Measuring problem

Discussion in 'General Electronics Chat' started by rsfoto, Aug 9, 2013.

  1. rsfoto

    Thread Starter Active Member

    May 14, 2013

    I have a general question in regard to some problems I have when measuring the input Voltage on my circuit depicted here

    When I connect my Multimeter to " CON EAST-WEST " and want to record the mV I am working with both control LEDs are lit (connected to EW-A1 and EW-B1). When I disconnect the measuring cable all is well. The measuring cable is about 3.2m long.

    Any tips what do I have to do in order to be able to measure trouble free the inputs of the comparator.

    Thanks for any tips
  2. #12


    Nov 30, 2010
    Your schematic is a PIB to look at. It will not allow moving the side bars so we can see the rest of it. Please post it in a different format, like .PNG or .PDF
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    The problem is that, with no leads measuring the voltage across con east west, connection point 2 on that connector is floating (it has no reference to the rest of the circuit). You are completing the circuit by measuring across the connection point. In general, hpthis is not a good idea. What voltage do you suspect the non-inverting input on svg1a (and inverting input on Svg1b) will be since it has no connection to the rest of your circuit? That is, it is not connected when nothing is connected to "Con east west"
  4. rsfoto

    Thread Starter Active Member

    May 14, 2013
    Sorry gentleman. Of course you are right. Apologies.

    It is just half of the LM339 which I am using (a screenshot of the whole circuit)

    On the Connector " CON-EAST-WEST " there are 2 inverse polarity connected LEDs Anode LED1 to Cathode LED2 and ANODE LED2 to Cathode LED1.

    The voltages going into the EASt-WEST and NORTH SOUTH vary from the LED sensors from -2V to +2V going through 0V.

    Look at this circuit

    On the Output 1 and Output 2 labeled with EW-B1 and EW-A1 there is connected a 1200Ω resistor and a LED to the +12V rail as control LED.

    I hope now it is a bit clearer. If not tell me and I will draw the whole circuit with all its components connected.

    Attached another schematic of the same circuit, now complete with the missing sensors as well as the control leds. but I took away the wires and labeled to where the end of each component is connected too. In that way it was easier for me to make the first soldered prototype.

    The outputs of the TLP521-2 just go to a common and close to ground when the comparator detects a volatge difference delivered by the sensor LEDs. The connectors are 3.3v and have a current of ~ 20mA.

    Gentleman, Apologies again, of course it was stupid from my side not to include the missing Sensors and the control leds.
  5. rsfoto

    Thread Starter Active Member

    May 14, 2013
    Here is an image of the circuit working.

    As the mount is standing full in the Sun I want to measure the voltages at the EAST-WEST and NORTH-SOUTH sensor inputs, from about 3-4m away so I do not get fried by the Sun whole day :D
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
    You could put op-amp voltage followers between the sensor's and comparator inputs to buffer the voltage.
    rsfoto likes this.
  7. rsfoto

    Thread Starter Active Member

    May 14, 2013
    Thanks, have read about the voltage follower but as this measurements are only temporary I think that is a high end solution.

    Anyhow I have a LM358 here and can try it as soon as I can, as my vacations in the Observatory are over. I am still trying to understand what does the op-amp voltage follower do in this case. I have read that I loose some millivolts and I am measuring in millivolts. I was able to measure once the voltage range in which the LM339 comparator was working and it is moving in the area of 6mV to 24mV. Somehow the lonmg cable introduces a signal which makes the input to get an additional signal in volts ¿?

    What about a shielded cable or something like that ? or just get a RS232 serial cable extension and keep the measuring cables as short as possible so I can plot the voltages ?