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Safety Beam Driveway Annunciator

Discussion in 'The Projects Forum' started by Mike H, Mar 23, 2009.

  1. Mike H

    Thread Starter New Member

    Mar 23, 2009
    I would like to use a set of garage door safety beams to make a driveway annunicator. I have a long driveway and a special needs student that needs to be ready to go when the bus gets to the house.

    The safety beams work on 18 to 24 vdc. When the beam is broken the transmitter / receiver set reverses polarity on the two wire power circuit.

    I have the safety beams, a power supply and sounder.

    I'm looking for a circuit design that supplies power to the safety beam set and senses a reverse polarity on the two wire circuit. When polarity is reversed it needs to activate a sounder. I'm using a sonalert as a sounder.
  2. Bernard


    Aug 7, 2008
    Just to break the ice: Powersupply is at the house ? , feeding a long pair carrieng 24 V DC to end of drive for a receiver abd transmitter. First look: Trans. & rec. are fed thru a fullwave bridge rectifier so they always have correct polarity. Rec controls a DPDT relay- cross strapped between fixed contacts . power is supplied thru relay contacts , when energised , powerlines from house are reversed. A diode and load resistor are placed across opposite fixed contacts so that no current flows on stand by.When rec. is tripped relay pulls in reversing power lines , with load resistor and forward conducting diode across line. At house increase in current is detected activating alarm.