R E M O T E  T H E R M O M E T E R
This article presents the design of a battery powered room thermometer able to periodically send measured data via IR light to a remote receiver. What the receiver does with the data is not discussed in details, but a few interesting ideas are mentioned.
Basic idea
A growing number of home appliances uses a trermometer probe as an input sensor using which they are able to self-regulate. For example, home heating systems need thermometers in order to keep room temperature inside the desired range. The purpose of this little device is to overcome limitations of fixed wired thermometers such appliances use. Remote thermometer transmits powerfull IR signals which can be received at distances up to 15 m, so placing it at any desired location inside a room is possible. This way, users can find optimal thermometer position from the aspect of actually measuring the temperature at the desired location, not having to bother with maximal wire length available, ugliness of wires dangling around the room etc.
IR remote thermometer block diagram
Remote thermometer uses three "AA" alkaline batteries as a power source. Standard quality cells provide more than a year of continuous operation. This is possible by limiting the time the thermometer is active to a fraction of a second while most of it the device spends in sleeping mode. Data is sent to the receiver in short bursts approximately every 20 seconds, which is, having in mind very low speed by which the temperature might change, more than frequent enough for home heating appliances to be updated to the current state of room temperature.
Accuracy of the measured data is better than 0,5'C - again, more than sufficient for the intended purpose. If better precision and accuracy is needed, it would not be hard to enhance hardware and software a bit but home automation sort of applications should do just fine with the existing design. The device uses a standard KTYxx temperature sensors which are rather cheap but very stable and reliable. Microcontroller software provides an auto calibration function so that any daviations of sensor parameters from standard values can automatically be compensated even if mass production of these gadgets is planed.
It is important to note that remote thermometer doesn't incorporate any displays or control inputs as this would increase battery drain much above any acceptable level. All it does is periodically sending temperature data in the prescribed digital form into the "free space". It is up to the receiver unit to process the received data and do someting useful with it.
As a demonstrational receiver device, a thermostat able to control the operation of an electrical fan has been designed. That module displays current room temperature on a 2-digit numerical LED display and provides means of adjusting desired room temperature via a couple of push buttons. What's more, the fact that it necessarily contains a standard IR receiver module by which it receives temperature data bursts emitted by the remote thermometer led to the idea that a universal IR decoding routine be inserted into the basic controller software so that the desired temperature can be adjusted by any standard remote control unit. Some more details on this default receiver device operation are given on the last page of this article.
designed by LP 2011