A LOOK AT THE KP-235
Please read the data sheet of the KP-235 before carrying on….
The KP-235 chip is housed in a special sub-miniature header package. A small soldering station to solder the 4 pins numbered 5 to 8 is a must, as well as a very good eyesight. The initial tests were done with the chip fastened to a small wooden board by means of double edge tape with a gap right underneath the connection pins. Then another strip of double edge tape held the small interconnecting wires: it was then easy to solder the wires to the pins, but this arrangement is only good for preliminary testing in the lab.
After some head scratching I found that the header pin spacing matched that of a section of the standard “Shield” boards used as interfaces for the Arduino Microcontroller. It was then very easy to build the Barograph circuit. Only pins 5 to 8 are used: pins 1 to 4 are used at the factory to digitally test and calibrate the device. In this way we have a purely analogue sensor based on very accurate digital technology, the rationale being that it is much better (and simpler for the user) to operate digitally at the factory and to operate analogically in the field.
THE SCHEMATIC DIAGRAM
The schematic shows the barometer circuitry to the left, connected to the Universal Interface and the DrDAQ to the right. As we shall see in the theory of operation, sensor output voltage range is 3000 to 4350 mV, too high for the DrDAQ input. Sensor output is at pin 7 and is PULLED DOWN ( KP-235 data sheet, page 15) by R103, R104 & R105. R104 & R105 bring output down by a factor of 2, thus the voltage range seen by the DrDAQ is 1500 to 2175 mV. R104 and R105 are 1% high stability resistors and they should be matched for exactly the same value for optimum accuracy. Since the DrDAQ Universal Interface input impedance is nominally infinite, the PULL DOWN resistor value is the parallel of 56 KΩ with 30 KΩ, e.g.19,5 KΩ, a load well within the range given in data sheet, table 6 of page 15.
Sensor supply voltage of exactly 5 V is given by Precision Reference IC 102 and reaches the sensor at pin 5. This voltage also needs to be monitored by the DrDAQ: therefore R101 and R102 bring it down by a factor of exactly 4, to a nominal value of 1,25 V. For optimum precision R101 & R102 (high stability 1%) should additionally be selected for the following values:
R101 = 8200 Ω
R102 = 2730 Ω.
Precision Reference IC 102 (Linear Technolgy LT1021B-5) has an output voltage tolerance range of 4,950 to 5,050 V. Multiturn trimpot R107 allows precise and stable adjustment of output voltage to exactly 5.000 V. My advice, if no calibrated 5 digit standard multimeter is available, is to make good the the DrDAQ calibration, because, as we shall see, the system will automatically compensate any Ratiomentric error due to small supply voltage deviations. The advantage of using the LT1021 is given by the voltage stability with temperature and time, once R107 is set. The DrDAQ and the Universal Interface are fed by the +5V USB bus of the PC (jumper between pins 11 &12 of M1 connected) while the Barometer board requires an external D.C. voltage of +7.5 to + 12V at less than 10 mA. Attempts to feed the Barometer board with the +5V of the USB bus by using a small low power booster failed because of ground loop problems around the DrDAQ and the PC which would alter the readings of the DrDAQ.
In PART 3 we shall discuss operation and Picolog channel set-up.
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