I have ICL7136 MQFC (the square one) chip driving an LCD display. it's measuring and displaying temperature data from a thermometer probe. I want to grab the analogue readings an log them with an Arduino as well. How what type of setup would yield something usable. Would I just measure from the IN HI or IN LO to the analogue in of the Arduino?
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\$\begingroup\$ This question is more suitable for Arduino SE. It is now in public beta: arduino.stackexchange.com \$\endgroup\$– akellyirlMar 2, 2014 at 14:07
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2 Answers
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It looks to me like the chip has reference voltage pins and then sensor voltage pins. So it is reading voltage changes from a sensor that reports back in that way, massaging those levels (amplification, scaling, whatever), and then outputting to the display.
Yes, I would think logically reading the same sensor outputs that the chip is reading on IN HI and IN LOW would allow you to access the data. I am assuming the mere act of connecting to those pins and reading them won't change the voltage, but I would defer on that to those more experienced as I could be wrong.
I think your issue, however, is going to be doing the scaling and amplification and conversion of the signals coming in so that the Arduino and the chip both think a given voltage represents the same (in your case) temperature reading. Once they both say that particular voltage level is that many degrees then it becomes a data logging issue.
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\$\begingroup\$ Thanks for your answer, I think I understand the scaling issues, but I'm still confused on how to hook it up. What am I measuring across? for instance do I connect the Arduino ground to the common ground of the temp circuit? and then the IN HI to an analog in on the Arduino? what does the IN LO represent. pardon my ignorance \$\endgroup\$ Nov 1, 2013 at 19:04
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\$\begingroup\$ My guess is the chip is designed to work with a number of sensors. I have a typical 3 wire water sensor in my sump, for example, that has V+, Gnd, and Vout. The Vout is where the data is. For my sensor all it does in 0v for dry and supply voltage for wet on the vout pin (digital, 1 or 0). I would think I connect the sensor output to the IN HI pin on that chip and probably put the IN LO to ground. With your temp sensor maybe IN LOW is the one to connect and read. But I am guessing so I will shut up. \$\endgroup\$– mikeYNov 1, 2013 at 19:25
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I wouldn't directly connect to IN LO and IN HI on the ICL7136 unless you haven't got an input filter - as shown in figure 11 the resistor is 1M ohm and an ADC input on an arduino is likely to upset it.
The other issue is the voltages on the two inputs - normally one could be expected to be 0V but this may not be the case so I'd recommend using an instrumentation amplifier to to take the voltage from the thermometer probe directly.
Somethink like an AD8221 but you'll need to generate the appropriate power supply rails for it - you can probably use the power rail for the arduino but you'll likely need a small negative rail unless of course the offset voltage coming from the probe is greater than 2V above 0V.
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\$\begingroup\$ There are a few ICL7136 one for temp, one for humid etc. I'm really just trying to figure out the simplest way to get the data that is going into the LCDs and log it. I does seem like there is a bit of a conditioning circuit before the ICL7136. \$\endgroup\$ Nov 1, 2013 at 19:26
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\$\begingroup\$ "the resistor is 1M ohm and an ADC input on an arduino is likely to upset it." That was a piece I didn't know and almost made me not answer the question- whether there was some sort of almost Heisenberg thing going on where the act of hooking up the Arduino to read the input would change the input and make it unreadable. \$\endgroup\$– mikeYNov 1, 2013 at 19:29
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\$\begingroup\$ @user379468 you need to establish what voltage is on IN LO. If it's at 0volts then no problem - you can wire to your data logger but you may find that one or both systems become noisy. An instro amp saves you debugging noise problems that may come and go and be annoying. \$\endgroup\$– Andy akaNov 1, 2013 at 19:41
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\$\begingroup\$ @mikeY I'm no expert on arduino but a lot of internal ADCs on MCUs require a decently low impedance offered by the voltage source they are measuring or they produce measurement errors and may induce other issues. Heisenburg LOL. \$\endgroup\$– Andy akaNov 1, 2013 at 19:56