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# LED + Battery without fading

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#### vascoribeiropereira

##### New Member
Hello everyone,
I'm searching for help since electronics are not my field of study...
I'm trying to build a simple project that requires an LED with a constant light output (the LED must be wired to a coin battery cell i.e. CR2032).
When reading the LUX from the LED I can see the LUX decreasing (faster at the beginner). What I really wanted was a steady and constant output. I've been suggested with using resistors (I didn't use at the time), but the problem remains - although the decrease of LUX is a little smoother.

After searching I've came up with an idea (so, now is the part where you may call me dumb): using a bypass electrolytic capacitor... I've read a fast decrease (about 5min) in lux to about 80% of the inicial value and then it stays around the same 80% until the end of my readings (3 hours). I've tried an 10uF, without resistors. When I add a resistor to the circuit with the bypass capacitor the result is the same as not having the bypass (it reaches 80% in about a hour of it being turned on).

So... Summarising my question:
Any ideias of a simple solution to have a fast and steady output of light from a LED wired to the CR2032 battery?

You can wire up an LM1117 and a resistor a current source (NOT a voltage source) and use that to the power the LED.

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Any ideias of a simple solution to have a fast and steady output of light from a LED wired to the CR2032 battery?
Unclear what you mean by 'fast'.
For steady output, you need a constant current circuit. Which will work constant for the time the power supply allows.
Like a water reservoir, the flow of its discharge reduces with time. And the smaller the reservoir (as a weak coin cell) the shorter time the current can be kept constant.

Unclear what you mean by 'fast'.

Sorry, by fast I mean like, the most instantaneous possible - Like the orange series of the chart or better (opposed to the blue and grey one).

You can wire up an LM1117 and a resistor a current source (NOT a voltage source) and use that to the power the LED.

Thank you. Will check it out.

You do not say what colour the LED is so we don't know what voltage it requires. (A red LED requires about 1,8 volts. White or blue ones require between about 3.0 and 3.6 volts.) A CR2032 cell is about 3 volts when new. so if it is a white or blue LED it will only just work. You don't say what value of current limiting resistor you are using or if you are just relying on the internal resistance of the cell to limit the current. To have any hope of limiting the current for a white or blue LED you would need a higher supply voltage so a constant current regulator had some extra voltage to work with. A step up switching regulator with the output current regulated would solve your problem but I don't know of any ready made ones that are available. (I suspect LED torches that use a single 18650 Li-Ion cell use this method.) Even if it was a red LED you would only have about 1.2 volts for the constant current regulator to work with. I think it would be difficult to design a constant current regulator using silicon transistors to work with such a low voltage. It may be easier to do using germaniun transistors.

Les.

The datasheet for a CR3032 coin battery shows that its standard load current is only 0.2mA which is way too low to power an LED properly.
You need a more powerful battery and a current regulator circuit so that the light output from an LED does not decrease as the battery voltage and current runs down. The current regulator needs some extra voltage for it to work.

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