REMOTE CONTROL via 2 wires

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

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.

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.

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.

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.

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.