Input signals to logic IC over distance?

Discussion in 'General Electronics Chat' started by cchilds, Feb 8, 2016.

  1. cchilds

    Thread Starter New Member

    Feb 7, 2016
    Hi guys, this is my first post so please go easy if I don't conform to etiquette!

    I'm looking to use a logic IC to AND two signals, the output of this will drive a relay which will signal my main control panel.

    At the moment all 3 signals (2 inputs, 1 output), are potential free or 'dry', I will obviously have to bring the Vcc through the inputs to drive the logic circuit, the issue I have is that these input switches are some distance away, sometimes over 30M and run in 1.5mm copper FP cable. I expect that the voltage loss over these distances will bring my signals outside the logic high range and will be useless.

    Would I have to introduce a higher voltage and use additional relays to switch my inputs or is there another solution you can recommend?

    I have 24VDC available, I would be using a regulator to drive the Vcc line.

    Many thanks.

  2. dl324

    AAC Fanatic!

    Mar 30, 2015
    Welcome to AAC!

    Many of the Users here have an EE background and we prefer to have schematics or block diagrams over verbal descriptions. It's easier to get to the heart of the problem without language interpretation issues.

    You need to bound the problem more to limit the solution space.

    Do you have a preference of logic family (CMOS, TTL, etc)? Neither of them will drive a relay directly, so you'll also need a transistor or MOSFET.

    What is the coil rating on the relay?

    You won't find any logic gates that operate at 24V. Do you have a preferred operating voltage?
  3. MrChips


    Oct 2, 2009
    The word for today is impedance.
    Impedance is a fancy word for resistance.
    Resistance is the opposition to DC.
    Impedance is the opposition to AC.
    You cannot send CMOS or TTL logic signals down 30m cable and get away with it. Logic gates have high impedance inputs. You need receivers with low impedance.
    You need to transmit current, not voltage. Convert your signals to 0-20mA. Use opto couplers at the receiving end. Then use the phototransistor output to drive your logic followed by high current drivers to drive a relay.
    cchilds likes this.
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    What are these 'signals'? If they are simply the states of manually-operated switches then the cable length won't be much of a problem.
  5. Papabravo


    Feb 24, 2006
    The LM393 Dual and LM339 Quad Comparators can be used to adapt 24VDC Input signals to logic levels, since the devices have open collector outputs. You can set the input threshold over a wide range and add hysteresis as well. It is the way we do it for industrial controls. As a bonus you can wire-OR the two OC outputs.
    cchilds likes this.
  6. Sonoran Desert Tortoise


    Oct 30, 2014

    Do you have your 24V power near the relay or near the logic that turns on/off the power?

    CMOS logic can be used at great distances if you switch it slowly. The key is that no current is flowing to drive the input, you just need a voltage signal. Since there is no current flowing in/out of the inputs (once gate capacitance is overcome), then there is no voltage drop over distance.
    cchilds likes this.
  7. cchilds

    Thread Starter New Member

    Feb 7, 2016
    Hi all,

    Thanks for all the replies.

    This is a smoke control system, the panel and relay are in the same location, the PSU is integral to the panel.

    There are two inputs, one from the panel and the other from a remote switch, the relay is then used to feed a volt-free input back into the panel. The panel outputs can have voltage pass through but are limited to 150mA.

    From your replies so far I believe I have enough to prototype a solution, I'll give it a go in the workshop.

    Many thanks :)
  8. MrChips


    Oct 2, 2009
    If the two inputs come from switch contacts and you are simply looking for an AND condition to energize the relay, simply connect the two switches in series with the relay coil and power supply, assuming that the relay coil is rated to operate at the voltage supplied by the power supply.