Speaking the Same Language as a Wireless Thermometer

Temperature is a delicate thing. Our bodies have acclimated to a tight comfort band, so it is no wonder that we want to measure and control it accurately. Plus, heating and cooling are expensive. Measuring a single point in a dwelling may not be enough, especially if there are multiple controlled environments like a terrarium, pet enclosure, food storage, or just the garage in case the car needs to warm up. [Tim Leland] wanted to monitor commercially available sensors in several rooms of his house to track and send alerts.

The sensors of choice in this project are weather resistant and linked in his project page. Instead of connecting them to a black box, they are linked to a Raspberry Pi so your elaborate home automation schemes can commence. [Tim] learned how to speak the thermometer’s language from [Ray] who posted about it a few years ago.

The system worked well, but range from the receiver was only 10 feet. Thanks to some suggestions from his comments section, [Tim] switched the original 433MHz receiver for a superheterodyne version. Now the sensors can be a hundred feet from the hub. The upgraded receiver is also linked on his page.

We’ve delved into thermocouple reading recently, and we’ve featured [Tim Leland] and his 433MHz radios before.

6 thoughts on “Speaking the Same Language as a Wireless Thermometer

  1. Yep, the super regenerative receivers found in most Weather station base units are very wideband, not very sensitive, -85dB or so and only have a single free running loop PCB coil as antenna. A super het receiver like the micrf002 with a proper crystal oscillator and you get at Least a mile range.

    1. Maximum allowed transmit power on 433.92mhz is 10mW for unlicensed use, so your sensor will at maximum transmit at that power, but possibly it is less powerful to conserve battery power, in the neighbourhood of 1.5-2mW. So a super heterodyne receiver with a 1/4 wave (whip) should be able to pick that up from a 1000′ in open space. A mile is doable if the transmitter is 10mW Another trick you can do, is take your transmitter apart to check if the antenna is wire stub (either a piece of wire or a trace on the PCB that goes from a capacitor to nowhere. Then you saw that off and leave a few mm trace after the capacitor and add a proper 433.92 1/4 wave antenna (173mm at 433.92mhz). That will quadruple emitted power because the transmitter will be loaded harder by the correct length antenna, but still be legal below the 10mW. If your transmitter has a loop on the PCB as antenna, you can also put a piece of wire as antenna where the loop is terminated by a coil (looks like a fat resistor), but such transmitters are not crystal controlled, so you will possibly detune the transmitter and risk getting outside the ISM band. You can retune it with the brass trimmer inductor on the board if you have an RTL-SDR or similar way of measure where it oscillates. A third option is to use a LNA (low noise amplifier) in front of the receiver and even a band pass filter to filter out FM stations between the LNA and the receiver. Then you will get several miles of range on 10mW. The fourth option is to up the power of the transmitter. The transmit part in these sensors work happily on 12V (but the rest of the sensor still needs 3V or whatever it is using). This certainly wont be legal, so you better not and it will also shift the transmit frequency up, even if the transmitter is crystal controlled.

  2. “Our bodies have acclimated to a tight comfort band” yea, we’ve really whimped out over the decades ever since air conditioning and central heating. Gone are the days when men could wear kilts comfortably while walking out in the cool morning, cold enough to see your breath.

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