For over twenty years we have built automated (animatronic) displays for the front porch to scare the trick and treaters at Halloween. Starting simple, with a ghost which makes a 'boo' sound when approached, these displays have become more elaborate over the years and now incorporate laser sensors, microcontrollers, and pneumatic actuators. Within the past ten years, displays have included witches who move, stirring boiling cauldrons, and skeletons which pop out of coffins. Our 2008 and 2009 displays, pictured here, featured a skeleton in an electric chair. When a kid approaches the porch the system triggers and the skeleton rises out of the chair accompanied by fog, a strobe light, and loud sounds of an electrical arc. The front window featured a rear-projection video of a large tesla coil operating with large arcs streaming everywhere to complete the 'mad scientist' appearance (also completed by yours truly wearing a lab coat and a wig of "shocked" white hair).

Electric Chair Prototype, Halloween 2008

The prototype for the 2008 skeleton is seen here in the workshop. The electric chair has a light rope for the "wires" connecting the skeleton to the chair. Upon triggering, fog is released by a fog machine behind the chair, illuminated by eerie green light and a strobe light.

The skeleton has taken various forms and each year it seems to get a bit more elaborate. In previous years, such as this display from 2005, the skeleton popped-out of a wooden coffin which, when triggered, is opened via a pneumatic cylinder. Once opened, and after a small delay, the skeleton suddenly pops right out, driven by a second cylinder.

The coffin itself was built from eighth-inch board with a 1-by-2 frame. Screen door closers operate as pneumatic cylinders - the adjustment screw was removed and a 1/8-27 pipe thread tapped into the end. A flexible line on compression fittings connects the cylinder to a 24V solenoid air control valve. Two cylinders and two valves are employed. When a laser trigger (described below) sets the display in motion, the first cylinder, which opens the coffin door, in activated. An RC delay circuit then provides a delay of about one second before the second valve is opened rapidly firing the skeleton out of the coffin. A commercial fog machine provides spooky effect by filling the coffin with fog while it is closed.

The cauldron and bones from a previous year's display sits on the right side of the porch.

Click Here to see a short movie showing the 2005 display in action


In 2006, the display combined the witch from 2003 and the coffin from 2005. Upon triggering the sensor (visible in the lower left of the photo) the coffin opened, and following a short time delay, the witch turned to greet the trick-or-treaters! Turning the witch was accomplished by mounting her on a piece of wood with a large bolt as a pivot. A pneumatic cylinder actuates the mount.


In 2004, the display featured a boiling cauldron 'stirred' by a witch. A hand, operated by a pneumatic cylinder, comes out of the cauldron when triggered. The display was improved two years later to feature the witch on a turntable allowing it to move quickly to face oncoming trick-and-treaters. Click Here to see a movie showing the cauldron in action

Costumes ...

As elaborate as our porch displays are, Jennifer (our youngest) also has had a number of elaborate costumes including a butterfly with colourful twinkling wings:

In the style of Disney's Spectromagic Parade, these wings are about 4 feet across and feature 140 bright, sequenced, lamps. Powering 140 lamps requires a large battery, in this case a 7Ah, 12V sealed lead-acid battery which runs a small inverter providing 120 volts for the light string.

The battery and inverter are mounted between the wings (which are made of a steel tubing frame) on a piece of plywood. Shoulder straps from an old knapsack serve as a mount.

Click Here to see a short movie showing the wings



An once again Jennifer had a lit costume. In 2006 we went energy efficient though - blue LED lights were used to surround the wings with power supplied by a 12V rechargeable drill battery. To supply the required 120 VDC the 12V was fed to a tiny inverter circuit previously used to power Nixie tube displays. Overall the wings weighed about half of the previous years' wings which featured a large lead-acid battery.

Twinkling wings Twinkling wings - circuit

For 2007's 'Tinkerbell' costume smaller wings were created which were made to twinkle by employing a PIC 16F876 microcontroller to generate random patterns on four sets of lamps, arranged so that the entire set runs off a 14.4 volt cordless drill battery. The photo does not do justice to the wings as it does not show the 'twinkling' effect - the effect is best with incandescent lamps as well, LEDs failed to show the gentle twinkling effect caused by the thermal inertia of the filament. The wings themselves were made from thick steel wire bent in the form of a wing and fastened to a piece of wood attached to straps from an old school backpack. Lamps were strung in sets of four across the frames and the outside of the wing covered with fabric sewn as a slip-cover: the slip-cover is pulled back on the lower right corner of a wing here to show the lamps underneath. 64 lamps were used in all. In 2008, the wings remained the same however a lithium-ion laptop battery was used to reduce weight: lithium batteries at 4.7Ah provide considerably more output than NiCads at about 2.2Ah and weight much less.

Mechanical Details ...

For the witch and cauldron display, the cauldron was made from paper-mache (I used strips of paper soaked in drywall compound) and is 22 inches in diameter. It features a moving hand which comes out of the cauldron when kids cross the laser-sensor (details below) by pneumatic power. The cauldron also features a moving stirring-spoon driven by a windshield-wiper motor inside - when attached to the witch it appears that the witch is stirring the brew. Pumped full of fog from a fog machine underneath, the addition of a red spotlight and a load of plastic bones on top gives the illusion of some sinister brew of bones!


2003's display was a plastic skeleton which rises from a coffin as kids approach the porch. The coffin itself is built from old pieces of 1/4" plywood with a 1*2 frame inside so it can easily be dismantled. An old storm-door closer acts as a single-acting pneumatic cylinder to open and close the door. A 3-way solenoid air valve operates the door on demand. This was the basis for 2005's improved display

An updated version, used in 2007, features a lift inside the coffin so that when triggered the coffin first opens then the skeleton 'sits up' abruptly to an upright position. Again, two pneumatic cylinders are employed - one to open the coffin and a second to prop the skeleton up - the action is seen here during prototyping in the workshop.

A fog machine fills the coffin full of spooky fog which is released when opened and a green floodlight inside the coffin draws attention to it.

The insides of the coffin reveal the skeleton (on a hinged board which lifts via a cylinder), a green floodlight, and a small fog machine. Previously, a larger fog machine was used which was mounted below the coffin and plumbed-in using ABS pipe.

The cylinder which opens the coffin is also visible in this photo (it is painted black to match the interior).


Electric Chair - Halloween 2008 The electric chair prop for 2008 consisted of a simple wooden chair built from low-grade, rough, wood. The skeleton is supported by two pieces of 1-by-2 wood hinged in the middle allowing the skeleton to move in a more natural way when lifting from the chair. The skeleton moves first in a bent-over position, when maximum extension is approached a chair pulls the support behind the skeleton's back causing the skeleton to move to a prone position. Two actuators could have been used however a single cylinder and two hinges proved to be a simpler solution. A close-up photo of the mechanism shot from the rear shows the arrangement of the cylinder, two support arms, and the chain which pulls-back on the arm to erect the skeleton. Wire ties hold the skeleton to the arms.

Electric Chair - Halloween 2008/2009

This shot of the rear of the chair was taken before it is covered with black cloth to hide the mechanism. A small fog machine was mounted on a bracket immediately behind the chair - when actuated it releases a programmable length blast of fog (the length set by a digital timer module). Beside the fog machine is a green floodlight which, when scattered by the fog, casts an eerie green glow all around and on the top is a strobe light (flashing associated with electricity). Also seen in the bottom of the photo is the solenoid air valve, timers for the fog machine, two power supplies, the electronics package, and set of computer speakers. The speakers are fed continuously from a laptop playing an audio file of "electricity" noises (loud arcing from a Tesla coil) and are simply connected to the same AC supply as the spotlight: when the display is activated the spotlight, strobe, and sound are all activated.


An old screen door closer, used as a pneumatic cylinders, is seen in the left photo - in this case the cylinder lifts a 1-by-2 inside the coffin (or on the chair) onto which the skeleton is mounted. The middle photo shows the two solenoid air control valves which direct compressed air into each cylinder (some displays used two cylinders, like the "skeleton popping out of the coffin" display while others like the "electrocuted skeleton" used only one). Both valves are connected to a simple manifold and a quick-connect fitting for connection to the compressed-air hose. Finally, the electrical controls in the right photo are seen here on the workbench. The laser sensor (held down here with a roll of duct tape during testing) sits atop one of two power supplies for the system. Beside that are two electronic timers used to control the fog machine (when used with the coffin, one injects fog into the coffin at periodic intervals and the second injects a burst of fog when the coffin opens. When used with the chair, only a burst is required when the display is activated). Finally, two relays on a board take the 'beam tripped' signal from the sensor and drive all DC (control solenoids) and AC (floodlight) loads. The electronics were designed to be generic enough to be used with various displays. The first relay (a 4PDT) activates immediately when the sensor is tripped while the second relay closes about one second later. Usually, the first relay activates fog and lights (and possibly the cylinder which opens the coffin for that display) and the second relay activates the skeleton motion.

The Laser Sensor ...

A key element of the display is an accurate sensor to trigger the display when a trick-or-treater approaches the porch. We had tried everything from PIR sensors to ultrasonics but these never proved reliable (the PIR, for example is quite unpredictable in cold weather when the kids are wearing thick jackets under their costumes). The laser sensor seen here proved to be the only reliable method of triggering. In addition to providing a simple trigger, the sensor was expanded to include two beams so that it can sense direction.

The display is triggered by a sensor which uses two infrared (780 nm) laser beams to detect which direction the 'victim' is travelling by determining which beam is broken first. A PIC16C84 microcontroller performs the necessary logic. The sensor then drives a 12V relay which activates the pneumatic air solenoid, a green spotlight in the coffin illuminating the skeleton, and sound effects from an inexpensive doorbell which screams when pressed (a commercially-available halloween prop obtained from a dollar-store for about $2.50).

The laser diode which provides the beam was scavenged from an old handheld barcode scanner. The laser includes a matching adjustable lens which proved quite useful in collimating the beam. The entire laser is housed in an old whiteboard marker along with an LM317T acting as a current regulator (The laser diode runs well at 55mA). Along with the laser diode the scanners also provided PIN detectors with matching dielectric filters to exclude all light except the 780nm laser's. The laser beam was collimated to cover both PIN diodes at the target distance.

An inside view of the sensor shows all optical and electronic components mounted on perf board inside an LED clock case. The PIN photodiodes along with matching filters are mounted behind a red bezel on the case front. The diodes are connected with 2M2 pull-up resistors to the input of two CMOS 4049 inverters before entering the PIC - by trial-and-error it was determined that the 2M2 resistance was optimal to provide a TTL-level signal (+5V when the beam was broken, 0V when the beam was complete). A PIC16C84 microcontroller runs a simple program to detect the direction of the motion based on which beam is broken first. Five DIP switches allow configuration changes such as direction (forward only or fwd and reverse, time for output pulse, retriggerable mode, etc.). Two DIP relays are triggered in response to the trigger event. To drive the entire display the output from the small DIP relay usually drives a much larger relay providing the current needed for the solenoid valves and spotlight.