Mini bluetooth thermal printer teardown - with schematic

These are really cheap and common devices on eBay, sold as toys for printing images onto thermally sensitive paper. This one came with several rolls including coloured paper and self adhesive label material.

The printer can only print one colour, usually black, dictated by the paper itself. The app does allow printing of photos, and uses dithering to get rough shades of grey. Given the relatively low resolution, the images are surprisingly identifiable.

The thermal printers are very minimalist and refined due to their widespread use in receipt and label printers. If you consider that almost every till has a thermal printer for ease of maintenance (no ink) and an absolute minimum of moving parts, then it makes them very cheap and available for items like this one.

The printing is done by heating up the paper with a row of tiny resistors. When heated the thermally sensitive paper changes colour.

Although the USB data lines are there, I plugged the printer into the USB port of my Chromebook and it didn't detect it. It may work with a PC.

Note that I've accidentally swapped the source and drain of the 2301A MOSFET in the schematic.
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