Perhaps subtle electronics issue

[Muse]

I have a device similar to this:

Computemp 5

That one is on sale used at Ebay. Mine is probably designed by the same folks, but is older, more primitive in design but probably does pretty similar things. Mine is the Computemp Plus. I couldn't find any links for it online, which surprises me, but I bought mine in 2005, also used on Ebay. Mine has an LCD instead of LED display. It also features two probes and I downloaded the manual years ago. I dig it because I have a probe dangling outside my bedroom (exterior to the house), and it shows me the current outdoors temperature. I can hit a button that tells me the high temperature for the day and another button that tells me the low temperature for the day. Those functions reset at midnight.

Well, aside from the fact that the LCD isn't super easy to read some of the time, my only complaint concerning this is that it has no AC adapter (the one in the link does have an AC adaptor). Mine uses one of those little rectangular 9v batteries. Around once a year the battery craps out and I have to:

1. Get another battery
2. Open the device (by removing a screw) and replace the battery.
3. Reset the device in terms of time and display options.

After 11 years, I'm tired of this, so I thought why not see if I can wire a wall wart to the battery terminal wires? Having a box of old wall warts I found one I figured would likely serve. It says 9v DC 300ma. I think it was for some phone accessory.

I have it all wired up now and after 2-3 days of service I'm finding that the Computemp Plus is not functioning properly. I press the HIGH button and I'm not getting the correct day's high temperature! It seems to have forgotten it. Apparently the wall wart's supplied voltage/current isn't what the device requires. This is surprising to me.

Is there something I can try that will fix the problem, maybe adding a resistor to the circuit or something similar? If not, I figure I can bid on the one in the link!

 

[A Casual Fitz]

I got you dude. Check it out:

https://forums.anandtech.com/forums/highly-technical.22/

 

[nakedfrog]

300mA seems pretty low.

 

[Muse]

300mA seems pretty low.

I figured it would have to be adequate. We're talking about a replacement for a little 9v battery that gets replaced around once a year (my guess, I haven't kept records). The screen is LCD (i.e. very low power requirement to keep data on the screen). Yes, some energy goes into servicing the thermometer probe, which is on around a 12 foot two conductor wire, and the internal electronics. I have the secondary probe removed, for some reason that one ever worked right.

There are 8760 hours in a year. The capacity of a 9v alkaline battery is ~550mah.

Now, I measured the voltage across the plugged in wall wart, unloaded, just putting my multimeter's probes across the naked wires and it said something over 16v. I suppose that under load that would drop to something closer to 9v. Still, I wonder if that has something to do with the problem I'm seeing.

Thing is, when I pressed the high button yesterday evening it said 57 degrees F. I know it got warmer than that, the high was reported at ~64 F in the newspaper today.

Anything I can try to see if I can make my DIY AC adapter solution work?

 

[deadlyapp]

I figured it would have to be adequate. We're talking about a replacement for a little 9v battery that gets replaced around once a year (my guess, I haven't kept records). The screen is LCD (i.e. very low power requirement to keep data on the screen). Yes, some energy goes into servicing the thermometer probe, which is on around a 12 foot two conductor wire, and the internal electronics. I have the secondary probe removed, for some reason that one ever worked right.

There are 8760 hours in a year. The capacity of a 9v alkaline battery is ~550mah.

Now, I measured the voltage across the plugged in wall wart, unloaded, just putting my multimeter's probes across the naked wires and it said something over 16v. I suppose that under load that would drop to something closer to 9v. Still, I wonder if that has something to do with the problem I'm seeing.

Thing is, when I pressed the high button yesterday evening it said 57 degrees F. I know it got warmer than that, the high was reported at ~64 F in the newspaper today.

Anything I can try to see if I can make my DIY AC adapter solution work?

Try measuring the voltage once you've got the load on it. Like you say, if the electronics inside are very sensitive to voltage differences (eg maybe it's been calibrated for a 9V battery), then you could be having this problem. Adding a resistor could help, or a voltage diode to better regulate the voltage to what you need.

 

[Muse]

Try measuring the voltage once you've got the load on it. Like you say, if the electronics inside are very sensitive to voltage differences (eg maybe it's been calibrated for a 9V battery), then you could be having this problem. Adding a resistor could help, or a voltage diode to better regulate the voltage to what you need.

Thanks. I am planning in the next day or two to visit a small independent electronics supply store that's been a staple in my town for decades. I am looking to see if they have a certain part I need for something else, but I will ask them about this problem. They may have a recommendation. Meantime, I will get that in-line voltage reading... before making that visit.

 

[Muse]

Well, I bid on that used Computemp 5 this afternoon, sniping at it and to my surprise I won. This evening it suddenly appears that the Computemp Plus I've had since 2005 and now have on my jury rigged AC adapter may indeed be working. The Jury is out! :Stupid looking emoji: [the new emojis just plain suck]

 

[Rubycon]

They make 9V battery eliminators for this purpose. Many wallwarts are unregulated and their powered devices are designed this way. Something that's designed to run on a 9V battery isn't expected to deal with ripple and other noise in the power supply! You can try a Lithium 9V battery. They can last up to 10 years in smoke alarm duty and surely will outlast standard alkaline 9V batts when asked to call for low load steady duty.

 

[Muse]

They make 9V battery eliminators for this purpose. Many wallwarts are unregulated and their powered devices are designed this way. Something that's designed to run on a 9V battery isn't expected to deal with ripple and other noise in the power supply! You can try a Lithium 9V battery. They can last up to 10 years in smoke alarm duty and surely will outlast standard alkaline 9V batts when asked to call for low load steady duty.

Cool! Didn't know they exist. Thanks.

Is a 9v lithium for smoke alarms going to differ from others?

The Computemp Plus is working some of the time, it seems. Today, pressing the High button, instead of saying 68 F is saying -- F 1:01AM, however. The Low button reports 54 F 7:07AM, which is probably correct. Now, I was up at 1AM and may have played with the device, pressed the High and/or Low button, but that should have had nothing to do with the anomaly. However, even when running on a 9v alkaline, it sometimes had similar anomalies. Not a lot, but the did happen. So, I need more data. Still, a 9v Lithium seems a good idea.

 

[Muse]

I'm finding that with the wall wart described (simple 9V DC output, 300ma rating, probably for a phone or a phone accessory originally), the Computemp Plus device is working OK some days, but not reliably. A couple days ago it said 61 F for my outside temperature (I have the probe hung outside, it's a wired probe), but it was obviously warmer than that. Pressing the High button made it update itself a couple of degrees. Within 1/2 a minute I had it saying 68 F, which was probably about right. Normally (i.e. when powered by a 9v battery, the kind used on smoke alarms), it doesn't do that, it doesn't need to be goosed to show the correct temperature. Since that occurrence, I'm not seeing this difficulty. Still, I'm thinking it's not dependable enough. Ruby's idea of a lithium 9v is very good. But before I order some (what kind? For smoke alarms OK?), I figure maybe I can make my AC adapter solution work. My Computemp 5 came yesterday and I'm disappointed. The reason is that the wall wart for it is using 8 watts! The one I have on the Computemp Plus uses 1 watt. This info by virtue of my Kill-a-Watt meter.

OK, so I looked up "wall wart ripple" a few minutes ago and came up with this:

A Dummies’ Guide to Working with Wall Warts

This site explains in detail how I can create a module that will regulate the output of my 9v 300ma wall wart. It should smooth it out and keep the output close to 9v. I am no EE. I have no training in circuits and design. I studied physics in high school and at the U. but it was all theoretical, no practical hands on work with devices and actual circuits. Do you think the approach explained at that site will let me modify my power supply for this thing so it works reliably?

It looks like what I'll be producing is this (I figure I want a fixed voltage solution, not an adjustable voltage solution, right?):

 

[John Connor]

low load steady duty.

Keyword there is low load. Not sure about his device though.

 

[lxskllr]

How about using a NiCad battery, with a charger always hooked up. You might have something like that laying around from an old phone or something.

 

[Rubycon]

A Dummies’ Guide to Working with Wall Warts

This site explains in detail how I can create a module that will regulate the output of my 9v 300ma wall wart.

The issue here could be RFI. Thermometers that use wired external sensors can frequently malfunction in the presence of medium to strong radio frequency fields. With the rapid proliferation of radiotelephony in the past decade (and this could also include multichannel wifi devices as well) the possibility of RFI increases particularly in sensor leads with simple untwisted pairs. Longer leads are usually compromised of coaxial cable and will also use small inductors and possibly spike protection to guard against static build up, etc. Adding a wire on the power supply even with adequate filtering and regulation could still augment circuit malfunction in the presence of radio transmitters.

The 9V lithium is physically compatible with the standard 9V part in other chemistries. Its open terminal voltage is a bit lower than a fresh off the shelf alkaline but given its relatively flat discharge curve to "empty" as opposed to the much more sloped curve of alkalines, devices will perform stable over its lifetime.

Keyword there is low load. Not sure about his device though.

It uses a non backlit LCD and can run for months off a single 9V rectangular battery so it definitely qualifies as a low drain device.

 

[Muse]

The issue here could be RFI. Thermometers that use wired external sensors can frequently malfunction in the presence of medium to strong radio frequency fields. With the rapid proliferation of radiotelephony in the past decade (and this could also include multichannel wifi devices as well) the possibility of RFI increases particularly in sensor leads with simple untwisted pairs. Longer leads are usually compromised of coaxial cable and will also use small inductors and possibly spike protection to guard against static build up, etc. Adding a wire on the power supply even with adequate filtering and regulation could still augment circuit malfunction in the presence of radio transmitters.

The 9V lithium is physically compatible with the standard 9V part in other chemistries. Its open terminal voltage is a bit lower than a fresh off the shelf alkaline but given its relatively flat discharge curve to "empty" as opposed to the much more sloped curve of alkalines, devices will perform stable over its lifetime.



It uses a non backlit LCD and can run for months off a single 9V rectangular battery so it definitely qualifies as a low drain device.

Yeah, I figure it's drawing in the neighborhood of 0.08ma, very low drain. Based on the extensive explanations at that site I linked in my last post I figure that the 9v 300ma wall wart is likely providing quite a bit higher voltage than 9v at the very low drain it is experiencing. Also, it's probably apt to suffer ripple from the AC supplied.

Lithium 9v batteries are evidently only around 2x as energy dense as alkalines. I'd maybe get 2 years out of them, therefore. That's less trouble, yes, but I'd like it if I could have adequate service from this 9v 300ma wall wart. So you think that constructing that filtering module and having it between the Computemp Plus and the wall wart isn't worth trying? There's an independent electronics supply store near me who I really like. I could get the parts from them. I figure I'd need a little board and hopefully a cover for the module, they may well have something for that too. I'm thinking the whole thing might cost me well under $10 in parts, if what I read at that site I linked is correct.

You are right that the device has a plain (not backlit) LCD. In fact, it doesn't do a great job in displaying the characters, but it's just good enough where I can make out what's supposed to be there, which is lucky! The other device (one I just bought used off ebay) has a really bright LED display, but draws 8 watts, likely more than 10x what the 9v 300ma wall wart is drawing. I think the functionality of the devices is pretty equal for my intents and purposes.

 

[RossMAN]

Speaking of smoke alarms, we recently upgraded to Roost and I love it.

https://www.kickstarter.com/project...battery-smarten-your-smoke-alarms/description

https://www.amazon.com/dp/B00ZWQHVP4/

Roost Battery:

http://www.smartroost.net/

Available for pre-order on Amazon: (ships September 1st)

http://www.amazon.com/gp/product/B00ZWQHVP4/

It's basically a smart 9V battery that connects to wifi, so it can turn a regular smoke alarm into a smart one. You can put it in a hardwired or battery-only alarm, and if the alarm is battery-only, you can also remotely silence it via the smartphone app. It alerts you when the battery is low as well, so it doesn't just chirp endlessly. It can also send you (and family & neighbors) alerts when the alarm goes off. Works on iPhone & Android. This is the easiest & cheapest solution to adding some smarts to your alarm. They say the battery lasts 5 years; not sure if that's how long the battery lasts or the overall usable life of the battery. A bit more reading:

http://liliputing.com/2015/06/roost-less-permanent-solution-smart-smoke-alarm.html

Pricing:

* Single 9V battery for $35
* 2-pack discounted to $65
* Replacement lithium batteries are $15

 

[Muse]

Well, unhappy with the occasional incorrect readings I was getting, I took the PDF printout of the linked site, https://forums.anandtech.com/threads/perhaps-subtle-electronics-issue.2488739/#post-38523007, to my local indie electronics supply store intending to probably buy the components to regulate and filter the DC coming from the wall wart I was using. The woman right away was talking about my buying another 9v PS. After discussing this for a bit, I figured it would be worth a try. So, I bought a 9v .650ma DC wall wart that's evidently solid state. The one I was using probably has a transformer in it. This new one (costing $6) measures ~9.15v no load, so that's an improvement. She said its filtering is probably pretty weak, though. I figure if I don't like the performance I can still build a little filtering module to place between the thermometer unit and the PS. I'd presumably eliminate the voltage regulator portion and just use the handful of capacitors (5, IIRC).

This new one is using even less power than the other one. I figure running it for a year would cost me under a dollar! So far, I'm not sure. The first day it acted strange, but I actually had it running as I made the solder connections (probably not a good idea). The last couple days it's been pretty OK. I need more data.

I'm wondering if I should shorten the leads. The power wiring is about 6 feet longer than necessary (under my bed), and the lead to the temperature probe is longer than it needs to be by something like 5 feet. I have it coiled and hung over a hook. I figure that's probably inviting picking up some RF or WIFI anomalous signal.

 

[Rubycon]

You can install F connectors on the PSU and display unit and connect them with slim coax assemblies like these:

http://www.l-com.com/audio-video-th...5-ohm-coaxial-cable-with-in-line-type-f-plugs

And a simple LC filter on the display unit can help with spurious RFI too.

 

[Muse]

You can install F connectors on the PSU and display unit and connect them with slim coax assemblies like these:

http://www.l-com.com/audio-video-th...5-ohm-coaxial-cable-with-in-line-type-f-plugs

And a simple LC filter on the display unit can help with spurious RFI too.

You are suggesting shielded wiring for the power source or for the temperature probe, or is it both?

Where would I get (or how would I build) a "simple LC filter?"

Thanks for this Rubycon. I hadn't thought of that exactly, but yes, I was wondering how I could shield the wires from absorbing radiation that might be causing the anomalous readings. The last few days there were multiple instances of bad readings:

1. At least two times in the last few days all of a sudden the unit didn't have a usable high-for-the-day reading, but showed "- -" for the high when the High button was pressed. That would persist until midnight when the high-for-the-day automatically resets to the current temperature (as does the low, of course).

2. Several times the temperature shown on the display was too high, higher than the actual temperature outside. It only corrected when I pressed the High button, would take a minute or so to come back down to a sensible reading. I have the unit configured to display one thing only, being the Main probe's temperature, and it's hanging outside the building under the roof line. The secondary probe never worked right for me, so I removed it, don't even know where it is. Years ago I had hoped the unit would serve to get me current and high/low for the day for inside temperatures as well, but abandoned that because the indoor readings were incorrect for some reason. Now, I just use the unit for the outdoor current and high/low for the day readings. To get the high you press the High button, to get the low you press the Low button. Otherwise, the unit displays the current temperature of the Main probe. You can have it alternate what it shows automatically every few seconds including Main, Auxiliary, Time. I don't like alternating info, I just set it to display Main.

Now, shielded cabling is what you are suggesting here, I believe, with the F connectors and that coaxial cabling. I have all that stuff including crimpers. I can make my own. I have run my own antenna cables from my two rooftop antennas, I believe this would be similar. However, thin would be good. Not sure I can get the thicker stuff through the hole I have in the wall now, haven't checked that, did it a long time ago.

Now, what I did yesterday (before seeing your post):

I brought the PDF Dummy's Guide to filtering wall warts, linked above (or whatever it's called), back to the indie electronics supplies store in my town a few days ago and asked if I could maybe build the filter illustrated (above), for the new solid state wall wart I'd bought there a week before (it is regulated to 9v DC, even with no load). The woman said as shown it would not be appropriate. It has a voltage regulation component in it. She thought that if I used the right part of that circuit, i.e. the 3 capacitors on the right (470, 0.1 and 0.001 uF), and placed that between the wall wart and the unit, it would add filtering. She wasn't sure how well it would work, but it was an idea. I decided to try it. WTH, I didn't like how it was working. What I do like is the fact that the new WW draws even less current than the one with the transformer. I can run the new one for less than $1/year!

Well, yesterday I installed my DIY filtering unit. I just built a little circuit board from a tiny piece of thin plywood, some wire, the 3 capacitors, used my trusty soldering iron. I also shortened up the wiring from the unit to the AC receptical by about 11 feet. It's been running less than a day, OK so far. Don't know if what I put in there is anything like adequate filtering, or really filtering at all. I looked it up and these filters usually contain more than capacitors, they contain diodes or inductors.

 

[Rubycon]

The only reason I suggested filtering and shielding was due to the fact that you have extended the power supply leads from a few cm to the battery connector to several feet to reach a mains powered converter. The sensor leads may benefit from this as well. L (inductance) C (capacitance) filter uses an inductor (choke) and small capacitor to reduce the effect of RFI on the leads. Unshielded and untwisted wire leads will have currents induced in them from nearby radio transmitters that can have adverse effect on your readings. Depends also on the type of circuit that's used as well as the sensor. Some are quite sensitive to induced currents caused by RFI. Older devices were built/designed before GSM/CDMA radios (your phones), wifi, bluetooth, etc.

 

[Muse]

You can install F connectors on the PSU and display unit and connect them with slim coax assemblies like these:

http://www.l-com.com/audio-video-th...5-ohm-coaxial-cable-with-in-line-type-f-plugs

And a simple LC filter on the display unit can help with spurious RFI too.

Why use F connectors and not just use the shielded cabling instead of the unshielded two conductor wiring I'm using now for the DC supply, and even for the sensor if the other doesn't work?

Using F connectors potentially could allow me to switch between a battery and the plug-in solution with some fancy stuff, I guess.

Well, it looks like I'll have to do something. Some days seem fine AFAIK. Yesterday was the red letter day saying, things are NOT OK! The actual High/Low for the day was probably about 64/57. The unit's readings when pressing High and then Low were 84/37 !!! When I saw that I thought I should maybe just buy some Lithium 9v, put one in and make sure the unit works OK with that. But think I should maybe try shielded cabling at least for the DC power. But why have F connectors and not just solder the ends of the shielded cable to the little DIY filter I made, which connects to the solid state 9v regulated PS?

May I also ask: That filtering you suggest? How can I come by that? Buy a unit (where/what?)? Build my own? Is the one I made from 3 capacitors inadequate?

 

[[DHT]Osiris]

Out of curiosity, do you need this to function? Or do you need to know the current temp, with highs and lows for the day? There's about a billion smartphone apps which serve this function.

 

[Rubycon]

Why use F connectors and not just use the shielded cabling instead of the unshielded two conductor wiring I'm using now for the DC supply, and even for the sensor if the other doesn't work?

Using F connectors potentially could allow me to switch between a battery and the plug-in solution with some fancy stuff, I guess.

Well, it looks like I'll have to do something. Some days seem fine AFAIK. Yesterday was the red letter day saying, things are NOT OK! The actual High/Low for the day was probably about 64/57. The unit's readings when pressing High and then Low were 84/37 !!! When I saw that I thought I should maybe just buy some Lithium 9v, put one in and make sure the unit works OK with that. But think I should maybe try shielded cabling at least for the DC power. But why have F connectors and not just solder the ends of the shielded cable to the little DIY filter I made, which connects to the solid state 9v regulated PS?

May I also ask: That filtering you suggest? How can I come by that? Buy a unit (where/what?)? Build my own? Is the one I made from 3 capacitors inadequate?

The leads can certainly be soldered or a variety of connection types can be used. F type connectors are common. Convenience factor only.
The LC filter typically uses a small ferrite cored choke in the millihenry range in series with the center lead with a few ceramic disc caps connected to the center lead to shield. They don't need to have a high working voltage rating and perhaps a capacitance of a few picofarads each. Again, this also depends on the sensor type. Increasing resistance on thermistor devices requires re-evaluation (two point calibration). Diode based devices should always be calibrated any time a change is made but will be stable. And those circuits are actually quite sensitive to RFI but mainly in HF region so a cell phone or wifi router shouldn't bother it unless it's laying right on the box.

A simple tight twisted lead no different than what you would get if you pulled the jacket off a piece of CAT6 wire may be all that's needed to prevent RFI/EMI from botching up readings.

If you're interested in further reading on the basics of why and how, you may want to start here:

http://www.murata.com/en-us/products/emc/emifil/knowhow/basic/chapter01-p1

 

[Muse]

Out of curiosity, do you need this to function? Or do you need to know the current temp, with highs and lows for the day? There's about a billion smartphone apps which serve this function.

Well, I suppose there's no necessity for the device to work. It actually has some advanced capabilities including sounding an alarm when a sensor gets to a certain temperature (good for farmers, etc.), has alarm clock functions too. I don't use that stuff. My phone is a Windows Phone, I will see what I can find in the store.

 

[[DHT]Osiris]

Well, I suppose there's no necessity for the device to work. It actually has some advanced capabilities including sounding an alarm when a sensor gets to a certain temperature (good for farmers, etc.), has alarm clock functions too. I don't use that stuff. My phone is a Windows Phone, I will see what I can find in the store.

Understood. Not trying to dissuade you from some fun EE work, just trying to make sure you are getting what you actually need

 

[Muse]

Understood. Not trying to dissuade you from some fun EE work, just trying to make sure you are getting what you actually need

I have several weather apps that I've had for a while. My favorite has been Amazing Weather HD, It's not a freebie, was recommended by a guy who had the same phone as me. Great app, especially in its hourly temp and rain likelihood %. It's most spectacular feature is the satellite radar. When precipitation is around you get a better view than on TV weather, and you can move all around, zoom, unzoom.

That app shows high/low for the day for any of several pre-chosen locations. Thing is, what I get in my back yard often differs significantly from what the weather apps indicate as predicted and current. So, I like to have this device operational.