REMOTE CONTROL via 2 wires

REMOTE CONTROL of 2 momentary contacts 1 at a time via 2 wires

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A problem, only 2 wires available in a conduit, yet need 4, to control 2 devices.

On one project, I had to control 2 devices, one at a time, by providing momentary contact closures to:

1) a Pedestrian Gate Strike release that already has strike power at the Pedestrian gate, and

2) to give a driveway gate motor a trigger to open, and and later another trigger to close.

Only two 26 guage phone cable wires in one available pair, from outside to the Phone equipment room, where 4 wires are needed, and 2 really need to be 16 guage or larger.  No room in a 1/2" conduit also. 

So I put on my thinking cap and was determined to find a solution to using the only 2 wires between the 2 locations the wire needed to be.

I thought the only thing that could perform 2 contact closures one at a time using 2 wires  would be one that could sense the polarity of a DC voltage. Also with the wires carrying no voltage until the 5 seconds of activation would last a whole lot longer than wires carrying voltage in outside conduits.

A DC relay would provide a contact closure and isolation from the device being controlled. Also a diode connected in series with one relay lead would make the relay polarity sensitive, like positive for one and negative for the other.

The polarity reversal could be accomplished by a DPDT relay in the phone equipment room. The  DC power supply would power up all three relays from the phone equipment room.

The logs document all the steps involved in getting all pieces tied together.

The background photo above is this same circuit used on a training board I designed to educate the technicians on DC relays, diodes for polarity, and another task to install a spike absorbing diode across the relay coil without causing the switching regulated supply to go into shutdown. Also if the switching supply was disconnected and reconnected with the polarity reversed, the power LED would light red. This photo shows the LED yellow, it should have been green. The color change is from a 2 lead dual color LED, one polarity is green, the other is red. It is assembled with a series voltage drop resistor and two 14 volt zener diodes wired in series with their cathodes connected. This provides a 14.7 volt drop across the zener diodes regardless of the polarity, the LED drops approx 3 volts and the resistor the rest from a 24 VDC switching wall adapter. I liked the really big 24 Volt contactor for the training board, so it set the voltage for all the board relays.

The jumper wires in the photo above are properly wired to connect the DPDT relay to the two remote relays on the right. The center off toggle switch is in the down position, powering the DPDT contactor coil.  The contacts have moved to the Normally Open contacts and the Red Led is lit on the lower relay on the right.
The dual banana connectors carry the spike absorbing diode. The DPDT relay and the polarity sensitive relays are connected so that when the center off toggle switch is up the DPDT Normally Closed contacts are made and the center off toggle will complete the circuit to energize the top relay on the right. The center off toggle switch represents the 2 Normally Open contacts to operate this circuit,  but on one toggle switch to maintain that both relays are off when off, and that only one relay is on at a time.

  • 2 × Relay 12 or 24 Volt DC coil, SPDT contacts Remote relay, both same DC coil voltage
  • 1 × Bridge Rectifier Provides Polarity for Remote relays and spike absorbing diodes
  • 1 × DC power supply 12 or 24 volts to match relay volts Local DC power source to provide the power for all 3 relays
  • 1 × Relay 12 or 24 Volt DC coil, DPDT contacts Local relay to provide polarity reversal
  • 1 × 1N4002 Diode Spike absorbing diode across the local DPDT relay coil

  • Thank you

    Boelens, Leland04/27/2018 at 04:50 0 comments

    I would like to thank and their sponsors for the opportunity to enter the 2018 Hackaday Prize Open Hardware Design Challenge.

  • Some Final Thoughts

    Boelens, Leland04/22/2018 at 21:34 0 comments

    This project solved a problem involving a momentary contact closure x 2 involving 2 wires.  Where 4 wires of the proper wire guage were needed.

    What  if you needed to remotely maintain a contact closure and needed the status of that contact closure. Then two computer chips communicating using tones on those 2 wires could talk back and forth. 

    A protocol could be devised where there is a normal banter back and forth in half duplex ( each taking turns talking ). Then the number of contacts could possibly be in multiples of 8, and each contact can be ON, OFF, PULSED, or TIMED as needed. With a status report on each and every one.

  • Operational Notes

    Boelens, Leland04/22/2018 at 21:22 0 comments

    Normally both remote relays are waiting for power to be applied to the 2 wires going back to the Phone equipment room.

    When power is present on the 2 wires, depending on the polarity, the designated relay will have coil voltage, and the relay contacts transfer.

    The operating states are 1) No power on the wires, both relays de-energized.  2) power present on the 2 wires, only one relay has coil voltage determined by its series diode and the applied polarity on the wires.

    The interconnection between the two schematics is Output 1 to 1 and Output 2 to 2. Now when Switch 1 is closed Relay 1 is energized.
    When Switch 2 is closed, the DPDT relay is energized, the DPDT relay contacts move, and Relay 2 is then energized.

    If both Switch 1 and Switch 2 are closed, Switch 2 will override Switch 1 every time.

  • The INSIDE of things

    Boelens, Leland04/22/2018 at 21:05 0 comments

    Inside the Phone equipment room the other end of those two 26 guage wires meet up with a DPDT relay with a coil voltage to match the 2 relays of 12 volts DC, and are wired to the Common switch terminals of the DPDT relay, one wire to each pole.

    The two poles of the DPDT relay, have their contacts cross connected
        ( N.O. to N.C. ) so that one polarity is present  at the two common switch contacts with no relay coil voltage, and then the reverse polarity is present when the coil voltage is applied, and the contacts switch on both poles.

    The DC power supply is wired to the Normally open contacts, one Normally open contact has the - polarity, the other has the + polarity, this + polarity is also wired to one of the coil wires. A Cathode of a 1N4002 spike absorbing diode also goes here, the Anode of the spike absorbing diode goes to the other coil wire. The other coil wire goes to one of the Business Phone equipment switch contacts, the other related switch contact of the same Normally Open switch goes to the - polarity on the DC power supply.

    The other Business Phone equipment switch contact has one connection to the
     ( - polarity)  on the  ( - Normally Open contact ) on the DPDT relay, then remove the cross connection wire  from this Normally Open contact, and connect the other related switch contact to the Normally Closed contact that the cross connect wire was connected to. Now when this Phone system contact is closed, the polarity at the Normally Closed contacts will energize the two wires, and thus energize the coil of one remote relay.

    The other Phone system contact will energize the DPDT relay coil and this DPDT relay will transfer its contacts and present the opposite polarity to those same wires and energize the other remote relay.

    The 2 wires to the Remote relays have no power until a Business Phone System contact closes.

  • Dealing with the outside situation

    Boelens, Leland04/22/2018 at 20:09 0 comments

    So I needed two small DC relays with SPST contacts and 4 diodes to provide each relay with a polarity, and a spike absorbing diode. I made a schematic drawing of the  two relays with a common connection to each coil, and a series diode one wired  forward and one wired reverse sharing the other common connection.

    When  I added the spike absorbing diodes, I noticed that all 4 diodes formed a Bridge Rectifier !

    Then on a small perf board I wired up (2) DC relays with 12 volt DC coils and a bridge rectifier. I provided screw terminal connections to connect to the 2 wires from the Phone equipment room, and screw terminal connections for the contacts on each relay to control the situation outside. This was able to fit inside the outside wire box.

  • Now what to do

    Boelens, Leland04/22/2018 at 19:55 0 comments

    The only 2 wires available to the Business Telephone equipment room was a single phone pair of 26 guage wire. I could not switch much current by these two 26 guage wires except power a relay coil.

    Then it occured to me that if I had a polarity sensitive relay I could power up the only 2 wires with a DC voltage to turn on one relay with one polarity,  and reverse the polarity to turn on the other relay, as required for normal operation. Both relays would normally be de-energized waiting for those 2 wires to power one of them up.

    The Business Telephone system provided 2 Normally Open contacts that would provide a 5 second contact closure when activated.

View all 6 project logs

  • 1
    Remote Relay Board

    On a small bread board or perf  board install the 2 relays and a bridge rectifier, make all connections. Bring out the relay contacts to respective terminal connection points. Bring out the 2 wire connections to a terminal.

    Install this board at the remote location.

  • 2
    Local Relay and DC Power source

    It would be easy to use a relay socket or a contactor type relay to have screw connection points for all wire connections. Just wire the external Normally Open switch contacts, and  the DC Power source to the relay socket, or contactor relay.

View all instructions

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