As seen at the 2006 Maker Faire in San Mateo, CA and DaVinci-Days in Corvallis, Or
A. An exercise bike with a speed/distance monitor. If it has this monitor, the motion sensor we need should be available.
B. A video game controller to hack. I've hacked two of them, so I can tell you exactly how for a Pelican TT Cobra racing wheel for PS2 or a Pelican Afterglow Pro for PS2 (after-market DualShock type).
C. The following components: 555 Timer chip, (2) 2N3904 Transistors, Resistors: 2K, 3K, 5K, 10K, 20K, Capacitors: 0.01uF and 5uF, a diode, a 100K Trim Pot or two. All are available at Radio Shack or online with a supplier such as DigiKey.
D. Basic electronics tools. At minimum you'll need a wire stripper, Digital Multimeter, wire set and breadboard or perf board and soldering iron.
I started with a used Tunturi recumbent exercise bike. It already had a speed sensor (as many exercise machines do), so it was only necessary to splice the sensor wires into the circuit. A continuity test (using the resistor value setting on the digital multimeter) confirmed that the sensor was a reed switch. The switch trips once a revolution and the multimeter registers zero resistance when the switch trips.
I noted the connections before removing the wires and then testing the resistances of the pots at various pedal positions. The cord to the unit is all you need. There are 4 wires which will be spliced into the circuit (shown below).
Pros: It's a racing wheel dude! And it runs ALL the racing games. And you don't have to solder anything.
Cons: The main problem is actually the wheel itself. It is very hard to pedal and have this thing on your lap. If you take the time to figure out how to attach it to something that won't get in the way of your legs, then it will work though. Another issue is that the accelerator emulates the X button, so unless you disable that button, cheating is too easy. But if you do disable the button, then you've got to pedal to select options and things. That's annoying. So read on, and I'll tell you how to hack a DualShock type controller...
Note: The resistor R2 (pink) seemed to need to vary a bit between games. You should start with around 2K.
After the racing wheel, I really wanted to hack a DualShock-type PS2 controller. The Afterglow had a clear case and I could see the solder points. Not that you couldn't hack a stock controller. If I were, I'd use the Multimeter in continuity (resistance) mode to trace the mylar buttons to the tiny little solder points. Clearly the Afterglow was going to be a lot easier.
I wanted to hack the R1 button, instead of the X button, because then you wouldn't have to pedal to select options. Was this a mistake? Maybe so, since two of my favorite games, Burnout Revenge and NFS Hot Pursuit can't use R1. But ATV2, Crazy Taxi, GT3 can. You decide. In any case, this was by far the better of the two hacks!
First I took off the cover and removed the R1 (and L1 button, since Crazy Taxi also uses it for Accel). Now you really have to pedal to go.
Next I identified the solder points and soldered three wires: Power (one side of the R1 button, Accel (the other side of R1), and Gnd. I tied the wires off for stress relief and routed them out the R1 button hole. Then I replaced the case.
Then I plugged those wires into my breadboard, turned on a game, and used a 100K Trim Pot to find the resistances for Stop and Go.
You will need to tune the circuit for your exercise machine. I used a 100k cermet trimmer (trim pot) in place of the resistor R1 (blue). Tuning is easy: Just exercise, play the game, and turn the trimmer screw until the desired response is achieved. My exercise bike trips the reed switch at about twice a second at a fast pedaling rate. For this frequency, the desired value of R1 was 44k.
The circuit I chose was a simple application of the 555 timer chip: The "missing pulse detector". This circuit has the effect of making our exercise bike pulses more "chunky". Check out the figure to the right to see what I mean.
Here is how the rest of the circuit works (see diagrams above). As the exercise bike is pedaled, the exercise bike pulses the circuit. If the timer chip output (pin 3) goes high, then the output transistor (red) switches on shorting the yellow resistor. This is equivalent to stomping the accelerator pedal. When pin 3 is low, the output transistor acts as an open circuit, and current again passes through the yellow resistor. This is the same as removing your foot from the accelerator. For the racing wheel, the brake is always off (we don't need no stinkin' brake!), due to the two 5k resistors (green).
All component values are approximate; plus or minus 10% will make little difference. If the rotary sensor on your exercise machine signals very fast (like 20 times a second or more), then you may need to try a smaller capacitor on pin 6. If you are new to electronics, your best bet is to use a breadboard (solderless prototyping board) and a wire set. Electronics suppliers should carry both for less than $20.