Serial Temperature Monitoring Kit


Introduction.

We now have two different inexpensive kits to measure up to eight remote temperatures. The data is then sent serially to either a PC or terminal or Basic Stamp. Each kit is $25.00.

Description.

KIT-1820 consists of a processor which interfaces with a Dallas Semiconductor DS1820. The kit includes one DS1820 in a PR-35 package and additional devices may be purchased for $5.50 each. The advantage of the DS1820 is that it requires only two wires; data and ground. The disadvantage is that it is more expensive than the DS1821 and the DS1820 is not available in a TO-220 package.

KIT-1821 consists of a processor which interfaces with the DS1821. The kit includes two DS1821T devices in TO-220 packages. Additional devices, either in a PR-35 package or in a TO-220 package may be purchased for $3.50 each. The disadvantage is that three wires are required for each remote sensor; +5V, data and ground. The advantage of the TO-220 package is that it is substantial and may be bolted to a motor or similar.

Note that the programmed PIC used for the two kits are different. DS1820 devices and DS1821 devices cannot be mixed on the same processor.

Each kit consists of a PIC16C558 with a capability of interfacing with up to eight Dallas temperature devices. Upon receiving either a logic one on MEAS or a logic zero on /MEAS from an interfacing PC, PIC or Basic Stamp, the unit will intitiate a measurement sequence and return the eight temperature measurements in ASCII text via a 2400 baud serial lead.

	27.2 28.3 23.2 -99.99 19.3 83.2 -5.7 -6.7

Temperatures are in degrees C with a resolution of 0.1 degrees C. Note that 0.1 degrees resolution does not imply that degree of accuracy. The manufacturer indicates an accuracy of 1.0 degrees over the range of 0 to 85 degrees C and somewhat greater error over the range of -55 to 125 degrees C. However, such resolution may be useful in considering temperature changes.

An option (grounding the 1_LINE lead on the PIC) is provided to output the text either as shown above or on multiple lines as shown below.

	0 27.2
	1 28.3
	2 23.2
	3 -99.99
	4 19.3
	5 83.2
	6 -5.7
	7 -6.7

Each line is separated with a nominal 1.25 second delay. The intent in offering this option is to minimize the memory required by users desiring to interface with the Basic Stamp or a PIC.

The processor automatically detects whether a sensor is present and if not, returns -99.99.

The baud rate is fixed at 2400. A MAX232 is provided to convert TTL to RS232 levels. This permits interfacing with a PC or any serial device. A series 22K resistor is included to interface with a Basic Stamp. No flow control is provided.

One measurement sequence is provided on detection of either a logic one on MEAS, a logic zero on /MEAS or both. The idea is for the PC to send a character causing TX from the PC to go low when it desires a measurement. Part of the MAX232 provides the RS232 to TTL level shifting. If MEAS is strapped to a logic one or /MEAS is grounded, the processor will continually loop, making measurements and sending the results.

The +5VDC required by the temperature devices is derived from an output on the PIC. At the begining of each measurement sequence this is brought low for 1.0 seconds and then high for 1.0 secs. The reason for this is to correct if a device latches. They aren't supposed to latch, but I have seen it too many times in the lab. The data lead just stays at a logic zero and the only way I have been able to clear it is to remove power. Thus, to save you that manual intervention, the processor does this at the beginning of each sequence.

Each measurement requires nominally 1.25 seconds. A measurement result for a device is sent immediately after the measurement is made. Thus, there is a 1.25 sec delay between each result.

Thus, each sequence requires nominally 1.0 + 1.0 + 8 * 1.25 = 12.0 seconds.

The kit does not include a printed wiring board. The intent is to use a solderless breadboard and then wirewrap or whatever technique you prefer.

However, it is pretty simple. Power and ground for the PIC. Lay down two nine resistor SIPs, one on each side of the PIC. Connect the ceramic resonator. Add the MAX232. Add the temperature device.

The basic $25.00 kit includes the programmed PIC, the MAX232, and all supporting components; ceramic resonator, resistors and capacitors. KIT-1820 includes one DS1820 in a PR-35 package. KIT-1821 includes two DS1821T devices in a TO-220 package.

The kit include lengths of 22 awg solid wire suitable for assembling the kit on a solderless breadboard and for testing, you may place the thermometer device on the breadboard. However, the kit will does not include the cable to wire to remotely located thermometer devices.

The kit includes assembly instructions and typical code for interfacing with a PC and with a Basic Stamp 2. The code for the PC is limited to a QBASIC terminal routine. Send a character and the results of the measurement sequence appears on the screen.

There are a number of commercial software packages that permit serial data to be directly imported into spreadsheet. But, the sophistication associated with these packages, with Visual Basic, Windows 95 and NT is way over my head. Please appreciate that this is a $25.00 kit designed to provide you with the string of measurements. You take it from there.

Additional Information

Ordering Specifics.

KIT-1820. Includes PIC processor, MAX232 and all supporting components and documentation plus one DS1820 in a PR-35 package. (The PR-35 looks like a TO-92). $25.00.

DS1820PR35. Additional temperature device. $5.50. Note that seven additional are required if you desire the full eight device capability. (7 * $5.50 = $38.50).

KIT-1821. Same as KIT-1820, but includes processor for DS1821. Includes two DS1821T devices in a TTO-220 package.

DS1821T. Additional temperature device. TO-220 package. $3.50.

DS1821PR35. Additional temperature device. PR-35 package. $3.50.

Note that with the KIT-1821, six additional devices are required for the full eight device capability. They may be any mix of the PR-35 or TO-220 packages. (6 * $3.50 = $21.00).

Ordering Information.

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