Abstract: The invention provides an equalizer or control unit to be used in conjunction with the present day type of parking meter. The control unit of the invention comprises an ultrasonic transmitter and receiver system which senses the presence of a parked vehicle, and which controls the parking meter in a manner such that the time remaining in the meter is returned to zero when the vehicle is driven away. The system to be described also controls the meter in a manner so as to prevent anyone from inserting coins into the meter after an initial parking interval has expired thereby to prevent the monopolization of the parking space by one vehicle over extended periods of time.