Earlier this year I have bought on the flea market a used car GPS system, an Amstrad gp1000.
(Un)fortunately it was so badly damaged, that it could not be repaired, so I thought I would reuse some parts from it. Then came the idea of building a portable GPS logger system.
The receiver runs perfectly from 3.3V, based on the NMEA V2. 2, 9600, 8, N, 1 protocol with a refresh rate of up to 1Hz and 16 channels. I’ve also reused the built in Li-polymer 1500mAh battery, some passive parts and connectors.
The whole unit was built from reused parts except for the two buttons, the Maxim MAX1811 battery charging IC, and the Linear Technology LTC3440 buck-boost DC/DC converter.
The LTC3440 generates a 3.5V voltage from a single cell Li-polymer battery. There is no real power switch, the unit is turned on and off from software. After removing power from the SD card and GPS using a single transistor Q1, the MCU enters sleep mode.
I have a 5V powered programming hardware, so a jumper is used to disconnect the rest of the circuit while upgrading the firmware.
The GPS unit originally used a tiny 3V Li-Ion cell for backup, which I’ve replaced with a schottky diode connected to the 3.5V line.
The AN0 analog input pin of the microcontroller is used for battery level measurment both in normal operation and during charging. The MAX1811 has a charge output pin which is used to detect the start and finish of a charging process.
Power consumption in sleep mode is around 1.5mA, while in active state between 60 to 70mA.
First thing I did was to solder the finest pin spacing component of 0.5mm, the LTC3440.
It was impossible to do the job right, without using some paper tape to cover the neighboring pins. The PCB is home made with toner transfer method, and no solder mask.
After installing the power supply and charging circuits, I’ve tested for proper functioning.
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