I’ve begun designing a Grave Jumper type prop that uses an electronic motor instead of pneumatics. There’s a few reasons I took this approach: one being the cost (I don’t own a compressor) and the other being to show that it can be done, if to anyone other than myself.
The concept works on the notion of a slingshot and uses a windshield wiper motor to cock it into position. Note that these are loose concepts and there’s a lot of variables to take into account such as the weight of the prop versus the tension on the bungee cords versus the torque of the motor.
The cocking mechanism is based on the Headless Flailer design over at Allen’s Halloween page. The mechanism is quite simple actually. The hammer spins freely on the motor shaft. Mounted below it is a disk with a pin sticking up. As the disk spins the pin catches the hammer and rotates it around to a cocked position and stops. When the motor starts again it pushes the hammer around until the tension from the bungee pulls the hammer faster than the motor is rotating firing the prop. The motor continues around, catches the hammer and goes back into the cocked position until the prop is activated again.
In Allen and Tony’s design, they used a limit switch and a series of relays to start and stop the motor. In the design here, I plan to use the Valeo 15094704 Windshield Wiper Motor (thanks to ScareFX at HauntForum.com). This motor has a unique characteristic that it always returns to the same parked position when the power is switched off. In this design, that position will be the cocked position. When the motor receives a momentary jolt of power it will spin one full rotation back to the parked position. This power can be applied by a manual trigger or PIR.
I don’t know if I’ll get the chance to experiment and build this one this year. But if anyone wants to take a stab at it go for it, just let me know how it turned out for you and give me some pictures so I can see it in action.
In the diagrams below, the blueish lines are cord and the orange lines are bungee. The grayed out bars show the hammer in various positions in relation to the rest of the mechanics.
