Over the last three weeks we've shown you how to install intake air and exhaust gas temp probes, and also covered a selection of digital read-outs that you can use to monitor the thermocouple temps. This week, we look at how to build an instrument that can take the input from multiple thermocouples and also has a factory-looking negative back-lit LCD display.
However, before I begin - a word of warning. If you have never put together any electronic kits or aren't familiar with building electronic circuit designs, don't try to make the display shown here. It's just too easy to order the parts, make a single wrong wiring connection, and blow 'em all up. Very disheartening. At the other extreme, if you are a top-notch electronics technician, making this display will be ludicrously easy. And if you're like me - you can solder, recognise components and can just follow a circuit - you'll be okay if you take lots of care and double-check all of your connections before applying power!
The AD595AQ
If you cast your mind back a few issues you'll remember that I said that the output of a thermocouple is a very low voltage indeed. To be able to read it on a meter, it must be amplified. Further, where the thermocouple wire (alumel and chromel in the case of a K-type thermocouple) physically joins onto your meter circuit, another voltage is created. This so-called 'cold junction' (as opposed to the hot one at the other end of the thermocouple cable) needs to be compensated for, otherwise the measured temp will be wrong.
Previously, thermocouple amplifiers and cold junction compensation circuits needed lots of separate components, but these days there's one Integrated Circuit (IC or 'chip') that does it all. It goes by the name of the Analog Devices AD595AQ and comes in a 14-pin package. There are two lots of good news about this chip - it's relatively cheap (A$18), works happily on car voltages, and needs no other components to make it work. By 'work', I mean that it can generate an output voltage of 10 millivolts per degree Celsius. So, you attach a thermocouple that's being subjected to (say) 50 degrees C, apply 12 volts and an earth, and on the output pins you'll find a signal of 50mV. It's the same sorta idea as a multimeter thermocouple adaptor (covered last week) but it's cheaper - much cheaper. (Incidentally, if you live in an icy part of the world, note that when powered like this, the measurement of temps below zero isn't possible.)
This diagram shows the pin-outs (ie function of each pin) of the chip. Note that the chip is viewed from above, with the chip orientated so that the notch is at the left-hand end! Pins 1 and 14 are at the extreme left - the thermocouple wires connect to these. Pins 13, 1, 4 and 7 connect to earth, Pin 11 connects to positive 12 volts and the 10mV per degree C output can be found at the join of Pins 8 and 9. If you intend building this display, I strongly recommend that you download from Analog Devices the complete pdf data sheets on this chip - you'll find them at http://products.analog.com/products/info.asp?product=AD595.
While the chip may well be quite happy connected straight to the car power supply as is shown above, car electrical systems often have voltage spikes on them - which may blow the chip. To prevent these getting through, two extra components are added. These are a 10 ohm 1 watt resistor and an 18 volt 1 watt zener diode. (Cost of these components is about a dollar from an electronics store). This diagram shows how these are wired in. Note that the band on the diode goes towards the 12 volt supply rail.
The Display
As indicated last week, I wanted a negative amber backlit LCD display for the thermocouple meter, so that it was an exact match for the factory LCD instruments in my Audi. In order that I could directly plug the AD595 output into it, the display had to be happy reading up to 10 volts (ie 1000 degrees C at 10mV per degree C) and the display also had to be able to operate off low voltages (not mains power, for example). Oh yes, and cost was also a consideration!
The one that I settled on is the DMO-553 3½ digit negative backlit amber LCD display from KNS Instruments
(www.knsinstruments.com)
of New Hampshire, USA. This display will take a direct input of plus/minus 20 volts, requires a 5 volt supply to operate, and comes with an in-built bezel, making mounting quick and neat. KNS Instruments also have the display available in negative red and green, as well as positive green displays. Bought as single units, the DMO-553 costs just US$56.60. Note that the maximum temp at which the display is happy operating is about 50 degrees C - the display goes a blank white over this figure, but returns to normal without any probs when the temp again drops.
Because this display is powered by 5 volts, some circuitry is necessary to provide this. The 5 volts is provided by a LM805 voltage regulator (12 volts in, 5 volts out) and this works with two 47uF electrolytic capacitors, a 1W 18 volt zener diode and a 10 ohm 1 watt resistor. Total cost for these parts is less than A$5. This circuit shows how these components go together. Note the pin-outs on the voltage regulator and the fact that the capacitors and zener diode have a polarity (ie go into the circuit only one way around). The capacitors have the negative lead marked on their bodies.
Putting it all Together
Just to stress again - while these wiring diagrams look very simple ('cos they are, basically), it's still very easy to make a mistake and so blow up either the chip or the meter. Check, check, check your wiring against the diagrams - as you do each step!
An important point is that the chip must be mounted away from the LCD panel's voltage regulator and 1 watt resistor. These two components get quite warm (about 20-25 degrees C above ambient) and if they are near to the chip, the chip's temp will also become elevated - which stuffs up the cold junction compensation. I mounted the chip (actually I used two, but I'll get to that in a moment) on a piece of pre-punched plain board, using a 14-pin IC socket so that the chip could be easily changed if I blew it up. (Confident, aren't I!?) I used a PC-board terminal block to take the thermocouple input wires, with the pins of the blocks wired to Pins 1 and 14 of the chip. The zener diode and resistor (only one of each needed, even if supplying two thermocouple chips) were also mounted on the board. The board was then mounted in a small 'zippy' box.
After you have carefully wired the chip up and connected the thermocouple, power and earth, measure the output from the linked Pins 8 and 9. You should read out a voltage that reflects the thermocouple's temp - ie 20mV at 20 degrees C. Warm the thermocouple and check that the output voltage increases - if it decreases, swap the thermocouple leads. Because I wanted to be able to monitor the output of two thermocouples, I used two ICs, but the details are otherwise the same.
Once the thermocouple chip is working properly, it's time to make the LCD display operate. I mounted on a (smaller) piece of pre-punched board the regulated 5 volt supply components, with the board then glued to a piece of high-density foam rubber, which in turn was glued to the back of the meter. When assembling this board, leave room for a heatsink on the voltage regulator - I also used a small piece of aluminium angle to increase the heat sink area and further support the regulator. The positive 5 volt output of the regulator circuit is connected to the '+5V' pin of the meter and also to the 'B/LV+' (ie backlight voltage positive). The negative wire from the regulator circuit output goes to the 'neg' and 'BLV-' pins of the meter. When you apply 12 volts to the regulator circuit, the meter should light up and display '000' or some similar low number.
With the thermocouple chip and the LCD display both working, all that's necessary is to wire them together. The output voltage from the linked Pins 8 and 9 is connected to the 'INHI' pin of the meter, with the earth wire from the IC connecting to the 'INLO' pin of the meter. Now, when you apply power to both the IC and the meter, you should see the temperature displayed! (If things suddenly go weird at this stage, swap the 'INHI' and 'INLO' panel meter wiring, in case you have got them around the wrong way.)
Because I wanted to measure both EGT and intake air temp on the same display, I used two thermocouples and two AD595's. The two signal output pairs from the chips and the input pair to the meter I wired to a DPDT (double pole double throw) switch, so that the meter could be switched to measure one thermocouple output or the other. This diagram shows how this wiring was done. (And no, I don't advise that you try to delete one chip and wire the thermocouple wires straight to the switch - the cold junction compensation will then be totally wrong unless you thermally tie the switch to the chip.) If you want to measure more than just two thermocouples, simple specify extra chips (one per thermocouple) and use a rotary switch to select the thermocouple to be displayed. You could them have eight, for example - one EGT per cylinder on a V8!
So, what did I end up with? For A$285 I have a backlit LCD display that matches the Audi instruments, can display temps up to 1000+ degrees C, and can show the input of two thermocouples, with selection by the flick of a switch. And that's exactly what I wanted!
TempScreen: Part 1 - Installing the Intake Air Temp Probe
TempScreen: Part 2 - Installing the Exhaust Gas Temp Probe
TempScreen: Part 3 - Displaying the Temperatures
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Getting the Stuff!
The backlit panel meter displays are available directly from KNS Instruments. Here in Australia I specified air freight, paid US$23 for the freight plus the US$57 for the panel meter (total A$133) had a panel meter on my desk 6 days after I ordered. No problems there. The AD594AQ's I ordered through my local Analog Devices dealer - find your local dealer here (http://www.analog.com/world/corp_fin/sales_directory/distrib.html) . Incidentally, my local distributor was totally hopeless - they forgot to send the order for a few days, didn't ring me when it arrived, didn't have any paperwork available when I went to pick it up, etc, etc. In fact, it took longer to get the chips from within Australia than to get the LCD panel meter from the US! The discrete components (resistors, zener diodes, capacitors, heat sink, punched board, etc) can be obtained from electronics stores like Dick Smith (www.dse.com.au) or Jaycar Electronics (www.jaycar.com.au) or your local equivalent. If you live in Australian and build an instrument exactly like the one shown here (EGT and intake air temp probes switched through to a single display) it will cost you about A$285. A thermocouple and LCD display to show just (for example) EGT will cost about A$200.
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