The LDB-1 provides a loop activated contact closure when used in series with telecom devices on analog phone lines. The LDB-1 will initiate a dry relay contact on any off-hook condition. Alternatively, add the optional PS-2 power supply to provide switched DC power.

The relay itself is phone line powered and does not need an external power supply. Because both TIP and RING currents flow through the relay, excellent line balance is maintained.