED with 1.8v forward voltage 20ma current rating and a 560 ohm resistor
- (same formula basis 14V 1.8 forward - 20ma required draw 560 ohm resistor), is running at 12v then below at 5.9v. Dimmer, but still fine.
The formula says I should be using a 200 ohm resistor to get the 20ma and voltage drop from 14 to 1.8, but as you can see it is not only fine at 12v, but still glowing at 5.9v

 

[RichD]

Here is a schematic that I quickly adapted from something else.

]

If you use a separate resistor for each LED with everything in parallel if a LED goes out the other two will still work. If the red LED burned out your counter would still work, if an IR LED burned out one lane would be dead. With the IR LEDs in series both lanes would go dead if one LED burned out. You would have to find out which LED had gone bad to fix that, so the complete parallel arrangement is best in the long run.

I have a power supply with a display that reads the volts to two decimal places and the amps to three decimal places. That comes in handy if I have LEDs that I don't have specs for. The voltage adjustment on that power supply is unusual in that the 10, 1, 0.1 and 0.01 volts can be adjusted individually. I can connect a LED that I don't have complete specs for and increase the voltage in small increments until the amp display reads the right value. It is the amps that will burn out a LED, the amps will go up as the voltage is increased. If there is only a volt specification I can set that voltage and read the amps.

 

[Savage]

Well seems its all for nowt

All wired and I can see the leds and the IR transmitters are working fine (using camera to see the IR.

Double and triple checked, pins 10 and 11 and 22/23 grounds on the IR recievers,connected but no go.

I am assuming it may be because I am using a PCI board with a DB25 on it, but don't know anyone with an old enough laptop or desktop with native DB25 port to check.

I am wondering how else to get this system working.

 

[scuderia_turini]

Hello Savage,

Can you try an alternative light source like a halogen desk lamp or a torch? What is the distance between the emitters and the sensors?

The parallel port has to be in bi-directional mode (EPP or ECP mode), perhaps that needs set up in the config for the board.

Leo

 

[Savage]

Hi Leo.

I think my wall lights are halogen, I could try that, but as I said I can see on the phone that the emitters are working fine, the receiver's I do not know how to test.

Would halogen work with the IR recievers ? I do have a matched 940nm TX/RX set.

The bi-directional mode rings a bell from last time, but that was set up in the PC BIOS as the last port was native. That PC is no more and the one I am using now uses a PCI card I will have to see if that has an option somewhere..

 

[scuderia_turini]

Hello Savage,

Yes, halogen light has high infra-red content so will activate the receivers\sensors. An old-fashioned torch or a white LED type will work also if held close enough. If I recall correctly, the receivers should be wired with the long lead (Positive/Anode) to ground and the short lead (Negative/Cathode) to 10/11. They should be robust enough to survive being wired the wrong way around...

Leo

 

[Savage]

I am unsure if this is the way to test, but I can confirm putting an ohm meter on the disconnected IR sensors reads zero and when placed in line of sight to the emitters it goes up, so to me, that says the receivers are working ?

I also changed the drivers for windows on the pci card to ECP printer. but still the software does not recognise any inputs for RC coordinator or laptimer 2000.

 

[Savage]

Ok I am now confused as heck !

I downloaded a parallel port tester program, according to that everything is working, the light goes out on the program to indicate pins 10 and 11 are receiving signals when the "beam" is broken, but neither lap counter 2000 or race co-ordinator register a change ?

 

[scuderia_turini]

Testing these via multimeter can be difficult, but sounds like yours is doing it ok. Seeing a change in the reading definitely suggests that the receivers/sensors are working.

If the wiring orientation is correct ( I just checked and definitely Anode to ground and Cathode to 10/11) then its down to the card being the problem. I have never been able to get a parallel to USB adapter to work but did think that a proper interface card (like you have) would work OK.

Have you defo got the parallel port address correct? No idea what it would be but I think Laptimer came with a port testing programme? I'll dig out an old (win98) laptop that has Laptimer installed.

EDIT - I see your update. Suggests to me that you have the wrong port selected in the software.

 

[Savage]

Windows Device Manager reports it is on LPT1, so are both bits of software. They do say 0x0378 (or 378 in RC case) where as the tester software says LPT1 is 0xEC00.