Ultrasonic range finder

The invention relates to an ultrasonic range finder comprising a transmitter (1) for emitting ultrasonic pulse bursts (I), a receiver (3) and a measuring and evaluating circuit (5) for measuring the transit time of the pulse bursts between transmitter and receiver. The received signal (U1) is compared with a variable threshold signal (U4), internally derived from the shape of the pulse burst, in a comparator. This threshold signal is additively composed of two signals (U2, U3), one of which (U2) approximately corresponds to the maximum positive amplitude of the received signals (U1) and the other one of which approximately corresponds to the envelope signal of at least the first negative halfwaves of the received signal. This signal can be advantageously formed by two peak value detectors (15 and 17, respectively) having different time constants. A threshold signal (U4) generated in this manner ensures that the received signal (U1) always intersects the threshold signal with the same positive halfwave. This edge-triggered point of intersection is used for determining the transit time.