Door Latches
Door 1 and Door 2 have mechanical latches fitted which when relaxed (off) leaves the doors Unlocked. A door is Locked whenever its latch is active (On).
The system is wired so that it is impossible for more than one door to be locked at any time.
In the event of a power failure, all locked doors will relax and assume an Unlocked state.

Door Latch Behavior
When Door 1 is Opened, Door 2 is instantly locked. It remains locked as long as Door 1 remains open,

When Door 1 is closed, Door 2 is still locked. Six seconds after Door 1 is closed, the yellow indicator in the control box will flash rapidly for Ten Seconds. (this will happen each time after a Door is closed)
If during this Ten second period movement of any kind is detected within the bathroom, the system will deem the room to be occupied and Door 2 will remain locked.

If during this Ten Second period No movement is detected, then the controller concludes that the room is empty and Door 2 will be unlocked.

The same process happens in reverse with Door 2 activity.

Input Sensors
The SM108b control system has Three input circuits. These are a Reed switch sensor on Door 1, a Reed switch on Door 2 and a normally closed contact on a PIR (passive infrared) movement sensor.

Output Relay Behavior
There are three output relays on the control system. Relay 1 selects which door to lock. If Relay 1 is OFF then Door 1 electric latch is selected. If Relay 1 is ON, then Door 2 latch has been selected.

When Relay 2 is operated it applies 12V power to whichever door lock has been selected by Relay 1.

(This method is intrinsically safe. It is impossible for both doors to be locked simultaneously)

Aux Relay Options
Relay 3 has a choice of functions depending upon the location of jumper JP2.
If JP2 is in position ‘A’, then Relay 3 contact is Closed whenever either door is locked. It can be used to switch 240VAC (5A max) to lights and/or an exhaust fan within the bathroom. In this way the lights and fan will activate automatically whenever the bathroom is occupied and deactivate after they leave. This contact will go OPEN Two Minutes after the room has been vacated to give extra time to operate the bathroom exhaust fan.

If jumper JP2 is in the ‘B’ position then Relay 3 behaves as an Alarm Warning contact. Relay 3 will pulse rapidly whenever a door is opened that is supposed to be locked. ie. Someone has overridden the door lock. This contact can be used to operate a warning lamp or buzzer in the room or elsewhere as an indicator that the occupant of one room has gained ready access to the second room via the joint bathroom.