Techniques and tools for controlling public output devices (e.g., public display devices) with client devices over a network. In some examples, a time-limited proximity code is displayed by a public display device, and a client device can provide the code via a public network to verify that the client device is present within a proximity zone and is allowed to control the public display. Once authorized, the client device can provide control data to control visual content on the public display. A relay service can be used to relay data over a network via communication connections between client devices and public display devices. For example, the relay service can be implemented on a server connected to the network and can use HTTP (hypertext transfer protocol) to relay data such as control data via messages from one device to another.