Idea for a portable sensor indicator

Everyone, I would like to build a project which is used to alert people who forget to retrieve their belongings like key, wallet, mobile phone, briefcase, laptop and etc. For example, you might put your keys on the table to grab some foods from the fridge and then forgotten about the keys and walk away. the sensor will trigger a small alarm that held by the user. Thus, you will remember to recover the keys and save the time for searching your lost keys.

I would like to have the tag on the item with an alarm, and the other part on your body, something like a wristband or somesort.

Can someone please tell me what i can use to make this possible?

Thanks in advance.

RFID is the answer. You put small tags on the items and the transceiver on your body. But you will have to carry a battery with the transceiver. Another factor to consider is that you need to use larger range RFIDs which consume larger current of the transceiver.

thanks.

I need to know how to be able to set the range of transmission (distance), can i have some advice from you guys?

There is little bit problem with rfids with short distance they operate or high power of reader needed to get higher distances .

As alternative you can implement microtransmitter in this these things which is active for gew millisec per let say 5 secs and wear a small receiver to detect signal . If such signal does not come in 10 secs you will get an alarm from your weared receiver . Power estimation - if 20 mW and transmitter works only 1/1000 of time (5 millsec per 5 secs), overall power usage will be 20 uW which is pretty good for small sized things as battery size will be reduced.

I have seen the item that artem is talking about in car security system. The keychain had an auto-transmitter that transmitted signal for a short pulse duration. If the owner of the car was not the one driving it, the car's engine stopped and the doors locked.