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single power supply + communictaion

Discussion in 'Power Electronics' started by ami85t, Apr 13, 2014.

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  1. ami85t


    Feb 19, 2014
    Hi everyone,
    I've build a metal detector circuit using a 555 timer as explained in this link:
    instead of using a speaker as the informer of detecting I am checking the frequency at the point were the speaker is shown in the diagram. The frequency is measured by a capture module of a uC (pic16f877) which captures the value of a timer (which is zeroed before capturing) every rising edge of the oscillator created by the 555. The data at the uC is been compared to the last value been measured and then summed up. After a few cycles the data of the summed information is been transmitted to a BT module (via UART protocol) and from there to a PC.

    For checking that the uC works well I have added in the programming a led flickering section in the main cycle of the system.
    When the led isn't flickering that's an indication the the system isn't functioning properly. for checking that the transmission works properly I have programmed the uC so to send before the main cycle a 5 4 3 2 1 packet.
    when the 555 circuit is disconnected and the capture module of the uC is connected to the external crystal oscillator (as a demo check) the data received at the PC's terminal functions properly ( 5 - 1 and then a fixed sequence).
    When I try to connect the 555's circuit to the uC circuit and using the same voltage supply for the different components (BT module, uC, 555 timer) the data received at the PC's terminal is very strange:
    Or that its a sequence of 5-1 5-1 .... or that it's a sequence of FF to FB ( -5 to -1 in HEX).
    My guess is that the problem for the disruption in the communication is due to the fact of not using multiple supplies or using a LDO. I have read about the issue of using one supply for multiple components and as far as I understand the simple effect of doing that will be that one of the components won't get the current it needs and therefor the system won't function, but why using the the same supply for the uC and the 555 causes for changes in the data?
    The power supply for the circuit is a DC power supply of 5v (from the socket) and I have in my disposable a few 9V
    batteries. Should I try using for every component a different battery with an LDO? (I am aware of the face of it being a great power-loss supply) or maybe to look in a different direction?

    Thanks, Amitai

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    Last edited by a moderator: Apr 13, 2014
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    It's difficult to say what the problem is. A proper, complete schematic and a code listing might help.

    The 555 circuit will cause noise on the power supply rail. It would be a good idea to try powering it from a separate supply of the same voltage that the microcontroller is using. Connect their 0V rails together, but not their positive rails. See whether that makes any difference to the data corruption.
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Following Kris' recommendation will give you a feeling whether the power supply is the cause for the problem.

    In addition to Kris' remarks: Do you have proper bypass capacitors on each IC in place? Typically 100nF with very short leads from Vcc to GND at each IC are good practice.
    KrisBlueNZ likes this.
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