So you want to make your own emergency button instead of buying a wearable to stay in touch? jeanotP1314 has shared a guide that shows how you can build a button that sends an e-mail through WiFi when pushed. The developer used a ESP8266 WiFi Module, LED, battery, and couple of other parts to put this together. You can use a SparkFun FTDI Basic Breakout or Arduino to program your device.
You can find all the parts and information about this project on Instructables.