DIY LED indicator for DIY 9V audio amplifier ?

[Jolly 7]

I am looking for the most efficient way to connect a 5 mm ultrabright led as a power indicator for a 9 volt battery audio amplifier I have constructed. It is bright enough connected in parallel with the amplifier circuit, using a quarter watt 1.2K resistor connected to the LED anode. As per my calculations, it is supplying only 5 mA to the LED. Need I worry about wasting power across the resistor or is there a better way ?

[Al (astral highway)]

It is bright enough connected in parallel with the amplifier circuit, using a quarter watt 1.2K resistor connected to the LED anode. As per my calculations, it is supplying only 5 mA to the LED.

Hello, when you say 'in parallel with the amplifier circuit', do you mean simply across the 9V battery?

5mA isn't very much for an LED. It probably won't be very bright at this current-- 25 to 30mA is more common.

A typical 3.3V blue LED that draws 25mA needs a 228R resistor in series with a 9V voltage source (your battery). 250R is a standard value and is fine. It will dissipate around 160mW, so a quarter watt resistor is fine.

Please check the voltage dropped by your LED at the rated current.

There are some surface mount types of LED that consume only 15mA, but they might be a bit fiddly to work with.

[Jolly 7]

Thanks. Yes I meant to say across the battery. There is some problem with my digital multimeter which does not allow me to measure current. However, my battery (six AA cells) is currently 7.8v and the voltage across the white LED is 2.7 V.

[Al (astral highway)]

Hello, 220R would be fine if this voltage were permanent.

The 250R I suggested originally is fine for the full battery voltage. It's not that critical.

[Jolly 7]

Great, thanks for confirming. Your proposed resistor of 250 ohms would allow an LED current of 30 mA to flow from a full 9 volt source as per Ohm's Law ?

[Biggles]

If it's an ULTRA bright LED there is no need to run it at such a high current. As a rule of thumb I run standard LEDs at about 5mA. I would experiment a bit around about the 2mA mark, unless you wish to use it as a torch as well! Try a few different resistors until you are happy with the brightness. Going to 25-30mA is going to drain your battery faster than the amplifier circuit will.
Alan.

[Jolly 7]

It is quite bright at my estimated 5 mA, correct. Since I will be using it only as a battery indicator, I will experiment with a higher resistor/ lower current drain till the LED lights up adequately for indication purposes.

[Argus25]

I have found many modern small blue LED's to be very efficient and even at 1.5mA quite bright. The formula to calculate the required resistance in this case is (9-Vd)/I, where I is the current you want and Vd is the voltage drop of the led which can be in the arena of 2v. In any case just try resistors and use the highest value you can get away with with reasonable LED brightness. For anything battery operated you need the current consumption as low as possible or the LED might end up drawing more current than your other circuit. I built a 9V powered effects pedal for a guitar once, the audio circuit consumed about 1mA and the LED's about 2mA !

[TonyDuell]

Incidentally, white LEDs will glow at quite small currents. Take a (normal, 5mm) white LED and hold the anode wire in one hand. Touch the cathode wire on the -ve terminal of a PP3 battery. Touch the +ve terminal of the battery with a finger on your other hand (effectively using your body as the series resistor). When I tried it I got a definitely visible glow from the LED. Not enough to be useable, but you could tell when you were completing the circuit.

As regards the LED taking more current than the rest of the device, I remember a project in a magazine (Everyday Electronics?) in the mid 1970s which flashed an LED briefly every couple of seconds. The circuit was designed to draw minimal current when the LED was off. The idea was to use it as a power-on indicator, it drew a lot less current than the LED (+resistor) would on its own and a flashing light is more noticeable anyway.

[Beardyman]

That was the LM3909 IC (still available) which would happily run a red led off a AA for about a year, I think the upper supply limit was 6V, but could be extended with a suitable resistor according to the datasheet.
Datasheet link here: http://solarbotics.net/library/datasheets/LM3909.pdf

[Top Cap]

I currently modify Chinese amplifiers that have an extremely bright Blue LED on the front panel, the brilliance is probably eye damaging if viewed close for a lengthy period of time. This LED runs from the 12V rail via a 820 Ohm and draws approximately 12mA. I remove the 820 Ohm and fit a 100k, yes you read that right, 100k and the illumination is perfect as an indicator. Current is around 100uA, so for battery operated gear it could be left in circuit while it is switched on

[mhennessy]

LEDs vary enormously these days - once upon a time, the answer was simple: start with 10mA and perhaps go up to 20mA if it's not bright enough. Today, that's all different. In this project, the green power LED is being fed with 150 micro-amps, and it's still too bright!

I suspect that the curse of over-bright blue LEDs comes about because someone simply changed the LED in an existing product and didn't think to change the series resistor. In every case I've met, the fitted resistor is ideal for a "traditional" green or red LED.

Sometimes in a project, you need a fixed voltage. For that, you might choose a zener diode, or perhaps a few of regular diodes in series. You have to provide operating current to these, so that's an "overhead" in terms of battery life. In these situations, the LED is the perfect answer - they made pretty respectable voltage references, and double up as the power indicator. Simple but effective

[Boater Sam]

You could put the LED in series with part of the amplifier, then the current used will be negligible if it still works!

[TonyDuell]

Originally Posted by mhennessy

Sometimes in a project, you need a fixed voltage. For that, you might choose a zener diode, or perhaps a few of regular diodes in series. You have to provide operating current to these, so that's an "overhead" in terms of battery life. In these situations, the LED is the perfect answer - they made pretty respectable voltage references, and double up as the power indicator. Simple but effective

This reminds me of a Zenith MDA computer monitor, the power supply of which was eccentric to say the least. I joked that it combined the reliability of a switch-mode PSU with the efficiency of a linear one, in that it started by rectifying the mains, then used a free-running chopper (no regulation applied to that stage), transformer and halfwave rectifier to get around 20VDC, then a discete transitor linear regulator to get 12 for the rest of the monitor. The relevance is that the reference for the linear regulator was the (green) power-on LED.

As for LED efficiency now, I made up a quick tester to identfiy which of 4 contacts on a plug a given wire went to. Just 4 LEDs, series resistors and a 9V battery. The Red/Yellow/Green LEDs were traditional ones and fine with a 1k series resistor, the more modern white LED almost blinded me with the same value resistor.

I am pretty sure the flashing power indicator circuit I mentioned used a couple of discrete transistors, not an IC. I will see if I can find it to check

[mhennessy]

I thought the LM3909 went obsolete many years ago. If it's available now, presumably it's been cloned, like the TDA2030 and others?

It used 4 transistors internally. While you could build a discrete version of it, it's very possible to use just 2. There are many variants, but my favourite 2 transistor flashing circuit is attached...

This is a self-contained 2-terminal circuit that is usually used in series with a load, but in this particular application, I put it in parallel so that it shorted out the LED when the transistors conducted - that suited me for various reasons at the time, but for a battery indicator, it definitely needs to be in series. Ignore the diode. With high efficiency LEDs, the tiny charging current will cause the LED to be on slightly during the "off" time - which might not be a bad thing.

With a very small amount of re-arranging, this circuit will flash a conventional red or green or yellow LED from a single 1.5V cell. The capacitor is used as a timing element and a voltage booster. You can expect a AA to last several years before it stops working. If anyone is interested, I'll draw up a schematic.

[julie_m]

It makes little difference to the battery whether you limit the current by pulsing the LED or just using a bigger resistor. Milliamps-hours are what matter; and the product is going to be the same whether you use fewer milliamps, or use them for fewer total hours by having the LED off for some of the time.

There might be a perception of more brightness with, say, a 5mA pulse and a 20% duty cycle, than with a steady 1mA. But if you are pulsing the LED at some frequency in the audio passband, there is then a danger of that breaking through into the audio signal.

I'd personally wire a 100kΩ pot as a simple variable resistor in series with the LED, set it to at least 1kΩ to limit the current enough to protect the LED, apply that across a battery and turn up the resistance until the LED was only just bright enough to see; then fit a fixed resistor of the nearest preferred value. If I couldn't find a suitable pot quickly enough, I'd use a succession of fixed resistors and a solderless breadboard. There'd be no need to try 68kΩ, if the LED was too dim with 47kΩ; nor 22kΩ, if it was too bright.

Side question, still just about near topic: In a single-valve amplifier, could you wire an LED in series with the second (screen) grid? Or would this adversely affect performance? (A diode will mean the G2 supply can only source current, not sink it -- if that matters).

[G6Tanuki]

Side question, still just about near topic: In a single-valve amplifier, could you wire an LED in series with the second (screen) grid? Or would this adversely affect performance? (A diode will mean the G2 supply can only source current, not sink it -- if that matters).

You could certainly put the LED (along with a suitable parallel resistor) in the cathode-circuit!

In the past it was quite common for valve/semiconductor 'hybrid' TVs to score a few free volts/milliamps for a transistor UHF tuner from the cathode-resistor of the audio output valve.

Back on-topic: I wonder if the OP's preamp draws a suitably-small current from its 9V supply that it would be possible to wire the LED in series? Then the amplifier essentially becomes the LED's 'dropper resistor', albeit meaning that the preamp has to survive on a volt-and-a-half less. The advantage of this approach is that it places no additional drain on the battery.

[Boater Sam]

Back on-topic: I wonder if the OP's preamp draws a suitably-small current from its 9V supply that it would be possible to wire the LED in series? Then the amplifier essentially becomes the LED's 'dropper resistor', albeit meaning that the preamp has to survive on a volt-and-a-half less. The advantage of this approach is that it places no additional drain on the battery.

This is what I was getting at, using it in series with the supply would give you reverse polarity protection too.

[Refugee]

Back in the day I found a stash of broken road lamps close to some big roadworks.
Some were two transistor and others were three transistor.
I did at the time reverse engineer the circuit and the one in post #15 reminded me of it.
They were 6 volt with a duty cycle of about 10%.

[Jolly 7]

Originally Posted by G6Tanuki

You could certainly put the LED (along with a suitable parallel resistor) in the cathode-circuit!

In the past it was quite common for valve/semiconductor 'hybrid' TVs to score a few free volts/milliamps for a transistor UHF tuner from the cathode-resistor of the audio output valve.

Thanks for the suggestion. I will attempt to do this over the next day or two and try to take a few current measurements with my new analog multimeter, my expensive digital multimeter won't measure current anymore for some unknown reason. I'll need to start a new topic for the meter