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I need help locating something to measure current on a circuit and relay that via bluetooth.

Discussion in 'Radio and Wireless' started by John_Thrust, Sep 30, 2015.

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  1. John_Thrust


    Sep 30, 2015
    Hi everybody. I'm not sure I'm in the right forum, but this seems a place filled with people who can help me.

    I'm developing a medical device for a competition. My design requires something very small in a circuit that can measure change in current over time, relay this information (in real time) to a bluetooth transmitter, and have that bluetooth relay that information to a phone. In theory this should be relatively simple- I find a small ammeter, put it on the circuit, and connect that to a bluetooth device.
    I need to stress how small this device should be. We're talking the size of a fitbit or a watch, all things included.
    Reality, as often in life, has proven much more difficult. For the life of me I can not locate a small ammeter that I can just put on a circuit- everything I'm finding is big, bulky, and often includes a gauge which I don't really need. Given that it's medical, we are probably dealing in rather small voltages (mA), so the ammeter's specificity and range is a relevant and real concern in picking an ammeter.

    I also need help locating a small bluetooth device, an explanation of how it works, and how I would connect it to anything I end up using.

    Conversely, is it possible to skip the ammeter altogether and just have the bluetooth part relay how much current it's receiving directly to the phone?

    If I have, in any aspect, been too general, please ask me to clarify. I'd rather clarify than have no feedback at all. The same applies to criticism.

    Thanks so much for your time.
  2. Old Steve

    Old Steve

    Jul 23, 2015
    Doing it in the size that you want is a big ask. Are you capable of working at that scale, to build it?

    For the current monitoring, a microcontroller and an operational amplifier would be needed. The basic current measurement side is easy enough - you put an accurate resistor in series with the load, between the load and ground, then measure the voltage developed across the resistor. I = V / R, where I is current, V is voltage and R is resistance.

    Due to the range of voltages you wish to measure, (mA), an op amp would be necessary to scale the voltage up to a level where good resolution could be obtained, (0 to 3.3V or 0 to 5V), then the microcontroller could measure the scaled voltage using an onboard A-D converter, convert it to a current value and store the sample until it's transmitted.

    When you say, "change in current over time", depending on the intervals between samples and transmitting the data, you might possibly need additional memory to store the results.

    An off-the shelf Bluetooth module could transmit the results. Add a suitable battery, along with the other necessary support components, (resistors, capacitors etc), and you'd be pushing your luck to fit it into the size of a watch.

    It's not as easy as "should be relatively simple- I find a small ammeter, put it on the circuit, and connect that to a bluetooth device."
    The 'ammeter' would need some means of communicating with the Bluetooth device - serial communications.

    Since this is a one-off for a competition, and you're not going into production, your best bet would be to find an existing module that fits your requirements, if one exists.

    Someone else might be able to suggest an alternative, but that's my 2c worth.
  3. davenn

    davenn Moderator

    Sep 5, 2009

    agree with Steve, without access to a micro electronics fabrication lab, you are pretty much asking the impossible
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    The size is going to be a problem, as stated by the others before me.
    However, since it is for a competition, prove of concept may be more important than actual realization.

    For the transmitter part you can use a bluetooth module with analog input plus some signal conditioning (mainly sensor, amplifier, filter) to convert the current into a suitable input signal to the BT module. Depending on your skill, you can start with a BT transmitter like e.g. this one plus a sense resistor and an operational amplifier to match the sense resistor's voltage drop to the transmiter's input range.

    If you're more into building the full circuit and size does matter, consider e.g. the RN4020 BT module from microchip. This module has a built-in 10 bit Analog-Digital converter. Microchip also offers software for I_whatever devices to acccess the module from your smartphone or tablet.

    Unfortunately I will not be able to support you in dealing with this module, only found it by a quick Google search.
  5. Old Steve

    Old Steve

    Jul 23, 2015
    I thought he wanted to measure current change over time, store the results, then transmit periodically, but re-reading the original post, I think he wants to stream it as you suggest Harald. That's a little easier - one less part. It'll flood the phone with data though.
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Old Steve likes this.
  7. Old Steve

    Old Steve

    Jul 23, 2015
    I'm amazed that they don't cook, at such a tiny size, (2.62mm x 2.14mm), and running at 26MHz. I suppose they're intended to be attached to something that would serve as a heatsink.
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