The original inspiration for this is Tinkernut's project.

• Power
• Size

I'm thinking of pulls from old mobile phones, esp. Nokia. Power use should be minimal. Availability (used) should be good.

What's available? Again power consumption should be small when backlight isn't used.

What's the power consumption?

• Adafruit Feather 32u4 Bluefruit LE, 2.0“ x 0.9” x 0.28“, 5.7 grams, battery charger, $30. Possibly all FOSS.
• Adafruit Feather M0 Bluefruit LE, 2.0” x 0.9“ x 0.28”, 5.7 grams, battery charger, $30. Possibly all FOSS.
• Bledino, 1.7“ x 0.8”
• RFDuino, 0.6“ x 0.6”, $17.99, $14.99

• Femtoduino, $35
• Microduino
• Pro Micro, 1.3×0.7“, $19.95 (official)
• Pro Mini 0.7×1.3”, $3 (clone) to $10 (official)
• Qduino Mini, battery charger, $30.
• Tinyduino, battery support, goofy I/O $20

• Use a Nordic BLE module for everything?

The two groups consist of “classic” BT and Bluetooth Low Energy (BLE).

• HC-06: $TODO, TODO mA, size is a bit big.
• HC-05: $TODO, TODO mA, size is a bit big.

• Adafruit Bluefruit LE SPI Friend, 0.9“ x 1”, possibly FOSS, $17.50
• Adafruit Bluefruit LE UART Friend, 0.8“ x 1.26”“, possibly FOSS, $17.50
• Itead HM-11, 0.7” x 0.53, $8.50
• Itead HM-10, 1.1“ x 0.5”, $6.50
• ReadBearBLEmini
• Seeed Bluetooth V4.0 HM-11 BLE Module, 0.5“ x 0.7”, $13.
• Seeed Micro BLE Module, $17.

• Lipo Charger Module for Arduino