LEDs and resistors

[Begadoc]

I was using a simple circuit in the church to communicate with the organist e.g "The bride's arrived". It needed to be visual rather than sound as he wouldn't hear a buzzer if organ playing loudly. Circuit is a 6v battery, push button and two LEDs wired in series - one in vestry and one in front of organist. LED in vestry acts as a check organist is receiving message - if vestry LED lights up, so must organist one. As I said, simple circuit.
LEDs have a forward voltage of 3.3V, so with two in series, no resistor is needed - or so I thought. However someone, more knowledgeable than me re electronics says I do. A resistor in that circuit would just drop the voltage available to the LEDs. Am I not understanding the purpose of a current limiting resistor as opposed to it simply being there to drop the voltage to that required by LED?

[David]

Hello.
All electronic circuit elements have ratings which must not be exceeded if they are to remain reliable, and leds are no exception. The most important is that of forward current (i.e. when the Led is illuminated) and the data sheet for the particular led will give you the manufacturer's recommended figure. Expect something in the low tens of milliamps. The danger of using two 3.3v leds and a 6v battery is that you could have - with, say, a new battery - a case where the leds are overloaded. Your circuit in fact does not have a good control of led current. IMHO a better circuit would consist of the two leds in parallel, with a current limiting resistor in series with each led. For 3.3v leds with an forward currect of 20ma, the value of each is 130 ohms.
There is an excellent calculator for the values of other combinations of leds and resistors on this sire under 'resistance calculators'.
Hope this helps.

[magpie]

I would stick with the idea of having them in series, that way when one lights up it shows the circuit is complete, and
working.You can make up your own battery pack using say 2 cells to start, seeing if the LEDs are bright enough,if not then increase the number of cells to 3.If you have to you can double the lights up on the receiving end to increase the light
output.This way of experimenting, will mean you will not need resistors in the circuit, but just go back to your original
concept.

[David]

I'm very sorry, but I do have to disagree with Magpie on this one. LEDs can be damaged by excessive forward current, and that is why they always need a series resistor (or other, more complex, current limiting circuitry). When they are treated correctly, they have a long life and I think that rather than wondering about whether the organist has seen the LED because of a possible LED failure (or not noticing because of turning over a page of music) it is probably best to adopt a system which is used in many theatres and auditoria. That is to have a duplicated circuit, acting in reverse - pushbutton by organist and another LED in the vestry (different colour?), so that there is a positive acknowledgement from the organist to the vestry to say "message received".

You might like to follow this link: http://dangerousprototypes.com/docs/Bas ... iode_guide

[magpie]

I have to stick to my original proposal. I have done the maths, simulated it, and find it is fine. What I have to make
clear is that is when I proposed to make up the power pack, I should have said the cells would be dry, say AA batteries
times 2 given 3 volts to start. This could then be increased to 3 AA's given 4.5 volts approx'.... as they have an initial
voltage they will be a bit higher. If you go for 3 LEDs, then 4.5 volts might be bright enough. However to improve on it
would require some feedback circuit.

[David]

I think we will have to agree to disagree on this one!
Nevertheless, I would have thought that a simple resistor to define LED current and prevent overcurrent damage would be a good idea ...

[MartinDavidson]

Hi,
I think 20mA is quite a huge current for a single LED... generally 20mA is the max current an ordinary LED can sustain.

thanks

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