Add LEDs to main power line

[Braden]

Hi, Maybe a really stupid question, but I dont know much about LEDs at all. I am working on a DIY Radio, I have added and on/off switch to the main power line, but was wondering how I would be able to go about adding LED lights after the switch to indicate when the radio is on. Is it as simple as adding a resistor and the LED to the positive line, just like splicing in more wire? Hope that makes sense, thanks for any help!

 

[Harald Kapp]

Is it as simple as adding a resistor and the LED to the positive line

Depends on what your "main power line" is.
If it is mains, the answer is no. LEDs cannot be powerd from AC without some additional circuitry.
If it is a low DC voltage (e.g. 5 V) the answer is yes - but you'll need a current limiting resistor. Read all you need to know in our resource.

 

[BobK]

Presumably, there is a low voltage power supply inside, run the LED off a low DC voltage.

Bob

 

[Doug3004]

Presumably, there is a low voltage power supply inside, run the LED off a low DC voltage.

Bob

For a LED it would be easier to run it off of an internal low-voltage line.

Instead of a LED, you could use a neon lamp with a suitable capacitor. The little rice-grain neon lamps are usually rated for 220 VAC, but they still do light up when connected to 115 VAC.

The neon lamps glow orange, so you don't really get to pick the color. They do have a very retro look about them. You may find yourself shopping for 3S4 and magic eye tubes eventually. There is something fascinating about watching electrons do circus tricks out in the open, where you can see them.... But anyway.

Also it is helpful to explain, if you did not know:
An LED is a light emitting diode, but it isn't made to work real well as a diode, and so you shouldn't use it that way. It's made to light up well.
Most visible LEDs can't withstand more than about 20 milliamps forward current, and can't withstand more than 10 or 20 volts reverse voltage. That's pretty lousy numbers for a diode.
Meanwhile, a very cheap and common 1N4007 diode is smaller, can withstand 1 amp forward current, and is rated for 1000 volts reverse voltage. And it's nothing special.

So if you must run a LED + resistor off an AC source, then you also want a 1N4007 diode in series too. Because that LED won't live long on AC power--even low voltage power.

 

[dave9]

If you supply a schematic of the circuit including details of the power supply or at least link to more info about the radio, someone could better advise about where a good place is to tap into the circuit for the LED and series resistor. I mean that as a visual indicator that it's turned on, you don't necessarily need it to indicate the presence of AC after the power switch, can instead have it indicating presence of voltage further along in the circuit.

If you have an LED in mind that you wish to use for a target visual effect then it would be good to link to that too.

 

[Braden]

Thanks so much for all the input! The neon light is probably going to be the best way to go, I didnt even think of that before! I am using a prefab. PCB bluetooth board, and the only spot I can see on it to place an LED would cause it to blink as an indicator.
I found a set of neon lights with resistors rated for 220 on eBay, so that should do the trick. One other question with that, I will be powering the PCB with a 12V 3.0A power supply, will the neon light and resistor have a major effect on the power supplied to the board? Thanks again!

 

[Braden]

I also found 12v edison bulbs, would that be a better alternative to the neons?

 

[Doug3004]

I also found 12v edison bulbs, would that be a better alternative to the neons?

Not really. The neon lamps can safely run right off the 220 (or 115) VAC power line. You would need a 10:1 transformer to use the 12v lamps.

Also the neon lamps may come with a resistor or a capacitor. Either is to limit the maximum current through the lamp. Way-back-when it seems I mostly saw them with a capacitor, but now most of the ones on ebay have resistors.

 

[Braden]

Thanks Doug!

https://www.ebay.com/itm/20pcs-Neon...361273&hash=item3ac84dc2a2:g:BS0AAOSwLN5WlmQO

These are the ones I found, with a resistor. So wiring two or three of them in series shouldn't have a huge effect on the 12V Power supply to the board, right?

 

[dave9]

Thanks so much for all the input! The neon light is probably going to be the best way to go, I didnt even think of that before! I am using a prefab. PCB bluetooth board, and the only spot I can see on it to place an LED would cause it to blink as an indicator.
I found a set of neon lights with resistors rated for 220 on eBay, so that should do the trick. One other question with that, I will be powering the PCB with a 12V 3.0A power supply, will the neon light and resistor have a major effect on the power supplied to the board? Thanks again!

Why would it blink? If you're using a 12VDC power supply all you have to do is wire the LED across the power supply output, with a series resistor whose value depends on how bright you want the LED to be (within its max current capacity). For example a standard 5mm red LED, with a 1K resistor in series would give it about 10mA. 10mA is "fairly" bright for a 5mm LED used as an indicator, you could go with a 2K resistor (1/8W or higher rated) for 5mA and so on.

Those resistor values would also work for blue or white LEDs since there is a large voltage drop, just a little less bright.

I would go with an LED instead of neon, because with an LED you can just drill a precise sized hole in a project enclosure and mount the LED in this fashion and have it look nice. if the hole isn't a tight fit or you don't want the LED sticking out as far, then you can keep it secure in the right position with some super glue, hot glue, epoxy, whatever, on the back of the hole.

There are also little plastic grommet looking holders (LED mounting bezel) for 5mm LEDs that allow them to friction-snap in and out of the holder, but those are more for thin walled steel and not needed if using a thick (hot glue or epoxy) method of attachment, but drilling painted metal can leave an unsightly shiny area around the hole so the LED holders also hide that.

LED Mounting Bezel

 

[Braden]

If using an LED resistor combo, do I wire them in series along the positive line? If I wanted to use two would it be like this:
Positive 12vDC line - 1k Resistor - LED - 1k Resistor - LED - Continue positive line to amp board

Sorry if I am really dense, haha I know this should be very simple, but it is a new project for me. Thanks!

 

[dave9]

Why do you need two LEDs? You can use two if you want to.

No you do not wire them in series with 12V PSU output. You wire across the PSU positive and negative output, from one to the other, a series of LED - 2nd LED - 1K resistor, or across the positive and negative you could do LED1 - 1K resistor, then do a 2nd group across positive and negative of LED2 - 1K resistor (or again, it might be too bright, could use 2K resistor instead of 1K), but if wired separately then you need 2 resistors instead of one.

 

[Harald Kapp]

will the neon light and resistor have a major effect on the power supplied to the board?

No, these will need to be added before the power supply, on the mains side,
However, it will be so much easier and safer to use an LED with series resistor and power it from the 12 V line. The additional 2 mA to 5 mA for the LED are probably irrelevant for the rest of the circuit. If they were, that would be a very bad design stretching the power supply's rating to the limits.

1K resistor

1 kΩ Is imho too small, resulting in approx. 10 mA of LED current. Modern LEDs work well with 2 mA to 5 mA, so 4.7 kΩ or down to 2.2 kΩ are suitable values. Wonder why the funny values?

 

[dave9]

^ I agree that 2mA to 5mA would suffice for an indicator light (in all but bright/outdoor use), but the text was about lights, and two LEDs in the last post, so I presume the desire is to make it brighter than usual.

A typical 5mm encapsulated LED is a 100mW part. That assumes good heatsinking (short lead to ample PCB copper) but even without, 10mA is manageable without heat issues and a long lifespan, unless it's one of those generic trash Chinese LEDs that uses the tiniest die possible.

 

[eetech00]

Hi, Maybe a really stupid question, but I dont know much about LEDs at all. I am working on a DIY Radio, I have added and on/off switch to the main power line, but was wondering how I would be able to go about adding LED lights after the switch to indicate when the radio is on. Is it as simple as adding a resistor and the LED to the positive line, just like splicing in more wire? Hope that makes sense, thanks for any help!

Hi

Use an AC opto isolator. AC Input across AC line with a appropriate current limit resistor.
Output is transistor with open collector. Use the collector to drive a low voltage LED indicator.

eT