Advice for changing LED's

[yidboy69]

Hi,
I am a complete novice in terms of electronics so was hoping someone could give me some advice. I have a bite alarm that runs off a 9v square battery and has a 5mm red led that lights up on rotation of a magnetic contact. The led also stays on for around 10 seconds after the initial contact. I wanted to change the red led to blue, so bought some super bright 5mm blue led's from Maplins today and they look identical in every way, even the element inside. To test the new leds, which i saw working in the shop, i opened the alarm casing and put the legs of the new led onto the solder points of the old led but it will not light up, just the old red one. I even pulled back the sleeving to get a better contact, but still the same problem. My question is therefore: is it possible that the new led would run once it was changed to the only main contact, could the battery only be capable of powering one led at a time, and will the new led, if working, stay on for 10 seconds(is this contolled by the board or led)??? Could there be compatability problems with the new led?? If anyone could help, or suggest a place i could take the units to in the Benfleet, Essex area i would be eternally grateful.

 

[JimB]

Red LEDs need about 2.2 volts to illuminate them.
Blue LEDs require about 3.7 volts.

When your bite alarm operates, there will be 2.2v across the red LED, not enough to illuminate the blue LED.

If you disconnect the red and replace it with the blue, I think you will find that it works OK.
LEDs are polarity sensitive - you have to connect then the correct way round or they dont work.

It is possible that your blue LEDs may not be as bright as you expect, it may be necessary to change a resistor as well as the LED.
Just sitting here not knowing the internals of your bite indicator, I dont have enough information to be sure.

JimB

 

[yidboy69]

Thanks Jim, that's a great help. The current battery life is very good, does this then mean that it will halve if the blue light works, or will it still only run off 2.2v with the existing resistor in place??

 

[Diver300]

The LED is probably driven from the 9V battery. LEDs always need something to limit the current, and the device probably uses a simple resistor.

As there is 2.2V on the LED, the remaining voltage, (9 - 2.2) = 6.8 V will be the voltage across the resistor. If you increase the LED voltage to 3.7 V that leaves 5.3 V across the resistor. That means that your current will be less, but not much less, because the current through a resistor is proportional to the voltage across it. Battery life will probably be slightly better.

A 9V battery is considered completely flat at 6 V, and that is still quite a lot above 3.7 V so there will still be a reasonable current in the LED.

Of course, there are lots of ways of controlling the current in the LED, and with other ways the blue LED might not work at all, and it might cause the battery life to be worse. Photos would help.

 

[audioguru]

First I thought it was a bike alarm spelt wrong. Then I thought maybe is an bite alarm for fishing.
At the park yesterday a fisherman caught a huge salmon that he could hardly carry.

 

[yidboy69]

Thanks for the information guys, and yes it is a bite alarm for fishing!! Any answers on the 10 second delay as to whether it is the LED itself that controls this or a component of the board?? Also, any suggestions on places i can take them for someone local to look at them in and around the Benfleet, Essex area??

 

[Mike_2545]

First I thought it was a bike alarm spelt wrong. Then I thought maybe is an bite alarm for fishing.
At the park yesterday a fisherman caught a huge salmon that he could hardly carry.

You mean spelled

 

[audioguru]

You mean spelled

Engrish is odd.
I spilled my beer or I spilt my beer are the same.
Why not spelt instead of spelled??