Wireless temperature sensor help.

Question

I Am looking for a way to build a temperature sensor network wirelessly.

I Would like to build a system to transmit data from a few DS18B20 and DHT11 or DHT22 sensors back to a central Arduino for display/log/web purposes. The initial idea will be using 2 wired sensors and 2 wireless sensors, but I would like it to be expandable for up to 8 wireless if I can.

how do I turn a digital sensor into a wireless module?

i Need are ware suggestions here.

I Am trying to keep cost low as well.

jvdberg@ieee.org · Answer

How to I turn a digital sensor into a wireless module depends on the wireless module you intend to use. If the wireless module has a digital input you could just connect the digital output from the sensor to the digital input of the wireless module. There also must be a way to determine from which sensor the data is. If the wireless module does not provide an ID, you must add identification to the data from the sensor. The sensor probably is providing continuous data. To be able to transmit the data this must be serial data. If you just want to log room temperature and outside temperature one measurement per minute is more than sufficient. If the wireless transmitter-module contains a receiver, the central Arduino can ask the transmitter to transmit their message. Then with each sensor you need some control circuit that is listening to the central Arduino and will transmit its ID and data when asked for. If the wireless module can only transmit or receive analog signals like audio you need a simple modem to convert the digital signal into an audio signal. For instance the TCM3105. see http://www.tcm3105.com/tcm3105.pdf The first step is to select a wireless module and look at its specifications. Depending on the specifications of the wireless module you can make the interfaces to the Arduino and the sensors. As a wireless module you could use a low power CB-radio. There are also digital thermometers for sale having a wireless outdoor sensor. Then you need to do some reverse engineering to get the data from the thermometer in your arduino.

jvdberg@ieee.org · Answer

Those are the radio-units that can be used for your application. They all operate in licence free frequency bands. That means that the same frequency is in use for more applications like wireless car keys, garage door openers, wireless LAN, wireless headphones and other low power applications. Depending how many users there are in your neighbourhood you must make a choice between the frequency bands 27 MHz, 433 MHz, 2.4 GHz or 5 GHz.. Unfortunately you can only find out by ordering a transmitter and receiver and try it out. The radio-units you found are cheap so you don’t lose much money if you choose the wrong frequency-band. For short distance you can try out the software even when there are many unwanted signals on the frequency. It is also possible to use more than one frequency band. For instance the arduino transmits at 2.4 GHz and receives at 433 MHz.. This way the units are able to transmit and receive simultaneously. For a long battery-life of the outdoor units current consumption might also be a reason for selecting a certain type of radio-unit. To prevent misreading when your arduino is receiving signals from another user the received signal must be unique and have identification and preferable checksum bits. The cheapest solution has only a transmitter at each sensor and a receiver at the arduino. A random timer than will switch on the transmitter and send a message about once a minute. The disadvantage is that two transmitters could transmit at the same time. By using random timers the next time the transmitters will not transmit at the same time. When two transmitters send at the same time you will either be able to decode the strongest signal or receive an invalid message. A more robust approach has a receiver and a transmitter at each location. The arduino transmits a unique code for each sensor and after that will switch off the transmitter and listen for a reply. The sensor only transmits its message after it has received its code. There must be something between the sensor and the transmitter. This could be a hardware circuit or a processor. This circuit must select a single message from the datastream provided by the sensor and add the sensor code to it. This circuit must also switch on the transmitter when necessary.