solid state signal selecting circuit

Discussion in 'The Projects Forum' started by lowrider2962, Feb 11, 2011.

  1. lowrider2962

    Thread Starter New Member

    Feb 11, 2011
    3
    0
    hi everyone, im looking for some thoughts and ideas.

    im currently working on a project circuit that will select between two different input signals and send them to a single output without altering the voltage. the input signals are both constantly fluctuating 0-1000 mV positive signals coming from two different voltage generating sensors. since theyr both sensor signals any alteration to the voltage with affect the accuracy of the computer. so alteration is unacceptable.

    the signal that causes the switch from signal A (the primary normaly closed circuit) to signal B is +12V. when voltage is applied to the switch signal the circuit needs to switch to signal B, and when the voltage is removed it needs to automatically switch back to signal A. the computer needs to be receiving a signal from the output at all times.

    i have worked with using an RS flip-flop for this job but it would require 2 nands gates and FOUR transistors (2 NPNs to use the positive output of the RS to switch the PNPs controling the signals). this design might work but im not sure.

    attached is a diagram of a different circuit ive designed that i believe may work, but the issue is that the output of the inverter circuit used to operate the transistor on input A is positive. since transistor A is of the PNP type a positive signal there does not work. perhaps using an NPN transistor inline with the base of the PNP would solve the problem.

    if you have any thoughts or ideas or corrections to my logic please do post a reply and let me know. ive been trying to find the simplest method to achieve my goal of switching between 2 unaltered signals and havent really gotten too far. i hope members of this forum can help me succeed with this project.

    thanks for reading

    jeff
     
  2. lowrider2962

    Thread Starter New Member

    Feb 11, 2011
    3
    0
    oops i forgot to upload the file i mentioned in my previous post. so here it is.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Does that mean the signals are analog in nature? And that they are always DC - that is, the signal polarity is always positive or negative?

    With regards to the computer - you must mean A to D converter when you speak of accuracy. What is the word size out of the A to D? If it's 8 bits, accuracy is in the 20 millivolt range (1 part in 256). If it is 16 bit, then accuracy is around 150 μV (1 part in 32,768). Quite a difference.

    The A to D's reference is probably going to be 5 volts, so you can actually use a couple of op amps to amplify the signals for better resolution. You might want to look into analog switches for doing the switching from signal A to signal B.

    What more can you tell us about the project?
     
  4. rogs

    Active Member

    Aug 28, 2009
    279
    37
    You could do the whole thing with just a single 4066, and one resistor.
    Use two sections as the voltage routing path, and one of the other sections to create the switching inverter. 4th section (unused) connect all pins to ground.

    Remember to use a 4000 series 4066, and not a 5V 74HC4066 version though!!

    Sketch of what I have in mind attached
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,129
    3,050
    Another approach would be a dual op-amp - voltage following - followed by a switch, DPST I think to select which one to send forward. This would buffer the inputs to a very high impedance and completely protect them from the outputs.
     
  6. lowrider2962

    Thread Starter New Member

    Feb 11, 2011
    3
    0
    hey guys thanks for the responses. the signals are positive analog and remain unaffected in theyr entine path to the computer module. the sensor is located seperately from the computer module and im putting the switch in between the two. the sensor reads oxygen levels in the air around it and provides a live signal to the computer. drawn on a graph with the x axis as time and the y axis as 0-1000mV the signal represents a constantly positive sine wave. since it is a live signal that has a great affect on the actions of the computer module neither voltage or time can be affected. time is alittle less crucial than voltage as the computer can accept a small delay but it must be VERY little.

    Rogs i like the looks of the circuit you drew. im gonna study it some more and study the 4066 and see if u might have presented a solution. thanks for your help.

    Wayneh im not sure i understand the circuit u explained. could u please elaborate alittle? thanks.
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You definitely get the prize for most alliterative topic tile with this.

    How much resolution do you need? You are apparently going to be digitizing the signals and there is a lot of difference in the voltage levels analog to digital converters can resolve.

    What is your definition of "small delay?" I have seen performance monitors built around 6502 processors as well as XT computers, both with not-terribly-fast A to D converters. What is significant about this delay?
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,129
    3,050
    I misunderstood your need for 12v triggering and recommended a manual switch. But, the idea of using an op-amp to buffer the inputs might still be useful if the impedance of the other solution is not high enough to maintain signal accuracy.
     
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