The temperature sensor is a fairly easy circuit to build. Generally consisting of just the PIC, temperature sensor, TL431 (low battery sense) and RF transmitter.
Parts list:
1x Enclosure
1x PIC16LF88 (RS components)
1x Bi colour LED
1x 330R resistor
1x TL431
3x 4K7 resistor
1x 1M resistor
1x Transmitter / receiver pair (Receiver not needed) - eBay (433 Mhz Europe or 310 Mhz USA version available)
1x Battery holder (Hold 3x AA or AAA) See below for more info on battery voltage
1x Dallas DS18B20 temperature sensor (you must get the version with B in the description)
1x Veroboard (approx)
The ASM file requires include files. Everything is in the proteus package at the end of this page.
Specifications: Operates on 3 to 5 volts DC.
Use 3x 1.5v batteries rather than 2x as you won't get full usage out of 2x batteries.
Low idle current. At 4.5v current is around 10 uA
Reads -55 to 127 degrees Celsius
Device is addressable - 254 combinations
How it works.
From power up, the device samples the battery state and temperature.
Once this is complete, data is output to the transmitter.
The LED will briefly illuminate green to indicate operation or
the LED will illuminate red when battery low.
The device goes into sleep mode for around 4-5 minutes. The whole cycle runs again.
Testing:
Once powered up, watch for the LED briefly flash after a second or two.
This won't happen again for another 4-5 minutes, so its worth powering down and up again if you missed it.
Check if the windows test software showed anything in the receive window.
If you have a 434 Mhz radio receiver, you could listen for the signal chirp being broadcast.
As the Transmitter/receiver are sold as a pair, its possible to connect the receiver up to a speaker and physically hear the audio on 434/310 Mhz. I will document this elsewhere. However, its simply a case of supplying 5v to the receiver, and the speaker is connected between data and ground.
Aerial information
External antenna: 17cm ordinary multi core or single core line (A strand of CAT5 cable is ideal).
Shorter aerials will work but with reduced range.
You can wind it into a spiral if this is preferable.
Additional
Recent testing on a 3v Lithium disc battery has shown fair results even on a short aerial (10cm). If you can afford the reduced transmitting range, the sensor can be built very small.
Since 2015 (now 2020) I've got typical results on battery performance/life which surprised me.
3v Lithium lasted for 11 months.
2x AA batteries lasted 3 years.
3x AA batteries are still running after 5 years
(2025 update, 10 years and still running on the same batteries!!)
I've a feeling that they will leak before they fail. Below is the module and actual budget batteries still installed.
Various versions of the circuit and housings.
 |
 |
 |
 |
 |
 |
As per usual, if you have Proteus by Labcenter electronics, you can run this in simulation.
All the files needed are here.
← Back to Projects