If you need more voltage than the pin output supplies. You can easily light a White led from a picaxe running on 3 volts, for instance, with a simple voltage booster.
The circuit is called a blocking oscillator and has appeared under various guises. Search "joule thief."
http://www.emanator.demon.co.uk/bigclive/joule.htm
He shows the CT coil differently but it is immaterial how it is connected.
or
http://cappels.org/dproj/ledpage/leddrv.htm
The inductor is a ferrite core (not important -even a ferrite bead will work) wound with 30 turns center tapped. The core size is whatever will accommodate the wire you happen to have (enamel magnet wire - but any insulated wire will work)
The easiest way to wind the coil is to wind a bifilar coil by looping a length of wire then winding both coils simultaneously and then cutting the loop and phasing them start to finish using an ohm meter.
A 2N3906 NPN transistor will work fine and any transistor with a low current gain of ~100 will work.
The cap and 1N4148 aren't necessary for operation but improve efficiency and light output.
One caveat: Don't run the circuit without a load, since the output voltage can exceed 80 volts and eat the NPN transistor.
Check the circuit by connecting it to 1.5-3 V before connecting it to make sure it oscillates. If it doesn't oscillate you may have the inductor windings phased wrong.
The circuit can also be used as a boost supply for turning on power mosfets, or by adding a third winding to provide power as an isolated high side mosfet switch. In that case, use a zener or LEDs totaling ~10 volts drop.
I've even used it for powering a picaxe from a single 1.2 V nickel hydride cell and 3.2 volt zener shunt regulator - all under solar power - in that case the sink source of the picaxe is minimal so external switching transistors are necessary to switch loads and the filter cap has to be 10uf electrolytic.
More power can be obtained by lowering the 10 K resistor down to around 1K - but don't exceed the current limits of the picaxe.
You forum gurus: Is there any provision for posting GIF or JPG artwork? - my ascii schematic work sucks
+------------+
| |
| C| Ctr tap inductor
| C| See text
| C|
| +------- +VCC
| | (or picaxe pin)
.- C|
10K | | C|
| | C| 1N4148
'-' |
| +----->|----+----+
| | | |
| |/ | |
+ - - - -| 2N3906 V --- .1 uf cap
|> LED - ---
| | |
| | |
+-------- --+----+
|
=== (or picaxe pin)
GND
The circuit is called a blocking oscillator and has appeared under various guises. Search "joule thief."
http://www.emanator.demon.co.uk/bigclive/joule.htm
He shows the CT coil differently but it is immaterial how it is connected.
or
http://cappels.org/dproj/ledpage/leddrv.htm
The inductor is a ferrite core (not important -even a ferrite bead will work) wound with 30 turns center tapped. The core size is whatever will accommodate the wire you happen to have (enamel magnet wire - but any insulated wire will work)
The easiest way to wind the coil is to wind a bifilar coil by looping a length of wire then winding both coils simultaneously and then cutting the loop and phasing them start to finish using an ohm meter.
A 2N3906 NPN transistor will work fine and any transistor with a low current gain of ~100 will work.
The cap and 1N4148 aren't necessary for operation but improve efficiency and light output.
One caveat: Don't run the circuit without a load, since the output voltage can exceed 80 volts and eat the NPN transistor.
Check the circuit by connecting it to 1.5-3 V before connecting it to make sure it oscillates. If it doesn't oscillate you may have the inductor windings phased wrong.
The circuit can also be used as a boost supply for turning on power mosfets, or by adding a third winding to provide power as an isolated high side mosfet switch. In that case, use a zener or LEDs totaling ~10 volts drop.
I've even used it for powering a picaxe from a single 1.2 V nickel hydride cell and 3.2 volt zener shunt regulator - all under solar power - in that case the sink source of the picaxe is minimal so external switching transistors are necessary to switch loads and the filter cap has to be 10uf electrolytic.
More power can be obtained by lowering the 10 K resistor down to around 1K - but don't exceed the current limits of the picaxe.
You forum gurus: Is there any provision for posting GIF or JPG artwork? - my ascii schematic work sucks
+------------+
| |
| C| Ctr tap inductor
| C| See text
| C|
| +------- +VCC
| | (or picaxe pin)
.- C|
10K | | C|
| | C| 1N4148
'-' |
| +----->|----+----+
| | | |
| |/ | |
+ - - - -| 2N3906 V --- .1 uf cap
|> LED - ---
| | |
| | |
+-------- --+----+
|
=== (or picaxe pin)
GND