Help me please

I have an Arduino Uno with a relay shield attached, the relay shield works with digital pin 4 off the arduino. I need help making the Arduino reveive PPM code from a receiver that will tell the pin 4 to go low, only for a very short time. Pin 4 needs to be set to always high until told to go low to activate the relay.

This is for wireless starting of a remote control lawn mower I am working on for a school project. Any help would be appreciated.

Take a look here:

Good luck!

If you're using an RC receiver to control the Arduino then it's not really ppm that you're using - it's just servo pwm. The Arduino can send and receive servo PWM signals easily enough. I suggest you look for examples in the playground.