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JBrock

Power-Up Latching Relay

[SIZE="3"][FONT="Verdana"]The purpose of the circuit is to create a switch which cannot change posi

  1. JBrock
    The purpose of the circuit is to create a switch which cannot change position after power is applied to the circuit.
    The position of the switch must be set before power is applied. After power-up, the position of the switch is locked.
    The schematic shows a SPDT "RUN/OFF" switch on the left, and the coil of a DPDT relay on the right.
    If the switch is set to RUN, the relay will actuate roughly one-half second after power-up. If the switch is set to OFF, the relay will not actuate.
    In either case, the switch will have no further effect after the power-up initializing period.
    This circuit has been built and tested. It currently works reliably in a computer with a second hard drive. It prevents power from being applied to or removed from the hard drive while the computer is on.
    The switch contacts of the relay control both +5V and +12V to the power input of the hard drive. There is an LED indicator on each of these rails to confirm power.
    The heart of the circuit is a set-reset flip-flop constructed from gates B and C of a quad nand gate.
    The timing diagram shows waveforms at points labeled in the schematic for the two conditions RUN and OFF.
    The assembly diagram shows the top side with components in the top half of the picture and the bottom side with wiring in the bottom half of the picture.
    This design is completely original by yours truly and it is free use (at your own risk of course!)
    Comments and criticism welcome.