400ER25U
400ER25U (receiver) is a ultrasonic sensor and it is usually used coupled to 400ET25U (transmitter).
400ET25U
400ET25U (transmitter) is a ultrasonic sensor and it is usually used coupled to 400ER25U (receiver).
250ER25F
250ER25F (receiver) is a ultrasonic sensor and it is usually used coupled to 250ET25F (transmitter).