Building a small D cell battery charger?
I would NOT design anything that would require direct AC power if I were you. Really. You can design your circuit to take 12/24VDC or whatever you need from a wall wart and then use that to charge your D cell.

Seriously. Do not go near the AC outlet. There are lots of design safety issues to consider, the first of which is using an isolating transformer. 

Oh one more thing. Do not design anything that takes AC power. Don't. 

As I said you can get a wall wart, or use any of the adapters you have sitting around if it supplies the DC voltage/amperage you need (which it should be able to) just use that.

There are lots of NiMH charger circuits out there you can use for reference. Your charging current is a function of your battery capacity, often expressed in terms like 0.1C meaning if your battery is 1Ah you want to charge it at 100mA.

You want a smart charger because NiMH are sensitive to charging current and voltage and have differing requirements depending on how discharged they are. If you ignore those you'll damage your battery.

I am fairly certain you should be able to find a commercial charger that can handle your D-cell. It may not be physically compatible in which case a set of alligator clips/wires can help you connect the two.