12v Battery Low Voltage Monitor Help

Discussion in 'The Projects Forum' started by JimmyJazz, Mar 15, 2010.

  1. JimmyJazz

    Thread Starter New Member

    Mar 15, 2010
    2
    0
    Hello

    I would like to build the circuit below:

    12-volt-battery-low-warning.jpg

    I have never built a circuit before but I am quite familiar with wiring and a soldering iron. But there are just a few things I would like to clarify first and hope somebody can help.

    1. Is the V In on the LED side linked to the 12v V In on the other side.
    2. Is it possible to substitute the LED with a 12v bulb. would you just remove the 500R.
    2. What is a 500R.
    3. Do you think this circuit would be OK to link into an alternator charge wire to operate the charging / not charging warning light.

    Apologies for the barage of questions and thanks for any help.

    Alan
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Yes, it is.
    No, 12v incandescent bulbs require much more current than the 741 opamp can sink or source.
    That means 500 Ohms. 500 Ohms is not a standard value of resistance, however you could use a 510, 560, 620 or 680 Ohm resistor with not much of a noticeable difference.
    Is this an automotive application? Most autos either have an alternator "idiot light" or a meter to indicate state of charge/discharge.
    If you decide to build this circuit, I suggest that you set the threshold to around 13.6v; below that indicates not charging, above that indicates charging. You will need to isolate the circuit from the alternator output using a filter, otherwise the electrical noise from the alternator will quickly kill it.
     
  3. JimmyJazz

    Thread Starter New Member

    Mar 15, 2010
    2
    0
    Wow! thanks for such a quick reply. It is for an automotive application. The problem I have is that the alternator has no wiring provision for an "idiot light" and there is no meter. I was kinda hoping I could use this circuit to take it's place.

    Will the 100K variable resistor be ok to set a threshold of 13.6v

    Can you suggest what filter and were it would go in the circuit

    Thanks again

    Alan
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Alan,
    Your thread galloped off the page so fast I lost where it was.

    Here you go:

    [​IMG]

    Radio Shack carries a 100uH inductor that will work just fine for this.

    You should connect the circuit to the ACCY or IGNITION circuit in your vehicle using a fuse of 1A or less. Do not forget to use a fuse.

    Use heat shrink tubing for insulation. Don't use electrical tape, as it will get all gummy and fall off in a year, then you will have problems.

    If you connect the circuit directly to the alternator or battery, you will drain the battery when the engine is off.
     
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