A friend wanted me to build a power supply that would enable him to permanently wire a GPS tracker into his van, with power supplied by the van's battery. The GPS tracker he bought uses a regular SIM card to SMS its location to a designated phone. It uses one of the 3.7V BL-5B batteries normally found in Nokia mobile phones. Charging the tracker's battery each week would quickly get boring, and leaving it on charge in the van permanantly would eventually degrade the battery, so I wanted a solution that eliminated the BL-5B battery altogether. I have an old Nokia kicking around so I thought I'd use it for testing.
You might think the task of building a power supply to give 3.7V from a 12V car battery would be fairly trivial but it's not without difficulties, chiefly the fact that even though mobile phones only draw a few milliamps most of the time, when they need to talk to the cell tower to make or receive a call or text message, they draw a significant current (several amps) for a short periods.
To deal with this I have used a heavily-decoupled MIC29152WT variable-output LDO regulator. In the diagram above, the left column pair of capacitors are 0.22uF ceramics, the middle column pair of capacitors are 10uF tantalums, and the right hand capacitor is a big 47000μF electrolytic. The choice of resistors selects the output voltage; devices powered by Nokia batteries appear to work best at around 4.1V; a little bit of experimentation with the resistor values may be necessary.
The components are probably over-spec'd for what they need to do, but I only need to make one, and space is not an issue (within reason!). The whole thing mounts nicely on a piece of stripboard, with no need for cutting tracks:
The new power supply connects directly to the battery terminals. An 82kΩ resistor between the middle terminal and ground is necessary to fool the phone into thinking a real battery is present:
Note: I'm disabling comments on this post because there seems to be a fairly repetitive trend going on. Comments fall into two categories:
- "Can I use different components?" - answer, probably. Try it and see!
- "Can I do this with a similar device XYZ?" - answer, possibly. Try it and see!