A customer ordered a talking skull, they requested that when 12v was applied the jaw would open. Usually servos would be used to make the jaw move, so getting completing this task took some creativity. Eventually, I ended up using an actuator that is used to lock/unlock car doors. The skull was retrofitted to mount the motor and a mechanism to hold the jaw and connect to the actuator was designed and 3D printed. 
When 12v is applied to the system, the actuator will extend, opening the jaw. When the circuit is open the jaw will return to its closed position. Included is a potentiometer to vary the speed of the jaw movement.
It was also a good excuse to look into an automatic jaw system, which will take the audio input and move the jaw accordingly. Essentially, it will listen to the line volume of the input and turn that into a voltage output to actuate the motor. Using an Arduino and a motor driver board this can be achieved. 
Both products are available on Etsy and eBay.
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