MD-22 dialer is a very small card with dimension 28 × 28 × 6 mm.
It is connected in parallel with the telephone line (PTT or extension of private branch exchange). It has two inputs for connection of switches, buttons, relays. It operates in two ways, according to the application.
1. Dialing of a number with the occupation of the line by the phone connected in parallel. When the handset of the telephone set connected in parallel is picked up, one of the predefined numbers is dialed.
This method is used in combination to a phone without keypad when an emergency call to a single number is needed. E.g. elevator.
Or in order to add pre-dialed digits to the number dialed by the user.
2. Dialing of a number with the activation via a switch.
When a switch is activated, then the card seizes the line, dials the number which corresponds to the input that has been activated and then sends (optionally) an alarm sound to the line for a pre-determined time and then it releases the line.
We can apply this method when we want to send alarm signals through the lines of internal phones of the building to PBX and its user. We can, for example, add bath alarm of fire detection functions to a hotel without installing new wire network.
PCB dimensions: 27 x 27 x 7 mm
Current consumption 1 mA
You can see the connection for MD-22 at the image on the right.