An Arduino and two insulated, twisted wires can be used to detect an approaching human (or animal). I set up a sample circuit to warn approaching people away from my desk.
The voice module in this instructable is optional--you can just use the twisted wire and an led to try this (or use the Arduino's onboard led on pin 13--change pin 5 in the sketch to pin 13).
Step 1: Parts
An Arduino Uno and two four inch long pieces of insulated wire are the minimum components to test this sensor (the onboard pin 13 led can be used to see the result).
I added a white led, 100 ohm series resistor, and a sound module to make it more interesting.
The speaker wire included with the sound module must be soldered to the included speaker. The speaker doesn't provide much volume, but you can hear the results.
Step 2: What's Going On?
Some background information can be found here (I call this the "Connor Nishijima Effect," named after the bloke who described this phenomena).
Here's what I think is going on. If you attach a scope to two twisted wires (the twisted wires are not connected electrically), you will get a 60 cycle wave--with a lot of "fuzz" on the wave. When you approach the twisted wires, the "fuzz" decreases. To eliminate the 60 cycle component of the wave, the time between the first and second "digitalRead" statements should be a multiple of 60 cycle waves (50 milliseconds, 100 milliseconds, 500 milliseconds, etc.)
The wire going into the Arduino's analog input pin must be twisted with a "reference wire." I put the reference wire into "gnd," but it works just as well if placed in "+5 volt." The system does not need any reference to earth ground--I have better luck if there is no power line connection (9 volt battery power works great).
If the power source is noisy (9 volt wall wart); this will not work (I tried). The low cost wall wart supply probably introduces a lot of noise. The usb supply from my Macbook works fine.
Here's the schematic for the setup I showed on the video in this instructable.
Power the speech board from +5 and gnd on the Arduino. Wire the "P-E" pin on the sound module to the output pin 4 of the Arduino.
Record up to 10 seconds of speech on the board using the board's "record" button.
Use the "human near" Arduino sketch to detect an approaching human.
Although my testing is very limited (taped this arrangement to cardboard and let it ride a robot), it appears to work from a moving platform--so that may be an interesting feature.