The objective of this exercise was to create a prototype quality battery operated charging circuit. This would be used to charge a pair of bluetooth earphones on the move.
The headphones are charged by USB input 5v @ 125ma. This is an ideal application of quite a basic charging circuit, so I've selected a simple linear regulator (78L05) for the task.
- Design a circuit based on a simple linear regulator
- Create a compact strip board layout
- Understand the electronics involved
- Fit the circuit into a suitably compact case
The result is quite a compact and fair useful circuit.
The hope is that I can develop something more suitable for charging a mobile phone in the move, again for the purposes of learning and utility.
Circuit Design
The circuit is built up of the standard reference layout for the 78L05 voltage regulator and a low battery indicator circuit.
Parts List
- 2 x 2N4401
- 2 x 10k ohm resistor
- 10k preset
- Red LED
- 390 ohm resistor
- 1mF tantalum capacitor
- 0.1mF tantalum capacitor
- 78L05 (T0-92)
- 1N4007 diode
Note: The 78L05 regulator (TO-92) appears to have its pins reversed. Take care to test you have correctly hit the desired regulation.
Assembly
After a little trail and error on the layout I find one I was happy with and moved onto soldering and assembly into a suitable case.
In this case a dental floss case works well enough for this prototype. If I were to scale this up, I would make a custom case for the project to get a better fit.
Breadboarded layout
Stripboard layout in case
Wires feed in through holes made in lid
Lessons Learned
Working to a specific case is a challenge, especially with close tolerances.
Cabling for a project like this is crucial. 9v clips really are prototype quality, cheap USB cables are inflexible.
9v PP3 batteries are limited in their application. Low capacity being the limiting factor.
Linear regulators fairly poor for battery operated projects. They produce a fair bit of heat, and at the 1.5A draw required for a mobile phone would need serious cooling.
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