Why is a 5V relay used if the battery is 9V?
The coil on the 5V relay is activated at 5V, but it’s rated for much higher voltages (I think up to 24 or 30VDC on the contacts, don’t know the upper limit on the coil itself). The 5v relay is used because it’s a ubiquitous component found in many low-power electromechanical switching applications (such as our humble little circuit).