Motion sensors come in a wide variety. You can get add on sensors for your existing alarm, as well as stand alone units for windows, doors and driveways.

PIR 6 LED Infrared Sensor Motion Detector Light Lamp
US $7.49

MOTION SENSOR DETECTOR HOME SECURITY DOOR ALARM SYSTEM
US $9.75

WIRELESS DRIVEWAY PATROL ALARM MOTION SENSOR DETECTOR
US $21.99

MOTION ACTIVATED ALARM W/ KEYPAD DETECTOR SENSOR
US $14.75

Infrared PIR 12V Switch Sensor Motion Activated Lamp
US $2.49

LOT MOTION SENSORS DETECTOR AUDIO SECURITY ALARM SYSTEM
US $14.95

BN Home Security Motion Sensor Alarm Infrared Remote IR
US $8.99

Infrared PIR Motion Sensor Detector 6 LED Night Lights
US $1.00

LOT MOTION SENSORS DETECTOR AUDIO SECURITY ALARM SYSTEM
US $14.95

Home IR Remote Control Security Motion Sensor Alarm
US $7.59

MOTION SENSOR DETECTOR HOME SECURITY DOOR ALARM SYSTEM
US $9.75

MOTION SENSOR BURGLAR SECURITY DOOR ALARM SYSTEM+LIGHT
US $9.75

MOTION SENSOR DETECTOR AUDIO BURGLAR ALARM SYSTEM 4 LOT
US $21.75

BN Home Security Motion Sensor Alarm Infrared Remote IR
US $8.99

Motion Sensor Security Alarm Siren IR Motion Detector
US $10.44