Needing help with an interface circuit

Discussion in 'The Projects Forum' started by jbarrington, Oct 7, 2010.

  1. jbarrington

    Thread Starter New Member

    Oct 7, 2010
    From what I understand, I need some sort of an interface circuit to connect two electronic items together.

    I am trying to take an accelerometer circuit (from SparkFun) and connect it to a sesmic kit circuit (from BG Micro) without using the geophone hardware that comes with the sesmic kit. The geophone hardware device within the sesmic kit can only detect up and down movement, but I want to detect X, Y, and Z movements; Up and down, Side to side, and Forward and backward.

    Normally within the sesmic kit with the geophone hardware device, as more vibration is made, the more LEDs are lit up. I think the sesmic circuit is just the same circuit that would be normally used as a bar graph to indicate sound level. Of course, there is a pot within the sesmic kit that can adjust the sensitivity up or down. Hopefully, the accelerometer can still be able to be allowed to be adjusted with it's sensitivity too, unless the interface circuit needs to be sensitive enough to transmit the signal from it to the sesmic circuit.

    I know for a fact the accelerometer circuit can provide these three movements. I also know for a fact the accelerometer circuit has been built using the sesmic kit, although I feel reasonably sure three kits were actually used for each of the X,Y, and Z movements. Although the person that built this told me they really did used the two circuits from BG Micro and SparkFun, the person didn't want to reveal how they were able to connect the two and make it work reliably.

    The primary items are these:

    Here's the sesmic schematic

    My question is there a kit or an existing circuit that I can use to connect the accelerometer and the sesmic circuit together, and where I can find it?

    Many thanks! I hope that I was able to provide enough information.
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
    Yes I agree that it is possible to mate one output from the accelerometer to the seismic circuit.

    To achieve this you need a scaling circuit via an op amp with suitable feedback resistors.

    Do you realise that you need either three seismic circuits or some form of switching to see the output of each axis as the seismic circuit is one dimensional?

    Also do you realise that the seismic kit is unecessary? You could simply buy 3 LM3914/5 bargraph drivers ics and the leds (I suggest different colours for each axis) etc.
  3. jbarrington

    Thread Starter New Member

    Oct 7, 2010
    Thanks for the reply studiot!

    Also, thanks for the Nat. Semi. link. I had seen this and was just going to build my own with ICs and parts. I wanted to provide the other links that showed the geophone circuit kit to hopefully show what I wanted to do with connecting the accelerator and the sesmic circuit, but without the geophone hardware device.

    I had felt that I would need to use three of the sesmic circuits to read the the signal from the accelerator. For my limited abilities, I thought this might be the easiest thing for me to do.

    You mentioned that I would "need a scaling circuit via an op amp with suitable feedback resistors." Would you happen know of a suitable circuit or kit that could do this for me? Perhaps a Kit link, or schematic design somewhere? I realize that I might need to build three of these as well.

    Oh, I should add that since the accelerator and sesmic circuit runs on battery power, I was hoping the interface circuit could run on some level of battery power as well. I guess that it goes without saying that I was wanting to place everything into a project box for portability.
    Last edited: Oct 7, 2010
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
    I haven't checked the output range of your accelerometer, but the scaling circuit is simply a voltage follower with gain. The gain (g) is such that if the output of the accel is zero to v and the input for full drive to the LM3915 is V then

    V = g times v

    The follower can conveniently be made from an op amp such that g = R feedback/Rin.
    Of course you can use 3 of the amps in a quad op amp ic for your 3 scalers.
  5. jbarrington

    Thread Starter New Member

    Oct 7, 2010
    Thanks for the follow up response.

    Much of your last message goes over my head, but if I had schematics I can normally build it if it something isn't too complicated. It's been about 35 years since I messed with electronics, back in high school, and 98% of it I have forgotten except reading schematics and soldering.

    Would you know where or if there is a electronic kit? If you are unsure, can you give me some circuit names so that I might reap the best results in a google search?

    I've seen you use:
    scaling circuit
    voltage follower with gain
    op amp

    Thanks again.
  6. gootee

    Senior Member

    Apr 24, 2007
    Go to and download the application notes AN-20 and AN-31 .

    For example, if you wanted to sum the three accelerometer axes' outputs and drive only one bargraph, you could do that, with a three-input opamp summing amplifier. (I am not sure why you would want to do that. But you could.)

    To convert the output of one circuit to have the range of values needed for another circuit, such as the ones you are dealing with, there are at least three things that you might need to do: 1) multiply by a constant (to adjust the size of the total range of voltages) with a simple opamp amplifier, 2) add or subtract a constant voltage (to align with expected voltage range) with a simple opamp summing circuit, and possibly 3) take the absolute value so that negatives become positives. But if your bar graph circuit shows both positive and negative, with zero in the middle, which seems desirable in your case, then you wouldn't want #3.

    You might want to download the free LTspice circuit simulator, from .