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Battery Check Light?

Discussion in 'Amps, Mics & Pickups [DB]' started by pmckinney4, Mar 21, 2006.

  1. I am currently working on an EUB project. Right now I am working on planning all of the electronics. I am going to start of trying to use Pick-up-the-worlds solid body preamp.

    This pre-amp requires a 9v battery. Forgive me if this is a stupid question, but I do not know a lot about electronics. Is there a way that I could wire in a small led to indicate the battery strength?

    I thought of this the other day. I was using a boss foot pedal and thought that the battery indicator light that is integrated into it would be cool to integrate into my bass so that the battery doesn't give out in the middle of a gig.

    Is it as simple as just wiring the LED inline with the power and as it gets dimmer the battery is lower?

    Any idea's on the best way to accomplish this?
  2. mje


    Aug 1, 2002
    Southeast Michigan
    You could, but even the smallest LEDs draw more power than most simple preamps, which means the LED would drain the battery much faster. To give an example, fdeck's one-FET source follower draws about 1ma; a small low-current LED will draw 2ma- that would triple your battery drain!
  3. I wonder if it would be possible to wire a small push button with it so that when the the button is pushed in, the led would lite up and you could check the brightness that way without leaving the LED on all the time to drain the battery.
  4. bannedwit


    May 9, 2005
    Buffalo, NY
    some company has a product where you drill into your electronics cover and install a LED that turns on only when the battery is low. I guess it just monitors the battery power and when it is below it triggers the LED... Some basses come with this option.
  5. mje


    Aug 1, 2002
    Southeast Michigan
    Yeah, but brightness alone won't tell you anything. You need a comparator and a reference voltage for a go/no-go indication. You also need to run tests to determine at what voltage your 9 volt battery is no longer usable, or will need to be replaced soon. A battery isn't just good or bad; it's good or bad for a specific applicaiton.

    Of course, you can also just plug in, play a few notes, and get a go/no-go indication ;-)

    Seriously, I think a package of fresh batteries in the gig bag is a better investment; if the LED tells you your battery is low, you're still gonna need the fresh batteries!
  6. mje


    Aug 1, 2002
    Southeast Michigan
    That's a useful little chip. But you'd still have to do some experimenting to figure out what your reference volatge level should be.
  7. mje


    Aug 1, 2002
    Southeast Michigan
    Sure. But you still need the voltage reference, the comparator, and the determination of the appropriate reference level. Looking at an LED won't tell you if it's supplying sufficient voltage for your application.
  8. JansenW


    Nov 14, 2005
    Cambridge, MA
    You can also wire the battery so that it is only on when you have a cable plugged into your bass.

  9. Yes, I did buy a switching jack so I can do this.

    I also agree with MJE that it is good practice to have a handfull of batteries in the gig bag. But if a battery dies in the middle of a song, you are without bass for the rest of that song....or at least until you can get your battery swapped out.

    I supposed I could just get an external battery tester and just pull it out and test it every once in a while, I was just hoping for an easier solution like a button or some kind of indicator on my bass that would tell me when to switch it.
  10. JansenW


    Nov 14, 2005
    Cambridge, MA
    Good idea. A built in batter tester that is only activated when you press the button.

    My Sadowsky has a active/passive switch. Perhaps you can install a preamp with this feature so can always fall back on the passive mode if your battery ever fails.
  11. Great!, How do I do this?
  12. pocket_groover

    pocket_groover Supporting Member

    Jan 21, 2004
    Northern California
    You could hook up a variable power supply to the battery terminals and starting at 9volts, play your bass. Then, start decreasing the voltage of the power supply until you hear something that you don't like (distortion, for instance). You could then set this as your reference voltage for the alarm. This is controlled by the ratio of R1, R2 and R3.

    Assuming you know your voltage to switch at, I could give you values for the resistors.
  13. mje


    Aug 1, 2002
    Southeast Michigan
    I suggest a calendar ;-) No, seriously; if you're gigging regularly, change your battery weekly, monthly, or whatever works for you. If you're like me, and play out once in a blue moon, buy a fresh battery for your gigs.
  14. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    I looked up the LTC1440 chip at www.linear.com, and there is a nice battery monitor circuit on the first page of the datasheet. Referring to that schematic, you set R1 = R2*(Vthresh/Vref - 1) where Vref = 1.182 V, and Vthresh is wherever you want your threshold to be. Wire the output through a 1k resistor to an LED, and activate with a pushbutton.

    I think 8.5 V is prudent. After the voltage really starts to fall, alkaline batteries sink like a stone. The resistors would be R2 = 200k and R1 around 1.4 MegOhms. Choose the closest values from the standard list of values.

    Pushbutton activation seems like a good idea. Ideally, it would be cool if the threshold is high enough that the bassist is guaranteed a gig's worth of remaining battery life. That way, you can check it while packing for the gig, and get a battery on the way if needed.

    A good thing to know is that a standard alkaline 9-V is around 500 mAh, i.e., 500 hours at 1 mA. Naturally, I recommend my circuit:


    Beware that I will be changing my URL soon.
  15. mje


    Aug 1, 2002
    Southeast Michigan
    For $29.95 I wouldn't bother making my own. Looks like a good solution.
  16. Yes, I agree. It will save me the trouble of trying to figure how to do it myself.

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