This project is very similar to the single-channel radio-controlled toy, but is a lot more sophisticated. Project cost: Around $100. What you'll end up with is a radio transmitter and a radio receiver that you can plug up to four vibrating toys into simultaneously. Each toy can be controlled from the transmitter individually.
You'll need a radio-controlled car of the variety that allows steering and forward-reverse control; not the expensive, upscale digital proportional kind, but the simpler variety with forward-reverse and left-right controls on the transmitter. You'll also need four small phono jacks (male and female parts), four small power diodes (1N4001 or equivalent; really doesn't matter too much) and of course four toys. The easiest to work with are of the variety that has a separate battery pack/speed control attached by a wire to the toy.
That's it for the radio. Now you'll need to modify the toys. Take each toy, cut the wires leading from the toy to its battery pack, and strip about 1/4" of the insulation from these wires. Starting with one toy, attach the two wires leading to the toy to the two terminals on one of your phono plugs. Put this one aside and do the same thing for the other three toys.
Now you're ready to test it out. Plug all four toys into the jacks you have connected to the radio receiver. Turn on the receiver; none of the toys should run. Now turn on the transmitter. It will have two levers: one to make the car run forward/backward, and one to make the car turn left/right. When you move the lever to make the car run forward, one of the toys plugged in to where the drive motor used to be should run. When you pull the lever to make the car run backward, the other toy plugged in to where the drive motor used to be should run. Ditto for moving the lever to turn the car left or right: when you press it left, one toy connected to where the steering motor used to be should run; when you press it right, the other toy in that pair should run.
Note that with this arrangement you can't run all the toys at the same time. You can make one out of each pair of toys run, so only 2 of the 4 can operate at once.
Troubleshooting: If none of the toys will run, check all your connections. Are there batteries in the transmitter and receiver? Are they both turned on? Make sure you leave the antenna connected to the receiver. If two toys turn on when you move one of the levers one way, and no toys turn on when you move the lever the other way, you have one of your diodes connected backwards. The anode of one diode should be connected to one jack for each pair; the cathode of the other diode should be connected to the other jack.
Note: Plans for a six-speed radio-control toy are available on Sartan's BDSM Workshop page located here. This unit is slightly different in design; it can drive only a single toy, at one of six pre-set speeds.