At the heart of Silicon Valley, Santa Clara County should be a pioneer in setting technology policy — including privacy rights, which increasingly are under attack by government agencies.

The Board of Supervisors on Tuesday should adopt Supervisor Joe Simitian’s proposal to govern the use of surveillance technology. The cutting-edge ordinance will require that the board and the public be informed before any new surveillance technology is purchased by law enforcement officers and that a policy is in place governing how it can be used.

Critics, including District Attorney Jeff Rosen, worry that this might interfere with law enforcement’s ability to catch criminals. He argues that the current proposal is too broad and that further discussion is needed to sharpen the ordinance.

Simitian’s proposal will not block law enforcement from purchasing new technology or putting it to use. It will guarantee that the potential use of a new gadget first be discussed in public, policies for its use adopted and oversight established, including making it a misdemeanor for law enforcement agencies or individuals to disregard the policies.

The need for oversight became apparent last year when the county Sheriff’s Office was given approval to purchase a cellphone tracking system for $500,000 without being able to fully explain to the supervisors how it works, let alone when and how it would be used. Supervisors were forced to rush to approve the request because of an imminent deadline for winning a federal grant, although the purchase was never completed.

The device impersonates a cellphone tower and can capture conversations, emails and texts of users in the area. In theory, it would be used only to track specific cellphone numbers, but the potential for abuse is high. At a time when the National Security Agency, FBI and other agencies are trying to broadly collect Americans’ private data, “Trust us” just doesn’t cut it.

Take drones, for example. They can save lives. Drones were used in the hunt for the suspects in Thursday’s attack on police officers in Fremont. They can be helpful in search and rescue operations, hostage situations and bomb threats. But they can also be abused — snooping over people’s backyards, for example. Elected officials need to set boundaries. Law enforcement should encourage this to keep the public trust. Otherwise, negative reactions to abuses could lead to voter-approved bans.

Tech advances constantly are producing new law enforcement tools. That’s great, but nearly all need some constraints attached. Because this proposal is an ordinance and not etched in stone by voters, the supervisors can revisit and adapt it as technology and social norms change.

Simitian’s ordinance provides transparency and accountability to this emerging field. The board should adopt it, and other counties should view it as a model. Eavesdropping gadgets may emerge from Silicon Valley, but they for sure don’t stay here.