Thermo fan warning light

Discussion in 'The Projects Forum' started by Allbert, Apr 17, 2010.

  1. Allbert

    Thread Starter New Member

    Apr 17, 2010
    2
    0
    Ive fitted after market thermo fans to a project car. They are switched on/off by a thermo switch that senses engine temp, and can also be switched on by a manual switch if required. They are powered through a relay. How i have them connected is shown in this image:
    http://img191.imageshack.us/img191/5251/thermodiag1.png
    (NOTE: I made a mistake on the image. the 10A fuse is in the fuse box under the dash and is only live when the ignition switch is ON. The 40A fuse is in the engine bay and is connected straight to the battery.).

    What i wish to do is create a LED warning light that glows on the dash if something goes wrong (A fuse blows, a relay stops working). Ive decied the best way to do this would be make the led glow in the following circumstances:
    (a) There is voltage at the un-fused side of the 10A fuse, but no voltage at the fused side of the fuse. (The 10A fuse has blown).
    (b) There is voltage from the switch/es, but none at the fan motors (The 40A fuse has blown, or the relay has stopped working).

    Im aware of how to use a transistor as a switch to connect the collector/emmiter when you apply current to the base. But to do this i would need to do the opposite (disconnect the collector/emitter when i apply current to the base). How do i achieve something equivalent to this?
     
  2. Allbert

    Thread Starter New Member

    Apr 17, 2010
    2
    0
    Can anyone confirm one/both of these circuit would work?
    http://img532.imageshack.us/img532/8572/thermodiag2.png
    Take no notice of the "dash fan switch" (Its turned off/not connected). Assuming both the "Unfused IGN On Input" and the "Thermo Switch Output" always have +12vDC, the "Dash warning light" would glow when either the "Fused side of 10A dash fuse" or "Fan motor input" have 0v (But are NOT connected to ground).
    If i have got this right, which is the best circuit to use, and can someone please help me with choosing resistors to suit? They will be running on about 12.5-13.5v and the dash warning light (the only thing they will be powering) will just be a standard 5mm LED.
     
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