Obstruction Sensing; Circuitry - Chamberlain LiftMaster Professional SW420 Installation And Maintenance Instructions Manual

18
System Features
CLOSE SINGLE BUTTON SELECT
The single button (programmable) control can be programmed to either function as a single button
or to function as a close button only.
DIGITAL MICROPROCESSOR
This is the main circuit board for the operator. It contains all the logic and intelligence for the
system. All the system programming is done on this circuit board. See Controls and Accessory
Install on p. 24.

Obstruction Sensing

INHERENT OBSTRUCTION PROTECTION
The pulley is equipped with an R.P.M. sensor. When the gate meets an obstruction, the loss of
R.P.Ms. will cause the gate to reverse. A second obstruction will cause the gate to stop. A
renewed wired input will restart the gate.
EXTERNAL OBSTRUCTION CIRCUIT
This circuit can be used with a gate edge. When the device mentioned activates, the operator will
react in a similar manner to the inherent obstruction described above.
NOTE: If external entrapment protection is required by the class of operator, both an
open and close protection device must be used.
SPECIAL NOTE ABOUT OBSTRUCTION SENSING FROM EITHER INTERNAL OR
EXTERNAL SYSTEMS
The operator will stop if it senses two sequential obstructions. It will not activate from any
automatic system, including the built in time delay to close. Either a manual device such as a
pushbutton within site of the gate and operator, or the stop button supplied with the operator must
be activated to resume the operator back to its normal operation.

Circuitry

OPEN ONLY CIRCUIT
Separate open circuits for line-of-sight devices and out-of-sight devices such as open loops or
radio controls.
LOOP CONTROL CIRCUITS
Vehicle control devices such as opening or security loop detectors are connected to this circuit.
TIME DELAY TO REVERSE CIRCUIT
Allows the gate to come to a complete stop before reversing direction. Approximately 1/2 second
between stop and reverse.
NOTE: This feature is defeated when either the inherent or external obstruction circuits
are activated.
Doc 01-G0610
Rev A