1. Defining the problem:

My wife and I misplace items such as keys, glasses, cell phones, etc. on a regular basis. We finish getting ready to leave for work in the morning and are about to head out the door when we realize one of us can't find the car keys. Ten minutes are spent searching before the keys turn up between couch cushions or under a newspaper. Lots of time could be saved if there was a small device attached to items that we could ping to locate them.

These devices would need to be about the size of a key fob (roughly 25 x 25 x 10 mm), run off a coin cell (ideally rechargeable), last for at least a month between charges, have a unique ID, and respond to an RF remote within 25 m. The Trinket Pro will be the brains of the remote to satisfy the hackaday competition requirements. The limiting factor in this project is definitely going to be the idle power of the dongles, which will drive most of the decision-making.

2. Choosing the best frequency:

Before starting the circuit design, I want to choose the frequency band that will conform to the following requirements:

  • It must be an unlicensed frequency to comply with the FCC
  • It must be an open frequency in countries other than just the United States so international hackers can use the same open design
  • It must be compatible with ultra low power options to enable long battery life
  • It should have low interference from external signals

Based on the first two requirements, 433 MHz seems like a natural choice. It is free to use in most countries and there are many options for low power ICs that you can buy. I'm a little concerned with interference from external signals (car fobs, wireless temperature sensors, etc.), but I guess we will see if that is an issue later on. I want to avoid 2.4 and 5 GHz since there are a lot of wireless routers within range of our apartment.

3. Basic concept:

In order to save on power, I want to design a dongle that will listen for a ping using a low power standby mode, check if the ping matches the device ID, and enable a piezo buzzer if it does. I was originally thinking of making some sort of range finding built into the remote, but since the contest deadline is only about a month away and I have zero experience in RF communication, I just want something that works. The following is a simple block diagram of the device: