The Noldus pellet dispenser has an LED light which indicates the status and can be used to identify an error:
An infrared photobeam detector senses whether a pellet is dropped after a “drop pellet” command has been given (either manually by pushing the blue button on the dispenser or with the USB-IO box and EthoVision XT). When no pellet is detected, the stepper motor moves the carousel within the silo one step further. If, even after a full rotation (15 steps), no falling pellet was detected, the system considers the silo to be empty. The pellet dispenser goes into error mode and the LED at the front of the pellet dispenser blinks continuously. Fill the silo with pellets and press the blue button at the front of the pellet dispenser to reset the device. It is then ready to be used again.
If the pellet tube is blocked, the pellet dispenser will go through the same process as above, but pellets remain in the carousel. Remove the carousel (by lifting it off the base) and check whether it is blocked by dropping a pellet into the hole. If it does not come out of the dispenser, the path is blocked. Use a thin, flexible object (e.g., a twist tie or cable tie) to clear the tube.
The LED indicator will remain on if the infrared photobeam detector sees something remaining in the path. This could be an obstruction as above. If clearing the path does not resolve the problem, the detector may be covered in dust. A pipe cleaner or a can of compressed air can be used to clear the tube. Using dustless pellets should prevent this problem from occurring.