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Wireless AVR Programmer (WiFi)
Blink a LED With Assembly Language & a PIC
DIY Arduino Soldering Station
Great design, and nice walkthrough of construction and usage. Good luck for contest!
LPD8806 VUMeter with PC & Arduino + GUI
Monitoring mains AC voltage is nothing like measure DC voltage like in this project. However, you'll want to look at the measurement that the open energy monitor project has documented (i.e. using a 9V AC transformer to do the measurement at a safer voltage - https://openenergymonitor.org/emon/buildingblocks/measuring-voltage-with-an-acac-power-adapter). You'll then want to have the main power to your appliance connected to the mains via a relay, which you can turn on and off, in order to connect or disconnect your appliance to the mains. This is of course just one of many possible ways - you could wirelessly control one of those wireless switch thingys (X1??) or a foot switch type switch for accessing hard to get power points.
NRF24 jr module for toy quadcopters
DIY Arduino Battery Spot Welder
ARDUINO FINGERPRINT SCANNER WITH 16X2 LCD
Arduino TFT Forecast Weather Station with ESP8266
Sleek ESP8266 Breadboard Adapter with Pin LABEL
Cheap Arduino WiFi shield with ESP8266
ATtiny Watch Core
ARDUINO MPPT SOLAR CHARGE CONTROLLER (Version-3.0)