Through the atom transmitter Dianthus bestows several gifts on Barbarella to help her with her mission. The first of these is the “portable brainwave detector…to test for Durand-Durand’s presence.” To operate it, Barbarella must press “a contact,” (Dianthus is offscreen when he indicates the contact, but later we see her operating the leaf-like button near the wrist) and if Durand-Durand is around, the ball of lights will glow and an alarm will sound.
The device is wearable, wrapping around Barbarella’s forearm, and held in place by a ring. This aspect of the design is good, since it means the device is ever-present for operation, and the design of it makes it lovely enough to be overlooked as a fashion accessory. In fact many characters see her wearing it and make no mention.
Manual activation is less than ideal, though, since this might tip off the suspect. This is especially true with the blinking, glowing ball of light and audio feedback. And, in fact, this is what happens later in the film when Durand-Durand trips over the device. The blinking light and audio catch his attention, betray the device for what it is, and blow Barbarella’s cover in the process.
The best feedback would be invisible, like a haptic vibration through the cuff to her skin. Ideally, the device would be constantly on, to detect the subject passively, the moment he came into range. But presuming battery life is the issue, the activation cue should be something much more subtle, like Barbarella’s touching the back of the ring with the thumb of the same hand. Such a gesture would match the existing design of the object, be discreet to an observer, and yet still discrete enough to prevent accidental activation.