My eChair

The most brilliant part of the electric chair is the knife switch.  You have almost certainly seen the gruesome movie sight of a convict getting zapped in an electric chair, but have you ever been the one to throw the switch?  My son Rob wanted me to make something really nasty for his Halloween haunted house. It  was a tour through and around his house for trick-or-treaters and his college friends. I think I overshot.  This one was absolutely horrible. Not only did you see someone get cooked to death, but you had to be the one doing it.  It was surprising how many people readily stepped up to the switch. 


(note the CRAFT'SMAN label)


girlfriend                                     boyfriend

I found that girlfriends (of the actor in the chair) will hold the switch down until the smoke starts coming out.  I'm still trying to figure out why, but it sure made us all laugh until our sides hurt.
Actually, this phenomena extended to all family members.  Here's a picture of Rob frying me until I was thoroughly cooked!   AND I BUILT THIS CHAIR FOR HIM!  That's gratitude for you.

The Design

Originally, my dad and I used normal relays and capacitors (see dad's drawing), but we soon realized, this was a job for an Arduino.  With the original design, the actor (usually me) had to push buttons under the hand rests to operate the flood light and smoke machine.  This required training.  Unfortunately, more than one of the wannabe actors had at least one beer in their system. This lead to too much confusion.  Also, I wanted the "shock" lights and the flood light to flicker like they do in the movies.  So I built this:

I would like to take this opportunity to thank and apologize to my wife Susan, for putting up with missing Tupperware containers. I have found that Tupperware is the way to go when you want to protect electronics. The "cover" is easy to remove, it's physically strong, it keeps moisture out and most importantly, it keeps those God damn arthropods out.  In the picture above, the circuit board is screwed to the breadboard through the Tupperware lid.  The rest of the container becomes the cover and is snapped on when you're done programming. 

The hardware is basically a Boarduino and four relays.  However, on this project I wanted to play with the on-chip EEPROM.  It's like having a little hard drive onboard.  I added two buttons and a pot for setting seven different parameters.  The parameters are things like "Cook Time" (the number of seconds a person should be shocked before the smoke turns on) and "Shock Speed" (how fast the shock lights should flicker).  The interesting thing here is how to update the EEPROM.  I noticed in the data sheet that any one byte in the EEPROM should not be changed more than one hundred thousand times.  That made me design a simple class that only re-burns a byte if it has changed (see perm_args.cpp). 

Another useful thing to note is that the relays take a little more power than one pin can deliver.  So I drive each relay from two pins.  Just turn the two pins on and off together and, voila, you get twice the current without the use of a transistor. 

The sound was one of the best effects.  An iPod played this sound track in a loop.  The right channel contained electrical buzzing and humming noises which stayed on all the time.  The left channel (which is much louder) is fed through one of the relays so that it can only be heard when the main knife switch is closed.

For details, please see my source code.