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enter image description hereHello friends

I'm working on a light detection system for the higher secondary summer term project, for which I'm using a simple Arduino board, Adafruit 16-Bit ADC, an LDR with some resisters, a 5V USB power source either the computer USB 2.0 port or a mobile power bank 5V-500mA/1A/2.1A, a USB mobile charging cable (not the data cable) and a Bluetooth module to communicate with Arduino system, an android or a computer (different from Arduino power source) to record the data.

The difference in maximum and minimum response is very poor when replacing the power source from the computer USB to power bank USB. With the same environmental and experimental conditions, the response goes from 50000 to 45000 counts (the changes in response are 5000 counts) with the power bank. For the same situations, the response goes from 50000 to 5000 counts (the changes are 45000) with the computer power source which is more suitable but it's better to use a small power supply.

How can I get the better and stable response? Kindly help me to solve out this issue. Thank you.

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  • Could you please provide a link to the ADC product page or datasheet?
    – Edgar Bonet
    Commented Jul 4, 2019 at 11:00
  • You could use the on-board ADC on the Arduino. See the Analog Read Serial and Read Analog Voltage built in examples.
    – sa_leinad
    Commented Jul 4, 2019 at 11:34
  • Thanks for your valuable comments. ADC must be at least 16 bits for my project while on arduino board it has 10 bit only.That's why I'm using ADS115 here. For datasheet: www.ti.com/lit/gpn/ADS1115.
    – AsTJ
    Commented Jul 5, 2019 at 7:19

1 Answer 1

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Based on limited knowledge of the design and limited knowledge of the requirements here are 3 possible reasons. More may exist.

  1. The USB 2.0 definition stats such a port provide 0.5A of current. Some devices limit the current as a precaution. Therefor the battery may provide more current then the USB source. The type of USB source was not specified and no voltage measurement data was provided.

  2. How fast does the device need to react to a change in light level? A light-dependent resistor or LDR can have a latency of as much as a second with respect to light level changes.

  3. With regards to the 16 bit ADC. What precautions have been made with regards to precisely controlling the ADC's voltage reference? Any fluctuation can cause large errors. What precautions have been made with regards to filtering out the ADC's random noise? For example, many ADCs only have a fraction of their resolution if averaging is not used.

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  • Source info.: 1) USB 2.0 5V 0.5A 2) MI and Intex original power bank has 3 port for 5V 0.5A/1A/2A supply. All 3 ports of each power bank are providing the same response. All data is being received only through the Bluetooth communication with all other things remain same. In my case the output should be at least first, so 5000 is minimum with a computer USB2.0.
    – AsTJ
    Commented Jul 5, 2019 at 7:00
  • In my case the output should be at least first, so the computer is 5000 minimum with USB2.0. But for similar situations, when the source is replaced with the above power bank, the output response for all the same conditions for the minimum response varies from 5000 to 45000. Again, with the computer's USB 2.0 / 3.0 port, it goes from 45,000 to 5000 for similar conditions.
    – AsTJ
    Commented Jul 5, 2019 at 7:00
  • Get a volt meter and measure the actual voltage across the USB power pins. Do this under the most current demanding situations. Likely when communicating with the Bluetooth device. If you can not do this, start simplifying the project. Start by removing the Bluetooth device and communicating with the Arduino over the serial port. Keep simplifying until you stop getting unexpected results.
    – st2000
    Commented Jul 5, 2019 at 11:56
  • Hello dear st2000 Thanks for your precious time and response. It is working perfectly with serial or Bluetooth when connected to the computer USB 2.0 port via cable or charging cable. But with above mentioned power bank or mobile charger, it is not showing the same response with Bluetooth and I can't go for the serial in case of power bank. Computer USB 2.0 port voltage = 4.91 V. Power bank USB port voltage = 5.17 V.
    – AsTJ
    Commented Jul 6, 2019 at 13:01
  • @AsTJ, this is not much of a voltage difference. Also, I looked up your ADC. Is this one the ADC you are using? If so the ADC chip contains an internal voltage reference. So the small voltage difference you measured between your computer and power brick should not make the difference you are seeing. I am at a loss. Perhaps try a different power brick or ordinary battery.
    – st2000
    Commented Jul 6, 2019 at 14:07

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