zondag 15 april 2018

Hacking the 4th generation IXO: white instead of yellow LED

I have a 4th generation Bosch IXO screwdriver and it is pretty good, aside from the measly LED that is supposed to help light up the thing you're working on. The LED is a yellow-orange color and is not very bright. It also has an uneven circular intensity pattern. In other words, it is pretty useless. I have been planning to replace it with a white LED for a while, and it seems I am not the only one with this idea. A bag of Nichia NSPW300DS 3 mm LEDs has been sitting on my desk for more than a year and I finally decided to have a go at it. These are probably not the brightest 3 mm LEDs currently available, but they were available at the shop where I usually buy components, and 15000mcd isn't bad at all.

However, as Joken mentions on his IXO teardown post, one cannot just replace the LED: the yellow LED is only supplied with 2 V through a 68 Ω resistor. Dropping the resistor only gains about 0.7 V, and 2.7 V is still way too low for a typical white LED that requires about 3.2 V.

I have a solution though: I bought some booster circuits from AliExpress a while ago, in fact these are buck/boost circuits that provide a steady 3.3 V output from any input between 1.8 V and 5 V. They aren't terribly efficient and I don't need the buck capability here, but the PCB is tiny which makes them a good candidate. They look like the following photo:

I did some tests by connecting the booster instead of the LED+resistor, but it is unable to reach 3.3 V with the white LED on its output. This is because the circuit that drives the LED seems to be a current source, limiting the current to either 11 mA (when pressing the switch without activating the motor) or 20 mA (when the motor is active). Due to P = U⋅I and some loss due to inefficiency of the booster, it can only push either 4.5 mA or 10 mA through the white LED. However, there is a way around this: by reducing the value of the 4.7 kΩ resistor that comes before the transistor, we can make it supply more current.

I simply soldered another 4.7 kΩ on top to reduce the resistance down to 2.35 kΩ:

This shifts the current source to 17 and 36 mA respectively. The booster turns this into 7.5 mA and 17 mA, which is much closer to the nominal current for the white LED, so this is the final configuration I used. There is no series resistor for the LED, I purely rely on the limited current coming from the transistor. I removed the 68 Ω resistor and connected the booster circuit. Instead of trying to solder onto the tiny resistor pads, I picked the connected through-hole pad. The little PCB (with the 3-pin header removed) fits nicely under the battery.

The result is pretty nice, finally the LED is useful! The lighting is also a nice even area instead of ugly concentric circles. It does draw more current now, but compared to the motor, 36 mA is still pretty negligible.

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