LOW COST, portability and versatility are the key features of this temperature recorder pro- ject. All components are mounted on a single PC board measuring only 57 x 60mm, which means that it could be placed just about anywhere that temperature monitoring is required. No external connections are required during operation, as the recorder is powered by an on-board battery and all measurements are logged in non-volatile memory.

Fig.1: the circuit is based on the Dallas DS1615 temperature recorder IC. The device is self-powered and is plugged into the parallel port of a PC for setup and data retrieval.

The recorder board plugs directly into the parallel port of your PC to allow setup and data retrieval. Windows-based software makes the task straightforward and even includes charting and graphing facilities.

The measurement range is from -40°C to +85°C in 0.5°C increments and a total of 2048 measurements can be logged in memory. Also included is a histogram feature which provides 63 data bins with 2°C increments. Both temperature logging and histogram tabulation can be programmed for sampling intervals of once per minute to once every 255 minutes.

Circuit details

A Dallas DS1615 temperature recorder IC does all the work (see Fig.1). The actual temperature sensor is contained on-chip, as is a real time clock/calendar, non-volatile memory, a serial interface and the associated control logic (see Fig.2).

The DS1615 can source power from either its V_CC or V_BAT pins. When the V_CC pin is higher than V_BAT, the entire chip is powered from V_CC. When the V_BAT pin is higher than V_CC, the V_BAT pin powers everything except the serial interface circuitry.

Fig.2: block diagram of the DS1615 Temperature Recorder IC internals. Even the temperature sensor is located on-chip.

Two TTL output lines from the PC parallel port supply power to the V_CC pin via a 100µF capacitor. At first glance this might seem to be a rather unorthodox approach but as the DS1615 draws little current it does the job.

A 3.6V lithium battery powers the temperature recorder when it’s not connected to a PC. With the serial interface powered down, current is really only consumed during a temperature conversion cycle, when it peaks at a maximum of 600µA. This drops to a couple of µA between conversions, which is probably less than normal battery leakage. As you can see, the sample rate ultimately determines battery life.