Can a LED be used to indicate voltage?

mlmcelligott

Hi all, I have a foundation level understanding of electronics and would love to know can an LED be used to indicate voltage

Say i have an old car dynamo that varies between 10 and 14 volts and i want to spin it at a certain rpm so it produces a specific voltage thats visually indicated by an LED lighting up.

Can that be done? Is it possible?

nibble

As in you want the LED to light at a specific voltage? An LED in itself will have a relatively wide operating voltage range so you're going to need to involve more circuitry, what is it exactly you want to achieve?

Capt'n Midnight

yes, but

LED's are semiconductor diodes and can be approximated as a perfect diode in series with a resistor

a perfect diode allows no current below a threshold voltage that depends on the material it's made and thereafter current DOUBLES with every 0.01V increase. This is why they need an external resistor , the internal resistance just isn't enough.

LEDs don't like reverse voltage , they are not rectifiers so if the generator is AC you will need a proper silicon diode in series.

10W LED's have 3 LED's in series. at 7V in a dark room the light is barely visible. a few seconds at 9V it's drawing an amp , getting really hot and you're blinded for a minute or two.


if the generator is an alternator then it will have build in circuits that use some of the power to energise the coils. This means they are less efficient , on a car engine that's perhaps 50Kw that's not that big an issue but will soak up lots of power from a human peddling or a small wind turbine

On the plus side it limits voltage to about 14.4V ( 12V in a car can mean 11.3V to 14.4 , battery discharged to battery charging )


If the generator is like a wind screen wiper motor or bicycle dynamo then the current is the limiting factor. Bicycle dynamos limit at 600mA but can go way above 6V , you easily get 60V out of one at turbo speed as long as you don't go above 500mA.





you could use a potential divider to divide the voltage by about 5
and put an LED in parallel with the lower resistor, the upper one would limit the total current to 10mA - 30mA say

or a potential divider with lots of LED's tapped at different points so they light up in series.

or put zeners and resistors in series with the LED's

or comparitors / op amps or get LM3914 and bargraph led's



There's no point in making a circuit to do this, unless you need it tomorrow.

If you have to buy the parts here in bricks and mortar then you could have a look at some of the cheap and cheerful stuff on the interweb

The down side is you have wait weeks for the stuff to arrive, but even on minimum wage it's not worth spending more than a few minutes on something you can get off the shelf

to measure the voltage you could spend €1.91 on this http://www.buyincoins.com/item/33616.html
or €1.06 on this http://www.buyincoins.com/item/45208.html

or 99c for http://www.buyincoins.com/item/44764.html or http://www.buyincoins.com/item/45333.html
which will regulate the voltage down to whatever you need - these things are way more efficient than a 7805 / LM317 - there are also step UP converters ,

if you can stretch to €3.74 there's this multimeter http://www.buyincoins.com/item/55422.html

you can also get this to show current and voltage at the same time http://www.buyincoins.com/item/32418.html


yes it's all cheap Chinese crap , but it's so cheap that you might as well have a few of these in your tinkering box for prototyping

locum-motion

Capt'n Midnight wrote: »

... human peddling ...

Why are you selling humans?


Or did you mean "...human pedAlling..."?