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Sherlock Ohms

Porch Light KOs Door Bell

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Charles Linquist
User Rank
Silver
Reflected power
Charles Linquist   9/9/2014 5:28:05 PM
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I needed to read the state of a toggle switch with a microcontroller, and the switch needed to be completely isolated (only one wire was available, and the return was through the chassis).  All the options that I could come up with involved getting some power across the isolation barrier to the switch so it could control the LED of an opto-isolator, or something similar. 

I thought of using a capacitor as an isolation means, but the chassis could have a lot of noise riding on it compared to the uC GND, and the capacitor would simply couple that noise.

Then I thought of using a transformer. I found a very small 1:1 audio type and toggled one  winding at about 100Hz through a 470 ohm resistor with a uC  output pin.  Another uC pin - this one connected as an input, was connected to the "transformer side" of that same 470 ohm resistor.  Finally, only the switch was connected across the other winding.  When the switch was open, the transformer was nearly an open circuit and the input pin toggled with the output pin.  When the switch was closed, the transformer was a near short and the input signal was too small to toggle the Schmitt-trigger input of the uC. 

 

The circuit has been working perfectly for years.

bobjengr
User Rank
Platinum
LIGHT KOs DOOR BELL
bobjengr   8/11/2014 5:52:44 PM
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C.Michael.   In 1976 we moved into a house built in 1953.  We have been here for 38 years.  I certainly know what you mean when you discuss wiring.  I can also add plumbing to that list.  The building codes have obviously changed over the years with many modifications to the NEC (National Electric Code).  I had extensive work accomplished to bring our home "up to code" electrically.  It was absolutely amazing as to what the electricians found when trying to trace and label the various "loops".   It was spaghetti. Switches in the basement actuating up-stairs closet lights; garage door switches turning on kitchen lights, etc etc.  All circuits were corrected but it really took time and rework.  One complexity; seal heat.  The house was originally designed with seal heat.  This heating system has been removed in lieu of a central system but the wires are still in the ceilings and floors.  Our house is one giant antenna.  

Excellent post.

William K.
User Rank
Platinum
Re: Found the same wiring years ago in my home
William K.   7/24/2014 10:35:55 PM
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N F, Actually, I have seen switches and outlets wired in paralell before, except that it was right across the line, so when the switch turned on the light went out.

The mistake was made by folks who had no clue about what they were doing and no understanding of circuits at all. BUt they didn't let that slow them down at all.

NedFreed
User Rank
Silver
Re: Found the same wiring years ago in my home
NedFreed   7/23/2014 10:07:14 PM
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I rewired the house extensively over the years; the walls and ceilings are all plaster on lath with many cross braces and wierd little compartments. And most of the wire is stapled in multiple places. It was very difficult to replace wire even in places where there was an attic above; this was in the basement with no access to the top of the wall at all,  meaning I would have had to tear up the ceiling from the wall to the switch to get another wire to that location.

My parents built the house; the desk was part of the original construction, outlet and all.


Finally, the outlet was in paralell with the switch, which makes it pretty clearly a wiring error. I canont imagine it was done intentionally.

 

William K.
User Rank
Platinum
Re: Found the same wiring years ago in my home
William K.   7/23/2014 9:50:54 PM
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In reality it is often not that difficult to pull wires through walls if they don't need to pass through those fire=stops or braces that buildings used to have. I have installed several switch boxes with wires coming up from the basement below. Also, boxes with wires going out through the top of the wall. So it is entirely possible.

But I really wonder why an outlet would be wired in series with a light in a closet. And the built-in desk may not have been in the original construction, at least not with an outlet.

NedFreed
User Rank
Silver
Re: What were they thinking?
NedFreed   7/21/2014 8:00:15 PM
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The Married with Children episode "User Friendly" (Season 9, Episode 23) may be of interest to you.

NedFreed
User Rank
Silver
Found the same wiring years ago in my home
NedFreed   7/21/2014 7:55:23 PM
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1 saves
Except it was an outlet, not a doorbell. The outlet was inset into the top of a built in desk and the light was a 75W bullb in an adjacent closet. The outlet had been used for years to power stuff like tape recorders or rechargers for  various electronic gizmos, which all worked fine because the 100W bulb allowed more than enough current to flow through.

Things got a bit wierd, though, when my father plugged in an electic typewriter. it worked, but ran a bit rough and the bulb was also glowing and flickering as he typed. It apparently never occurred to my father - a doctor - that this wasn't how things were supposed to be.

This went on for years, until one day I happened to be getting something out of the closet when he turned on the typewriter. It was immediately obvious to me that the circuits were intertwined, but I didn't figure out exactly how until I traced out the wiring. It turned out that the AC line, switch, and outlet wires all ran to the light socket, so it was easy to move the outlet neutral to the line neutral where it belonged rather than to the switch leg it was on.

I'm not sure what I would have done if the outlet wire ran  to the switch. This wiring was new circa 1950, so needless to say there were no ground wires in sight that could be repurposed. About the only option short of tearing into the walls would have been to put a pull string on the bulb and put a blank cover where the switch used to be.

Mr. Wirtel
User Rank
Gold
Re: What were they thinking?
Mr. Wirtel   7/21/2014 6:37:32 PM
It is not necessary for a house to be 100 years old for it to have wacky wiring. We just moved into a house built in the late sixties and have found that amongst the many updates and improvements we have switches in three rooms that do nothing. At first we went around the house trying to figure out if anything was being turned on and off. Then I got an electrician in here to do some other things and he found numerous loose dead wires in the attic along with empty junction boxes. We talked to the previous owner and she told us her husband was in the process of redoing things when he got called out of state for a different job and they needed to sell the house with the changes incomplete. So the switches, wires and boxes were part of a plan never completed, but at least there were no live wires exposed anywhere.

dougs
User Rank
Iron
reflected impedance works
dougs   7/21/2014 5:17:48 PM
been a long time since i have used this trick.  put in a 24 volt doorbell transformer and a 24 volt coil ac relay in series with it.  add some fuses then run the line out to say the pumphouse put in another couple of fuses and an electronic grade transformer (similar wattage to the relay coil) with the 24 volt secondary connected to the signal line.  connect the primary to the unused terimanls on the pressure switch.  you have unused terimanls because the switch drives a contactor which runs the pump but only uses one of the two contacts of the pressure switch. when the power is off to the pump because you are on emergency power you can sense the pump condition without powering the pump. when the presure switch closes the low impedance is transferred across the transformer isolator to the low voltage sense circuit and the far end relay closes.  the low voltage circuit is isolated from the mains voltage by the transformer and protected by the fuses.  but try it on the bench with the signal wire you intend to use so the signal path inductance is not excessive too often we only look at the loop resistance and then have to backtrack. the alternative is usually a second pressure switch at the generator location but then you have to keep the two switches adjusted so the pump switch operates higher and lower than the remote switch not something to mess with durring an emergancy. or you can use the dc sense used by marine generators for start on load but now you need a relay state machine several timers and to not put line voltage on your dc circuit.

William K.
User Rank
Platinum
Good job of diagonsing the problem
William K.   7/9/2014 10:14:00 PM
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This is an xample of good diagnostic thinking, and solving a quite interesting problem. Old wiring that has been worked on by folks of different skill levels can produce very interesting results, as you discovered. Good job also in using the story to make a class demonstration more interesting.

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