matched impedance question

roughshawd

Jul 13, 2020
289
If I have a board that has a pinout for a reed switch that diagnostics about 5 volts into/or stops from/ going into the circuit to make an alarm bell sound when its high...

ipqr- a door switch that says when the doors open.... The board is getting a 4.88v reading and the doors are closed and maybe its supposed to change when they are closed to 0.00v

So to fix the issue without changing the reed switches out---
I unplug the unit, [check|get] the resistance at the two input pins --- and then install a 5v resistor of the same resistance in the circuit pinout, where the two contacts for the pinout for the reed switch is,
will it silence the door alarm? acting like a closed circuit, or could it be harmful to the board to install the extra resistance?

kellys_eye

Jun 25, 2010
5,511
I've never seen a 5V resistor - got a link?

Jul 13, 2020
289

Bluejets

Oct 5, 2014
6,199
Provide a circuit diagram, include power supplies, explain what you think the problem is, not what you think might be a cure.

kellys_eye

Jun 25, 2010
5,511
Either short the reed switch out or put a magnet to it. It will simulate the door being open/closed.

5V or 4.88V doesn't mean diddly squat. It's the same thing as far as a detected signal is concerned.

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,098
Might not be 5v
That's 5 %, the tolerance. It has nothing to do with voltage.
then install a 5v resistor of the same resistance in the circuit pinout, where the two contacts for the pinout for the reed switch is
Nonsense. A switch is either open with a resistance (for practical purposes) of infinity or closed with a resistance (again for practical purposes) near 0 Ω. No impedance matching is done in such an application.
On the contrary, it will be detrimental: The circuit usually "expects" a voltage near 0 V or near 5 V, depending on the state of the switch. With a "matched impeadance" the voltage will be around 2.5 V and the circuit is likely to behave erratically.

roughshawd

Jul 13, 2020
289
So what you are saying is that the proper impedance is likely to cause more problems than the clip is worth!!
Got it! So if the signal is supposed to be 0(zero) when the door is closed the switch is maybe bad so check the switches.
Now I have to take all four doors off...

Where is your inbuilt compensator that says... "cut work time in half, install a resistor" when you need it!!

This door work is going to take a while, so I will probably not be posting for a while !!!
Anyway, it seems to me that if the switches are good, then a resistor might do the trick!

roughshawd

Jul 13, 2020
289
Update on the magnet test.... The dead lights in the freezers turned on!!!! Ok! I will test the volts tomorrow, but the alarm is still bonging!

roughshawd

Jul 13, 2020
289
I will try stronger magnets and see if the lights turn off when they are applied. There is something funky with a set of switches that appear to work backwards...

Martaine2005

May 12, 2015
4,539
Only one way to find out if the switches are working backwards. Drill a hole in each fridge door to see if light comes on or goes off when doors are closed!.

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,098
Only one way to find out if the switches are working
I can imagine a less destructive way: put a camera into the fridge, record a movie while closing the door. Then watch the movie to find out what the light does.
Or use e.g. an ESP32-cam module and watch live via WiFi.

bertus

Moderator
Nov 8, 2019
2,857
Hello,

An other way to detect the light would be a light sensor like an ldr inside the fridge on long wires going outside the fridge.

Bertus

hevans1944

Hop - AC8NS
Jun 21, 2012
4,792
I will try stronger magnets and see if the lights turn off when they are applied. There is something funky with a set of switches that appear to work backwards...
Backwards? All a reed switch (or ANY switch) does is present an open or a closed circuit. How this is interpreted by whatever electronic circuit uses the switch has nothing to do with "forwards" or "backwards" in how it works. That is up the the interpretation of the designer. So, yeah, a closed reed switch might be closed because the magnetic seal on the refrigerator door actuated the switch, but it is the circuit designer who must decide what a closed reed switch means.

Your successful troubleshooting of whatever problem seems to be occurring depends on your discovery of (1) how the reed switch is actuated and de-actuated and (2) whether it is actuating and de-actuating according to plan. This is most easily determined by disconnecting one of the two reed switch leads and measuring the continuity between the two leads of the switch with a volt-ohm-milli-ammeter (VOM or multimeter). This effectively eliminates the rest of the circuitry to which the reed switch is connected IF the switch tests okay. If the switch fails the continuity test, i.e., it tests closed no matter whether a magnet is held near it or moved far away from it, OR it tests open no matter whether a magnet is held near it or moved far away from it, then the problem is NOT the switch. It could be a problem with the magnet not being strong enough, but large permanent magnets are readily available. Try different magnets if the test fails. Note that the north and south poles of the magnet should be alighned with the axis of the switch to be reliable and effective. The switch contacts inside the glass capsule are part of the magnetic circuit. This is why the switch does not open immediately when the magnet is slightly withdrawn from the position that caused it to close.

Reed switches usually fail when arcing inside the glass capsule welds the two contacts together. In very rare instances they can fail from metal fatigue of the moving contact. Or the glass capsule can be damaged (broken), which will cause the contacts to mis-align and not function properly, even if the solid wires connecting to the contacts through the glass capsule are soldered firmly in place. I am sure there are other failure modes that I am not aware of, but I have NEVER observed a faulty, intact, magnetic reed switch that was not physically or electrically abused in some way. Magnetic reed switches are very robust devices rated for hundreds of thousands, if not millions, of operational cycles before failure.

Good luck on getting your beer-keg cooler working properly again.

roughshawd

Jul 13, 2020
289
The panel was staying lit all the time. with stronger magnets the door alarm silences and the lights in the freezers operate properly. I need two(2) 2 pin connectors for the doors, my wire nuts should be replaced with suitable connectors. The refrigerator is not reading properly now. it is reading about 10 degrees high, and the french(the small part between the two large refrigerator doors that folds out of the way when the doors are opened as one large opening...) was slightly warm due to my stupid testing of the forced operations!!!!(get a laugh.. they are far and few between...) I think with testing, the proper connections and some good old fashioned elbow grease, this old smart fridge can be a great fridge... and soon too!

roughshawd

Jul 13, 2020
289
I don't feel right spending 22 dollars on two little connectors, it's not American! So I have taken apart a video cable and an old VHS player that doesn't need them any more. There will simplify the connection, and should give smartkegger adequate signal to the board.
Are the skinny video cables 5v stable or are they signal only?

dragon

Oct 31, 2022
229
if they are copper they are probably ok.

roughshawd

Jul 13, 2020
289
They are rca but I remember reading something about the video connectors on the RCA plugs being a superior material so there's less noise.

kellys_eye

Jun 25, 2010
5,511
Those are screened cables - I suppose they'll work but not exactly the right thing to use.

hevans1944

Hop - AC8NS
Jun 21, 2012
4,792
They are rca but I remember reading something about the video connectors on the RCA plugs being a superior material so there's less noise.
There is a LOT of misinformation, not to mention outright lies, concerning connectors and their associated wiring. The RCA connector is a coaxial-style connector, similar for example to a BNC-type connector. The BNC is impedance-controlled by its manufacturing design. The RCA is simply a coaxial, shielded, small-signal connector, widely used for audio and NTSC analog composite video, but with no specified impedance. It was also used to connect radio-frequency signals, mostly between units in a single chassis, as between a rotary channel tuner and the televison main chassis. Sometimes the RCA male connector was permanently soldered to a chassis-mounted female RCA connector. I believe (without proof) this was just to ensure the reliability of the connection and prevent an accidental disconnection. I still use RCA connectors to this day because they (1) do the job and (2) are usually dirt cheap.

During the "hi-fi" craze that occurred after the end of World War II in the previous century, there appeared a class of listeners who possessed "Golden Ears" that could allegedly detect the slightest distortion in sound reproduction... regardless of whether or not such low levels of distortion were actually measurable with sensitive electronic equipment. To satisfy the "needs" of this elite group of individuals, RCA connectors were manufactured and sold with gold plating on their contacts instead of the usual tin plating. The Golden Ears insisted they could hear the difference between ordinary RCA connectors and the gold-plated RCA connectors. More than that, these folks were willing to pay exorbitant prices for the gold-plated connectors. That wasn't enough. The next "big thing" was zip-cord speaker wire: originally just ordinary 14 AWG lamp cord, the Golden Ears demanded that it be made from "oxygen-free high-conductivity" or OFHC copper wire. Later, with lower-impedance speakers becoming popular, because of power semiconductor output stages that needed no transformers to match impedance between pentode power tube plate circuits and loudspeakers, a larger gauge wire was also deemed necessary. Some aficionados even went as far as using ultra-stranded, ultra-flexible, welding cable to connect their speakers!

I and, every other electronics engineer or technician that I ever worked with, denied the claims of those who said they had Golden Ears and could hear residual distortion that even the most sensitive electronic instruments simply could not measure. I kept my mouth shut and refused to become embroiled in the controversy. But you can bet your ass that if I was selling "hi-fi" waaay back when, I would try to up-sell to the Golden Ears whatever they asked for.

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