Recently I have been watching my poor old keyring light slowly die on me and have begun to wonder what I could replace it with..
Well the obvious answer of course is a Lithium Polymer powered XP-G keyring mod! So this is the plan so far:
1. Take a section of square black anodized aluminium tubing of 10mm internal diameter and cut to around 60mm in length. From there, cut one third of its length so that it has the similar tapered end that I have used for other projects. I will leave a ‘tail’ section so that I can attach it to a keyring later.
Aluminium Key Light housing
2. Next I will make bend a flat section of aluminium into a U with the bottom of the U being flat. This U shaped aluminium bracket will fit snugly into the tube so that bottom flat section of the U points towards the front. Onto this flat section I will thermal epoxy a 10mm Metal cored printed circuit broad (MCPCB) with the XP-G R5 ‘pill’ install on it. Onto this MCPCB will go the 10mm square optics. This assembly should fit snugly into the aluminium tube with only the optics showing out the end like a flash light.
Left: The MCPCB & XPG. Middle: The optics. Right: Combined.
3. Next a hole will be drilled on the top for a toggle switch to turn the light on and off. A second hole will be drilled on the bottom towards the rear (not the eyelet keyring hole) into which a micro USB (Type B) connector will be placed later to allow for charging of a LiPo Battery.
Micro USB Port and 1.5A Toggle Switch.
4. Next is shaping up to be the most difficult part, the lithium polymer battery. At this stage it is likely be two 100mAh cell’s in parallel making a 2P pack. These tiny 100mAh cells can be found here
. These two cells will connect to a chip known as a “AMC7135” which is a constant current regulator. This will drive the LED at around 350mA regardless of the cell’s voltages keeping a steady output as the battery runs down during use.
A 100mAh pack and constant current IC.
5. The addition of a protection circuit for the LiPo cells is undoubtedly a must for something that I will be carrying around in my pocket. Even the tiny 6 grams of LiPo is enough to cause severe burns if it was to “vent with flame” due to over/under voltage or over current. This over current, over voltage and under voltage protection will be administered using a S-8211D series IC. This disconnects the battery from the charger if it goes over voltage. It also disconnects the battery from the load (the LED) if it under voltages. Over all a very useful little chip.
S-8211D series IC Dimensions
6. Lastly, there needs to be a way to recharge the battery. This will be done by taking a USB cable with a USB Type B on one end and plunging it into any computer and then into the keyring. An hour later the two 100mAh LiPo batteries will be fully charged. The constant voltage, variable current linear charging will be done using a “MAX1555” IC chip. This fantastic little chip takes up to 7V DC input, controls the charging process specifically designed for small single cell LiPo batterys and has thermal protection. The chip is so small that it can be housed in the light it self.
A USB cable and the very small MAX1555 IC
To sum up, this will be a 130lumen micro light with a half hour run time, USB rechargeable and all in something the size of a AA battery.
What more could you want!