HeaterMeter Hardware v4.2.4


 
Yeah the A+ looks pretty great. I'm not sure how much the lack of an Ethernet port or second USB port is a deal killer for people. The lack of ports getting in the way opens up the flexibility in the shape of the HeaterMeter significantly though.
 
Yeah $20! With $10 shipping :/ I was considering buying 10 just to get free shipping.

The larger question about these is if they're going to produce them in any quantity or will they be as scarce as the original Model A, which is to say "good luck finding one".
 
Since the weather has gotten bad here in the Midwest I want to report that my HMv4.2 is registering my probes nice and steady right down to 20F (unfortunately!). I am no longer having the probes drop out when the temps dip low like on previous versions of the HM, even when run over a 50Ft CAT5 cable. I'm really happy about that, 'cause I used to have to set the blower to manual mode when firing up the pit until the the probes warmed up and started registering, and if I turned off manual mode too early the probe used to drop out and make my servo damper do a violent dance... I'm really glad those days are behind me...

On another note, my TC bottoms out at 37F, I know that is to be expected but I am wondering why, and if there is anything I can do to make the TC register lower temps (for monitoring the weather). I think the TC amp and TC both should be able to register temps way below 37F?
 
Not if the thermocouple has hit a physical limit in measuring temperature. You'd have to get another probe for the temperature range you want to measure.

Ripped straight from Wikipedia:

Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges. Properties such as resistance to corrosion may also be important when choosing a type of thermocouple. Where the measurement point is far from the measuring instrument, the intermediate connection can be made by extension wires which are less costly than the materials used to make the sensor. Thermocouples are usually standardized against a reference temperature of 0 degrees Celsius; practical instruments use electronic methods of cold-junction compensation to adjust for varying temperature at the instrument terminals. Electronic instruments can also compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements.
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The problem is that the thermocouple amp can only pull the voltage down so close to the rail (0V in this case). The lowest the AD8495 is speced to measure with 0V ground is 25mV typical, or 5C. You're getting a little better than that. The solution would be to add a small bias voltage to make the temperature you want to measure come out over that 25mV threshold, or to use a negative power supply for -Vss on the amp.
 
OK, thanks for the info Bryan. If I were to add a bias voltage would I have to use a negative offset in the HM setup to make it read the right temp?
 
Exactly!

Note that the datasheet explicitly recommends against using a simple resistor voltage divider as a bias voltage due to the high impedance affecting the output. A precision voltage reference should be used and you're only going to want like 100-200mV (20-30C) because you can only subtract out 127 degrees at most
 
Thanks for the follow up, my first thought was using a resistor voltage divider, guess I will have to go another route....
 
Bryan, If/when you are designing the next HM board if you could include some small solder pads attached to the 4 dead pins on the CAT5 jack that would be great. Either through hole for wires or just a surface pad to solder to would make using those spare CAT5 wires much easier, 'cause piggy back soldering wires to those 4 pins in such close proximity can be a bit of a PITA and it is easy to get a bad solder joint because you can't use very much solder to attach a wire.
 
I 2nd that idea. Although, its not hard to solder the wires to them, as wrapping wire fits nicely into the holes already, it would make it much easier to get access to the extra cat5 connections without accidentally shorting anything.
 
4x through holes can take a lot of space but I certainly can try to see what I can fit in, pads or holes. What else was I going to put on the todo list? Oh yeah! Add a straight cuttable trace for the LCD backlight.
 
Instead of adding space to the HM board why not design a breakout board. A quick look at the footprints suggests to me that a ~2 in^2 board would get 2 RJ45 connectors and an 8 position .1" pitch header.
 
I would be for a break out board and maybe an external TC amp board so it could fit in something like the roto damper. The one from adafruit i am not liking like others.
 
DaveyR said:
I would be for a break out board and maybe an external TC amp board so it could fit in something like the roto damper. The one from adafruit i am not liking like others.
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Keep an eye on the thread John created recently about the TC amp board he found on OSHPark, it uses the same TC amp chip as the HM and John seemed to indicate the circuit structure was the same but you just need to install the component values from the HM TC amp circuit rather than the project parts list. I wish I could figure out how to do board layout with EAGLE, the schematic diagram I can manage but the learning curve on board layout seems to be too steep for me to dive into...
 
RalphTrimble said:
Keep an eye on the thread John created recently about the TC amp board he found on OSHPark, it uses the same TC amp chip as the HM and John seemed to indicate the circuit structure was the same but you just need to install the component values from the HM TC amp circuit rather than the project parts list. I wish I could figure out how to do board layout with EAGLE, the schematic diagram I can manage but the learning curve on board layout seems to be too steep for me to dive into...
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I ordered 3 of those boards and I will know next week or so how they work
 
I've pushed up a new .brd file for v4.2.4 to github. The original silkscreen was pretty high DPI so the board house sort of hit or miss if you'd get a boxy robot or you'd just get a few lines. This one meets the DPI maximums with some room to spare and looks good on the most recent boards I've received. You can tell it is slightly lower resolution but it still is crisp. The zipped dorkbot files have been updated with these new cam'd gerbers too.

The new brd file is in the new EAGLE v6 XML format so it can't be read by v5 any more. Not sure if anyone else was on the ole' v5 like I was for so long.
 
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