Hot-Dot: An open thermal haptic interface for smartphones

Principle of Operation

Hot-dot is a thermal haptic interface that converts content from a smartphone screen to thermal feedback. It leverages the actual smartphone CPU to do most of the heavy lifting, i.e. it analyses the content under a user's finger and converts the region of interest to appropriate thermal signals which are fed to an MCU to actually drive the microheaters. The system is wearable, low-footprint, low-cost (targeting a cost of under 40 eur.) and open-source. It is composed by:

• Wearable finger-band: A Kapton-based device which consists of an array of 0201 SMT thermistors arranged in a 2x4 grid working as heaters, (possibly) three ultra-fast photodiodes, an RP2040, peripheral electronics and a USB-C connector.
• Android script: The main tasks of the Android script are: (a) to read the screen content around an area that a user touches, (b) to determine the appropriate thermal "translation" (either Brille or a custom hybrid mode for texture feeling) and (c) to communicate information with the finger-band.

Technical Specifications

Hardware
Actuators	2x4 Array 0201 Thermistors as microheaters (i.e. B59421A0115A062)
FETs	Typical small-footprint n-type MOSFET, driving the thermistors with VUSB coming from the Android phone
Firmware
MCU	RP2040
Protocol	USB CDC (Serial/UART)
Software
Android	Native or a simple framework
MCU	Raspberry Pi Pico SDK
PCB Design	KiCad