The disclosed flow sensor includes a pipe segment forming a flow passage, having vanes that cause the entering liquid to spiral in passing through a flow-detection zone. An orbital ball is captive in the zone, orbiting in proportion to the flow velocity. A localized capacitive flow detector and a companion capacitive electrode sense the orbiting of the ball. The capacitive electrodes are coupled to the frequency-determining portion of a feedback-stabilized oscillator whose output is modulated each time the ball travels past the sensing electrode. A feedback-stabilized oscillator having a sensing coil is similarly modulated by the effect of a vaned rotor moving past the coil. Different forms of feedback having a long-time-constant maintain high sensitivity of the oscillator to the effect of the flow-activated device.