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Messages - odledm

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IR Pens / Considering adding a joule thief to an IR pen
« on: July 08, 2008, 01:52:49 PM »
I am new here and planning on making my first IR pen but was curious as to how long batteries are lasting with your IR Pen designs and if anyone has considered adding a Joule Thief circuit to your IR pens.

For those that don't know what a joule thief circuit is, it is essentially a circuit that steps up the voltage of a voltage source so that it can be used to power something that needs a higher voltage than provided by the source.  It can be used to "steal" energy out of batteries once the batteries have dropped below their typical useful output voltage (hense the name).  For more details check this site out ( which shows how to make a circuit to power a blue or white LED.

Now obviously the circuit in the link above would need to be changed to output the correct voltage and current to drive an LED, but I was thinking that it might be useful for prolonging the battery life of an IR pen.

I've never made either the pen or a joule thief so I don't know:

a.) If the frequency of the led powering on (according to the article it is 40 kHz) will cause problems with the whiteboard software.  Supposedly it is not noticable from the human eye and it may be smaller with an IR LED since the driving voltage is typically less.

b.) If there will be any noticeable delay when pushing the momentary switch to lighting the LED.

c.) If this will actually cause "new" batteries to be drained faster.

What are your thoughts?  Is battery life even an issue for those of you that use your pens daily?

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