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Question on relays

Discussion in 'Electronic Basics' started by Raul Rivera, Aug 21, 2003.

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  1. Raul Rivera

    Raul Rivera Guest

    I want to install a relay that will light up my third brake light on my car
    when a certain signal is generated by the ECU. The problem is, the brake
    light circuit is also wired to the brake pedal and if I activate the light
    with the ECU signal, won't the current also flow back throught the brake
    wiring? Do I have to install diodes on both circuits so this won't happen?

    Here are more specifics...

    I have an -88 Honda CRX Si car with an engine swap. The new engine uses
    Honda's VTEC technology. I put a VTEC sticker on my third brake light so
    when the brakes are depressed, the VTEC sticker lights up.

    VTEC is activated by the ECU at 6000 rpms and I want to use this signal via
    a relay to light the VTEC sticker. My concern is that when that happens, the
    current may pass back to the brake circuit. The same when I depress the
    brakes - the current will flow back to the ECU.

  2. DaveC

    DaveC Guest

    Install 1 diode (100v silicon rectifier such as 1N4002) in each wire from the
    brake switch and the ECU:

    1n4002 1n4002
    from brake switch --->|---+---|<--- from VTEC signal (ECU)
    to 12v ^ |
    | |
    | 8
    relay contacts \ 8 relay coil
    | 8
    | |
    bulb @ |
    | |
    | |

    Good luck,
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