Amazon.com: Logitech USB Unifying Receiver, 2.4 GHz Wireless Technology, USB Plug Compatible with all Logitech Unifying Devices like Wireless Mouse and Keyboard, PC / Mac / Laptop - Black : Electronics
Logitech K540 Wireless Keyboard Works with unifying Receiver Very Good 5466 376329876436 | eBay
Unifying Receiver Logitech
Logitech K540 Wireless Keyboard Without USB Unifying Receiver - tested working 376329876429 | eBay
Logitech Unifying Receiver for Connecting Multiple Devices