Design of a Reliable Multiple Contact for the Coin Telephone Totalizer

The totalizer performs many of the logic functions in handling a call from a public telephone set. It registers the denomination of the coin deposited and pulses appropriate signals back to the central office by transferring four sets of contacts, which are operated by a falling coin. The energy available from the coin is quite low, so contact force and cam rise of the switches must be kept low. Satisfactory operation of the contacts is essential to reliable service from the telephone. Public telephones are subjected to extreme environments, and early trials indicated contact contamination might cause the totalizer to fail to meet desired performance levels. The contact design was reviewed from three aspects: analysis of field failures, analytical investigation of design parameters, and laboratory testing to establish relative reliability figures. Considerations leading to the adoption of a brush of four precious-metal wire contacts as the design to be used in production are summarized.