Two machines for changing dollars to bitcoins were on the show floor at the Bitcoin 2013 show in San Jose, though only the machine from Lamassu Bitcoin Ventures was up and running when CoinDesk toured the expo.

Lamassu's machine is a small, glossy white box about the size of a microwave oven, with a slot to accept bills, a screen and a camera that can read QR codes from a user's wallet app on a smartphone screen.

Attendee Evan Rose fed two $20 bills into the machine and saw the bitcoins hit his digital wallet within seconds.

"There goes my cab fare," he joked.

"Convince your cab driver to accept bitcoin," advised Matt Whitlock, one of the company's three co-founders.

The trio plans to sell the machines for $4,000 each to wholesale distributors, and suggest a $5,000 retail price. Then machine owners can operate the machines independently, setting their own fee structure, Rose said.

The RoboCoin Kiosk, a much larger machine the size of an arcade game, was powered down and unattended Saturday afternoon. We'll try to catch it in action some other time.