Minecraft redstone observers and pistons

Solution 1:

You are looking for a stabilizer circuit. It is a simple circuit which requires two repeaters and some redstone dust.

Here is an image to build it:

Note: One of the two repeaters is set to a 4 tick delay. This solution will pulse the piston twice and then stop.

If you absolutely need the piston to only power once, you have to use a more complicated circuit. Each cycle the observer pulses 3 times. Once when a block is placed in front of it, once when the piston extends, and once when the piston retracts. You have to use a pulse divider to divide pulses by 3. This means the circuit will output 1 pulse for every 3 pulses in. This can be accomplished with 6 droppers, a comparator, a repeater, and some redstone dust.

Here is an image to build it:

Place 1 item in the dropper which is being tested by the comparator. As the observer pulses, the item moves through the dropper system.