Therm - HP48 Temperature Logging Application

 

Introduction

This little project revolves around the interfacing of the HP48 with Parallax's Basic Stamp microcontroller and Dallas Semiconductor's DS1620 digital thermometer chip. The DS1620 is an inexpensive(~$6) and fairly accurate(+/- 1 degreeC) chip packaged neatly in an 8-pin DIP. The chip uses a 3 wire serial communication which can be interfaced directly to the Basic Stamp with minimal parts support(1 resistor and 1 capacitor).  The Basic Stamp reads the temperature from the DS1620 and outputs the reading via a RS-232 port to the connected HP48. The HP48 then takes the temperature data and does some conversion then you can have the option of doing something with the data. Like displaying it or return the temperature as a value to the stack or time stamp each measurement for logging purposes.

 

Construction

Making a probe out of the DS1620 required a little ingenuity. First, I mounted a 8-pin IC socket on a small piece of PCB cut out to fit into the slotted end of a 9-pin DSub shell. The PCB was etched out for the 8-pin socket where wires would also be attached from the backside. I then
proceeded to mount the capacitor on the backside (copper-side) of the PCB.  Using a 5 conductor shielded computer cable, I attached the resistor to one of the leads. With all the leads cut to the same length, I soldered each to their respective pins on the backside of the PCB. With all the connections made, I housed the sensor in the plastic 9-pin DSub shell.  If you plan to use the sensor in a hostile environment, I would recommend filling the shell with epoxy. With the shells I used, it was necessary to shim the wire anchor, since I was using a narrow diameter cable. With
all the support components mounted within the shell, it was only necessary to attach the wires to the Basic Stamp. In my case, I was using a Basic Stamp II with the carrier board which conveniently has a built-in RS-232 port, a 9-volt battery clip and a male header for external pin connections.  I soldered the wires from the probe to a set of female headers and plugged
them into the carrier board's header pins. One set for the I/O pins and the other for power. The same RS-232 port that would be used in programming the Basic Stamp would also be used to connect to the HP.

 

Software Interface

The Basic Stamp requires a small program in order to get readings from the DS1620. The DS1620 has a few undocumented features which allows you to get temperature readings with a higher resolution. This particular feature is useful when you want to know the relative temperature change.  Take note, this has nothing to do with accuracy. The rated accuracy is +\- 1 degreeC over the range of -55 to +125 degreeC. Basically, the stamp will read three parameters, the temperature and two counter values. The three values along with a start marker will be sent serially to the HP48. The start marker is necessary to mark the beginning of the three parameters in order to separate each successive reading.   On the HP side, I wrote a small library to read the serial port and to take the three parameters and convert them to a meaningful temperature. Depending on which command you run, you can either display the temperatures as they are being read-in, or take one reading and return the converted temperature to the stack, or take one reading and return an array containing a Julian day time stamp along with the converted
temperature.

 

Practical Use

There are countless applications for this project but one of the more interesting features is the ability of the HP to log at certain times and intervals. You could set a control alarm to log anywhere from 10s intervals to weekly intervals. 10s is the minimum interval because it takes about 5s to acquire a reading. When you finish logging or want to inspect the data, you can graph it directly using a scatter plot and observe any trends, anomalies, etc. How you want to implement the project is up to you, so send me a note if you have done something interesting. Good luck!

 

Download Software

Therm v1.0 - HP48 library to interface with the Basic Stamp.

bstherm.bs2 - Basic Stamp II code to interface with the DS1620 and HP48.