Issue 99, October 1998
X-Y Graphing Data Logger

INSIDE THE BOX

The final circuit diagram is shown in Figure 4. Since this is a low-power, unshielded, mixed A/D circuit, the overall result depends on the quality of the layout.

Even though we’re all accustomed to 12-bit converters, you must take great care to correct grounding so you have stable readings of the least significant bits. The analog and digital grounds must be kept separate. They have to be joined only at one point (as near as possible to the regulator ground). Bypass capacitors are mandatory, and capacitors on the inputs are equally necessary.

The MAX186 has a good pin layout, which helps separate input lines from data lines, and a stable readout is easily obtainable. But in noisy environments, consider averaging in software to further reduce uncertainty.

The eighth analog input is left unconnected on the prototype. This arrangement enabled me to bring the power out to the nine-pin input connector, thus powering external sensors. However, it’s fully supported by software, so if you need it, simply add an input capacitor and feel free to use it.

Of course, power supply is critical in every battery-operated device. Power-up rise times must be short because there is no external reset circuitry for the micro. The MCLR pin is tied directly to VCC.

The low-power regulator is more delicate than a regular 78L05. It requires an output electrolytic capacitor—better if it’s tantalum.

The required current is impulsive, consisting of a 50-mA offset (sleeping micro and ADC), 1.5 mA during serial communication with the calculator, a single pulse of 1.5 mA for a few milliseconds (A/D conversion), and a 1-ms peak for LED flashing (current given by the LED series resistor).

The LED flashes once per conversion. I recommend a red LED because it produces more light with the same current.

I brought the power supply to input pin 8 in case some sensor needs it. Be sure to take sensor power requirements into account when estimating battery life. If the sensor sinks a significant amount of current, consider powering it through one of the free PIC I/O pins, which will power off when not in use.

I assembled the whole circuit using a prototyping board, and it fits inside a small plastic box that’s only the size of the calculator. The box contains all the circuits and the battery. Only the micro jack connector (connecting to the calculator serial port) hangs out.

As you see from Photo 1, there’s a lot of space left on the board. If possible, use a 90° jack to keep the unit even more compact and rugged.