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PICAXE humidity using the HopeRF Recent SILICON CHIP articles on HopeRF 433MHz data transceivers have drawn our attention to other monitoring modules made by this Chinese firm. Amongst their offerings is a well-priced and calibrated humidity sensor, the HH10D. This has shown itself capable of extremely simple interfacing to even the humblest PICAXE, although factory calibration values first need reading via an I2C-level PICAXE such as the 18X. A lthough atmospheric humidity levels may not be considered as interesting as temperature, numerous environmental situations arise when humidity insights can be crucial. Relative humidity (RH) levels range from a bone dry 0% to 100% (when the air is so saturated that dew forms). Although temperature dependant, human comfort levels typically range between 40-60% RH. Drier air may cause skin conditions, static discharges and thirst due to excessive sweating. Higher RH levels are associated with food spoiling, disease, moulds and human discomfort, due to an inability for perspiration to evaporate. In contrast plants may wilt in dry air (and hence usually favour higher humidity levels) but food preservation, equipment storage and crop drying best suits low RH levels. Naturally the likely onset of rain may be associated with a rise in the atmospheric RH levels too – a technique used by many home weather stations. Relative humidity is classically measured by techniques ranging from paper and hair stretching to more sci- entific dual “wet and dry bulb” thermometers, with simple instruments based around this latter approach still capable of very good results. Although inconvenient, a couple of similar thermometers and a moist cotton shoelace can readily give doit-yourself RH insights. The moist bulb cools as the evaporating water takes thermal energy from it, with the degree of “wet” temperature drop being inversely related to the RH. (Refer to www.picaxe.orconhosting. net.nz/humtable.jpg for the resulting wet/dry tables). Electronic humidity measuring has predictably developed, being normally now done with specialised capacitors, since an exposed and porous dielectric slowly changes its moisture content in response to humidity levels in surrounding air. If used in an RC oscillator circuit, small frequency changes can be linked to the altered capacitance value. Humidity module The HopeRF HH10D relative humidity sensor module, available from Microzed and Futurlec for around $13, utilises a capacitive-type sensor element linked to two on-board ICs. The first is an M24C01 EEPROM which is used to hold two factory calibration related values required to calculate the relative humidity as a percentage. The second IC is a humble CMOS 555 timer whose output signal varies in frequency with the change in capacitance as the humidity level changes. Of course, such an RC oscillator is a classic 555 application, although the frequency is usually altered by resistor variation. Accessing the EEPROM calibration data The EEPROM requires an I2C communications bus to read the calibration data but as these are fixed values, the data needs only be read once. If you only have a small PICAXE-08M then tedious “bit-bashing” may be needed to read the two calibration values. Larger PICAXE chips (such as the 18X) however handle I2C commands directly and just a few lines of code will display these constants via a DEBUG screen. The HopeRF datasheet does not give the EEPROM I2C slave address, which is the same as other EEPROM and FRAM chips, however their data- (Left): an enlarged photo of the HopeRF HH10D humidity measureing module, with the physical component layout shown at right. The same module can be seen from the opposite side in the breadboard layout on the opposite page. Note the label fixed to the back of the PC board – this is explained in the text. 76  Silicon Chip R1 SENSOR POSITION JB R2 U1 EEPROM C2 C4 R 3 U2 CMOS 555 C 1 X SCL X SDA (NC) (NC) FOUT +3V C3 GND HopeRF HH10D HUMIDITY SENSOR – COMPONENT SIDE siliconchip.com.au measuring HH10D module By Wayne Geary and Stan Swan sheet does indicate that the EEPROM external address pins are configured to address 01. While the HH10D datasheet indicates that there are three calibration values, only two are in fact used: • The Sensitivity value is stored in EEPROM locations 10 and 11 • The Offset value is stored in EEPROM locations 12 and 13. Here is an example of the program lines required to use a PICAXE with inbuilt I2C communications bus. This has been written using commands that any PICAXE X, X1 or X2 type chip can use. I2CSLAVE %10100010, i2cfast, i2cbyte READI2C 10, b0, b1, b2, b3 DEBUG The PICAXE DEBUG command will display theses byte variables in a Programming Editor window for recording. Those experienced with the PICAXE chips will know that word variables (w) use the byte variable (b) with the odd number as the Least Significant Byte (LSB) and the byte variable with the even number as the Most Significant Byte (MSB) The HH10D humidity module however has the even location as the LSB and the odd location as the MSB. As an example, using the above lines of program, we might see the byte variables in decimal format as: siliconchip.com.au Almost any classic PICAXE layout could be used, since only a single monitoring wire is needed! Here’s a hybrid PC board and breadboard approach, with the PICAXE-08M mounted on a trimmed AXE021 protoboard. That TO-92 package is a 78L03C low power 3V regulator. Although PICAXEs are more tolerant,the HH10D must not be supplied any more than 3.3V. b0 = 01, b1 = 85, b2 = 30 and b3 = 29 From these byte values we can determine the calibration values as: Sensitivity (Sens) = 1 * 256 + 85 = 341 Offset Value = 30 * 256 + 29 = 7709 It’s suggested you make a note of these calibration values, perhaps printing them on a small label to stick to the rear side of the small humidity module PCB. An example is given below: HH10D Sens 341 Offs 7709 For users without I 2 C capable PICAXEs at hand, it may be tolerable to just use values approximating ours – testing of several HH10D units showed most modules were close in value to these anyway. However cross-checks against readings from a known good weather station (or even a classic dual thermometer “wet and dry bulb” hygrometer) could aid later fine tuning calibration. Humidity Module connection and output. Once the calibration data is read, the HH10D humidity module is very easy to use. Of the 5 connections only three are required for VDD supply (3V), Gnd (0V) and FOUT (Soh). Note that the 3V rated module must June 2009  77 Don’t you just love how PICAXEs make circuitry so simple? The HopeRF HH10D Humidity Module interfaces dirctly with PICAXE chips, albeit with a bit of fiddling around with programming to overcome some of the lower-end PICAXE chips’ limitations. Note the supply voltage in this circuit – 3V instead of the usual 4.5V – even though the PICAXE is quite happy at the higher voltage, such would make short work of the HH10D (absolute maximum 3.3.V, as shown in the spec table below). CON1 DB9 6 7 8 9 2 3 ON 1 22k 4 5 10k TO PC SERIAL PORT 2 1 7 IC1 3 PICAXE 6 -08M 4 8 5 0 1 2 HOPERF HH10D HUMIDITY SENSOR SCL FOUT +V GND 3V 3 4 I/O PINS (CHANNELS) SC 2009 8 4 1 Picaxe HUMIDITY SENSOR NOT be run on any more than 3.3V – a supply value that fortunately PICAXE-08Ms still readily work with. For testing, two AA 1.5V cells should hence be suitable to supply the entire reading setup. For breadboard or soldered prototype mounting, the module’s 5-pin SIP header terminal strip connectors may better suit being resoldered from the other side of their holder PC board, as this then allows the capacitive sensor to be more conveniently mounted away from other circuitry and upright into clear air. Ensure the correct module connections are being used if this simple modification is made! Right-angle SIP strips could also be used for vertical mounting – the exact choice depends on your application. The output signal on the pin FOUT (called Soh in the datasheet formula) is a frequency of approximately 6-8kHz which varies as the humidity level varies. The PICAXE COUNT command, which rapidly counts the number of times a designated input pin changes from a low to a high state within a given time period, can readily help here. This makes use of the frequency of a signal being of course the number of such cycles per second (recall f=1/T), so simply COUNTing the HH10D signal for 1 second should give the frequency. From the HH10D module datasheet we are given the following formula to calculate the Relative Humidity as a percentage value: RH(%)= (offset-Soh) * sens/212 Substituting the calibration values extracted from the sample module gives: RH(%)= (7709-Soh) * 341/4096 However, a few minor problems experienced with PICAXE maths now arise: • only integer (whole number) maths are performed • the largest number held by a word variable (W) is 65535 • it does not support brackets for precedence. #PICAXE 08M ‘Sample HopeRF HH10D humidity module program for June 2009 SiChip ‘Ref resources => www.picaxe.orconhosting.net.nz/hopehum.htm ‘IO DEFINITIONS SYMBOL humid =3 ‘ ‘ VARIABLE DEFINITIONS SYMBOL axefactr = b2 SYMBOL Soh = w2 ‘w2 = b5:b4 SYMBOL diff = w3 ‘w3 = b7:b6 SYMBOL RH = w4 ‘w4 = b9:b8 ‘ ‘ CONSTANTS -strictly need to be read via I2C for each module, but can be assumed close SYMBOL Offset = 7709 ‘HH10D offset calibration constant - a second module has 340 SYMBOL Sens = 341 ‘HH10D sensitivity calibration constant - a second module has 7762 ‘ ‘ MAIN PROGRAM Main: COUNT humid, 1000, Soh ‘read the frequency (ie cycles in 1 second) diff = Offset - Soh axefactr = diff / 19 + 1 ‘factor prevents number roll over error if >65535 RH = 10 * Diff / axefactr * Sens ‘int. result (x10 gives possible 0.1 resolution) axefactr = 4096 / axefactr ‘a factor to prevent number roll over error RH = RH / axefactr ‘final value for RH% RH = RH / 10 ‘divide by 10 (for now) as just whole integer RH% SEROUT 1, N2400, (“RH% = “, #RH) PAUSE 5000 ‘wait for 5 seconds until next reading GOTO Main 78  Silicon Chip siliconchip.com.au As might be seen, multiplying a number greater than 192 by 341 will result in an overflow of the values! Likewise, dividing by 4096 may result in loss of accuracy as no fractional part is retained. To improve the accuracy of the maths, allow one decimal place and also avoid overflow within the PICAXE maths, we have introduced a factor. This factor is a variable number defined to keep the intermediate results as large as possible which will help minimise error. Our variable called Axefactr (short for “PICAXE factor”) is a value determined as follows: HOPERF INCLUDING THE HopeRF HH10D HUMIDITY SENSOR USED IN THIS ISSUE! Axefactr = (offset- Soh) / 19 + 1 This number will calculate correctly with the PICAXE. The result will be a number from 1 to 64 inclusive. Using this factor and multiplying the early part of the calculation by 10 makes it possible to extract 1 decimal place ( if required), at the completion of the calculation. Digital Sensors RH10(%)= (((offset - Soh) / 19) + 1) * 10) * Sens / (4096 / Axefactr) This awkward calculation needs to be broken over several lines of code in the PICAXE. To save on the limited number of variables available to the PICAXE chips, some are reused for a new variable part way through the calculation. After all the number crunching and formula tweaking, just a single PICAXE-08M input pin (here 3) suffices for eventual reading of this final value, with a SERTXD showing %RH readings on the Editors F8 terminal. The code shown at left can also be downloaded from www.picaxe.orconhosting.net.nz/hopehum.bas. In future, we plan to extend use of this humidity module with the HopeRF 433 MHz wireless data transceivers. Article resources and references can be linked to, from www.picaxe.orconhosting.net.nz/hopehum.htm SC RF IC & Modules Sensor Performance Specifications and HH10D Humidity Module Characteristics Parameters Min Typ Max Units Resolution 0.3 0.08 0.05 % 3 Accuracy Repeatability -0.3 % 0.3 % Uncertainty 2 % Response Time 8 s Hysteresis 1 % Interchangeability Fully Interchangeable Humidity Range 1 99 % Temperature range -10 +60 °C Working voltage 2.7 3.3 V 3 Semiconductor Devices SAW Devices Distributed in Australia by Power consumption 120 150 180 A Microzed Computers Pty Ltd Output Frequency Range 5.0 6.5 10 kHz Phone: 1300 735 420 Fax: 1300 735 421 1% Stability versus time per year Here are the manufacturer’s specifications for the HH10D Humidity Module. All it needs are supply and data output lines to interface with the PICAXE. siliconchip.com.au www.microzed.com.au June 2009  79