Imagine you have a black box with 5VDC and ground inputs and you have to create one output that is an oscillating signal. What is the simplest circuit that can do so? Can you create a tank circuit with an inductor and capacitor?
The output signal will be detected by a PIC. The frequency is not important but should be rather low (between 10 and 500Hz). The PIC will not measure the frequency but only detect if the oscillating signal is present or not present based on whether this "box" is connected or not. That means the signal can be sin, square, saw-tooth, whatever, the shape doesn't matter.
Bonus points for the cheapest, lowest component count and lowest real estate solution!
Answer
Lowest component count I can think of:
The 74HC1G14 is the single gate version of the 74HC14 in SOT-23 package.
OK, I lied. You can do it with less. Take a microcontroller with an internal oscillator and write this incredibly complicated program to output a square wave. Number of components: 1. Board space: 6 mm\$^2\$.
If you drop the frequency restriction you can use a LED: f ~ 374740572500000 Hz. ;-)
Also out-of-spec is the Schmitt-trigger inverter with the output connected to the input. That's also a 1-component solution. Should oscillate at a few MHz.
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