I Need A Simple Voltage Threshold Switch/Relay/Circuit

Discussion in 'General Electronics Chat' started by Travis Rayner, Mar 11, 2016.

  1. Travis Rayner

    Thread Starter New Member

    Mar 11, 2016
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    Hello,
    New member here...first post. For several months, off and on, I have been reading posts on this forum trying to find an answer to a problem I am having. So far, no joy. :) I am sure there is a simple answer to my problem and I am in hopes that someone can provide the answer. Please understand that I am comfortable with AC and DC wiring...but my knowledge is "very" limited with electronics and circuitry. But...I keep trying! :)

    Our house is about 200' from our driveway and walk gate. The original owner ran some underground wire from the house out to the 12 volt gate opener to keep the battery (at the gate) charged. Charging is supplied by a battery maintainer located in the garage of the house. No problems with any of that. I discovered the underground cable had an unused third wire that I decided to use as a trigger wire for a gate switch that would in turn sound a 12 volt buzzer in the garage letting me know the gate had been opened. I connected to the ground at the gate control box (battery) and ran a wire from there to the gate switch (magnetic switch) and then back to the control box where I connected it to the third leg of the underground wire. Back at the house I connected a 12 volt buzzer between the 12 volt (+) and the trigger/ground wire (-) (the third wire). So, when the gate is opened it closes the circuit on ground wire (trigger wire) and sounds the alarm.

    After a week or so the alarm would stop sounding...so I would replace the alarm and everything would be good for a few weeks...then it stop working, again. While discussing this with my wife she pointed out that the buzzer always made a low humming noise that I could not hear. I then checked the voltage and it appears I have a leak somewhere between the ground wire and the trigger/third wire. Depending on how damp the soil is, I have a steady voltage between 1 and 2 volts indicated when the gate is closed. Open the gate and I have 12.5 volts. So...this low voltage has been destroying the little buzzers.

    Digging up and replacing the existing wire is not an option. So, Is there a simple circuit or relay that would ignore this low voltage and only allow the full voltage to pass when the gate is opened, thus sounding the buzzer. Your help and guidance would be appreciated.
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,009
    3,233
    If you could post a diagram of the circuit it would help.
    Don't understand how the leakage is damaging the buzzer.
    You likely could add an amplifier of some sort to isolate the buzzer from the leakage.
    Do you have a multimeter?
     
  3. Travis Rayner

    Thread Starter New Member

    Mar 11, 2016
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    Thanks for the reply. Yes, I have a multimeter. I will try to draw a diagram as soon as I can. The best I can figure, the constant 1-2 volts DC that is always present is damaging the buzzer.
     
  4. MrSoftware

    Active Member

    Oct 29, 2013
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    Instead of running the buzzer directly from the leaky wire, maybe run a relay with the leaky wire and let the relay turn the buzzer on and off. 2V is surely below the latching voltage for most 12v relays. There's surely a more elegant solid state solution, but this might just solve the problem with minimal complexity. :)
     
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Also, all buzzers, beepers, sounders, and alarms are not created equal(ly). Manufacturer/part number/distributor stock number will help.

    AND - why is there a battery at the gate?

    This is not a difficult problem to solve, but a more complete picture of the entire setup will save about 50 postings.

    ak
     
  6. Travis Rayner

    Thread Starter New Member

    Mar 11, 2016
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    Thanks! I have a couple of 12 volt relays in the shop. I will try them and see what the reaction is with the low voltage. That would be a simple solution! Thanks. With that thought in mind...I looked at a solid state relay: http://www.amazon.com/uxcell-Single...57726384&sr=8-2&keywords=solid+state+relay+dc That has a 3 to 32 VDC range. Somewhere in the kingdom I have one of those. Since the low input range is below my leaky voltage level...do you think this might be a better option than the mechanical relay? Thanks again...the help is greatly appreciated! :)
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    I would say you have AC voltage picking up when its damp, especially if there is a hum on the buzzer, best bet is a simple 12v automotive relay, and let that put the buzzer on.
     
  8. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    And this is, "why". The SSR is guaranteed to operate down to 3 volts. It might also work a little below that. A stupid relay will surely ignore the 2 volt signal while the smart relay might not.
     
  9. Travis Rayner

    Thread Starter New Member

    Mar 11, 2016
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    0
    In regards to the question about the battery at the gate. It is located inside the control box that controls the automatic gate opener...it powers the gate system. #12, thanks for the comment on the SSR. Good point. I will try the stupid relay and report back on the results. Thanks! :)
     
  10. MrSoftware

    Active Member

    Oct 29, 2013
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    I grew up out in the sticks, and often the gate was a long way from the house (or any power source). A 12v gate opener is convenient because a solar panel can be used to keep the battery charged. I'm guessing whoever installed it used one of these 12v systems, but for some reason ran wires down to the battery instead of using the solar option. Maybe the gate isn't too far away, and the 12v system is just what was readily available at the time?
     
  11. Travis Rayner

    Thread Starter New Member

    Mar 11, 2016
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    Same assumption here. Considering some of the things we have found...it appears there were a lot of projects where they just used whatever they had on hand. As a side note on the 12 volt gate...we have a number of folks around us with 110VAC gates. We get a lot of lightning around here. Most folks around us have to change their control panel/gate controller a couple of times a year. We have never had a problem with the 12 volt version. :)
     
  12. Tonyr1084

    Active Member

    Sep 24, 2015
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    Without knowing more about what you have - guessing at what may be going on is difficult.

    On the comment about a low AC hum, if it's line crossover (induced voltage from a local higher voltage AC source) then I ASSUME (a bad thing to be doing) you're getting a 2 volt AC feed on your DC buzzer. It's possible the damage is being caused by the reverse current on a very low level. At some point the buzzer is just getting tired of bucking that low voltage. PERHAPS (and this is guesswork here) a Diode will solve the problem. It will naturally drop a few tenths of a volt (typically around 0.7 v), thus blocking the AC from reaching the buzzer AND bleeding off that extra (nearly) one volt may do the trick. No need to wire in a relay. And if one diode works, two would work even better, thus eliminating nearly 1 1/2 volts from your circuit. Likely not going to cause a problem with the buzzer.

    FOOD FOR THOUGHT.

    One last thought: I post a lot of drawings. I do them with 'Paintbrush'. Takes a short while to draw out a circuit but if I discover something wrong with my drawing - it's easy enough to correct and repost - or edit an original post.

    Here's a drawing of a "Delay Start Relay" circuit I did to delay the start of some gauges on my truck.

    Time Delay Start.png
     
    Last edited: Mar 12, 2016
  13. mcasale

    Member

    Jul 18, 2011
    210
    12
    One quick comment on the posted circuit: it's good practice to put a flyback diode across the relay coil. When the relay is turned OFF you might kill Q1 with a big reverse voltage.
     
  14. Travis Rayner

    Thread Starter New Member

    Mar 11, 2016
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    0
    Tony,
    In regards to an AC hum. That was a comment that was mentioned by someone else. There is no AC current involved in this. It is a 12 volt DC system. The low humming noise I referred to was/is being caused by the 1-2 VDC that is on the line...not enough to sound the buzzer but enough to make hum, buzz. Thanks! :)
     
  15. Tonyr1084

    Active Member

    Sep 24, 2015
    535
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    @mcasale: In practice you're right about the flyback. In this case Q1 is buffered by C1. This circuit has been doing its thing for over 2 years. But yes, you're right, a quenching diode WILL prevent flyback caused by the collapse of the magnetic field in the coil of K1 (which is a 24 volt dc coil, powered on a 12 volt (13.8) car battery system). Nevertheless, the post isn't about my circuit, it's advice to Travis Rayner who might consider using PAINTBRUSH to draw his diagrams. My drawing is only meant to illustrate that paintbrush can be used to draw out circuits. I use it all the time.

    @travis: OK, I understand, this is pure DC. I still think (without good reason)(I said "WITHOUT" good reason) the diodes may drop just enough to quiet the buzzer. Just advice from a novice, not from any kind of expertise. And the circuit is yours to build any way you wish. I hope you can find an answer to your problem.

    Best of luck Trav.
     
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