This kit has been discontinued. Over the years we have developed better designs.
Introduction
We have completed the development of Microchip PIC based system which can, in theory, accommodate up to 256 DS1820 temperature sensors on each of two twisted pair runs. Thus a total of 512 temperature sensors. Note that the user may simply add sensors on a run and the PIC will "automatically" find them when a measurement sequence is initiated.
The design consists of a programmed PIC12C672 (8-pin DIP), a 4.0 MHz ceramic resonator, five resistors and DS1820 sensors. The kit includes one DS18S20. Additional sensors are available at $4.50 each.
(The kit does not include a PCB. However, even a three web finger person like myself can put a single 8-pin DIP and resonator on a small piece of perf board.)
Detailed Description
When the GO input to the PIC momentarily goes high, the PIC proceeds to find the devices and make measurements. The PIC determines the first address of a DS1820 on twisted pair run 0, performs a temperature measurement and returns the result using 9600 baud serial. It then continues to determine the second address on run 0, performs the temperature measurement and sends the result and this process continues until no additional devices are found on run 0. This process is then repeated on the second twisted pair run.
Note that a complete measurement sequence is initiated by bringing the GO input momentarily high. The may be the PC sending any character. If the GO input is tied high (or left open) the PIC continually performs measurment sequences.
Each temperature measurement requires nominally one second.
The format of the returned data is;
RDD TTT.TT SSSSSSSS SSSSSSSSwhere R is the run number, either 0 or 1.
DD a sequential hexadecimal number in the range of 00 - FF.
TTT.TT is the temperature result in degrees C.
SSSSSSSS SSSSSSSSS is the unique 64-bit serial number of the DS1820 expressed on hexadecimal
For example;
000 27.32 10D6F33A 00000036 001 26.54 10773B3B 000000CE 002 -5.43 etc 003 105.34 100 -32.33 101 16.12Note that in this example, four devices were found on Run 0, and two were found on Run 1.
Note that fields are delimited by single spaces and the line is terminated by a new line consisting a CR and LF.
The range of the temperature is -55.00 to 125.00 degrees C. The unit provides a resolution of 0.03 degrees C. Please refer to the DS1820 data sheet at for absolute accuracy over the full temperature range.
The serial numbers of the devices are provided to permit the user to ascertain which sensor is which. Note that the two digit device number DD is simply a running number beginning at 00 which is assigned as each device is found using a binary search on each of the twisted pair runs.
Thus, 00 might correspond to a DS1820 in your PC, 01 to the living room, 02 to the freezer. Assume you then add one on a cold water pipe in the basement. You may well find 00 is still your PC, but 01 is now the water pipe, 02 is the living room and 03 in the freezer.
However, note that once a system is in place, 00 will always be the PC, 01 the water pipe, etc. But if your cat pulls off the sensor in the living room, 02 will then be the freezer.
If you don't wish to use the serial number information, you might consider initially connecting the sensors on a breadboard and use your finger to determine which device is 00, which is 01, etc and then label each of the sensors and then install them with 00 in the PC, 01 in the living room, etc.
Note that all DS1820 devices are operated in the parasitic power mode (the DS1820s use the signal lead for power) and thus a single twisted pair may be multipled from one device to the next.
The PIC is theoretically capable of finding and measuring the temperature of each of 281,000,000,000,000 devices on each of the twisted pair runs. However, the limiting factor is the capacitance of the cable and the capacitance of each DS1820. My guesstimate is that 16 devices on a total of 200 feet of twisted pair is realistic. Note that the use of the parasitic power mode does not limit the amount of cabling nor the number of devices.
The serial data is 9600 baud inverted. The intent is that the PIC may be directly interfaced with a PC COM port with no intermediate level shifter such as a MAX232 or DS275. In the past I have been hesitant to take this approach. However, the very popular BX24 (Basic Stamp Super Clone) uses this approach and I have yet to see a complaint. (But, if you should find it does not work for you, we will provide a modified PIC and a MAX232 at no charge).
The maximum current drain of the PIC and all of the DS1820's is 3.0 mA. This offers the possibility of powering the circuitry from the COM port and the kit includes a 78L05 regulator and associated capacitors.
The $25 kit includes the programmed PIC16C672 8-pin DIP, 4.0 MHz ceramic resonator (3-pin SIP), one DS1820 and five resistors, a 78L05 regulator and associated capacitors, the Morgan Logic Probe for debugging, wire for bread boarding and documentation.
. This was provided by a satisfied customer in New Jersey. Note that the 7805 regulator is to the rear, the logic probe in the middle and the PIC12C672 in the front.
Interfacing with a UNIX Workstation/a> by Ottmar Klaas at RPI.