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Testing Digital Display

Discussion in 'General Electronics Discussion' started by AutoNub, Oct 18, 2012.

  1. AutoNub


    Oct 5, 2011
    I have an old legacy digital display and I'd like to wire it such that all of the digits light up when I press a momentary push button. This is for testing the LED display to verify all are in working condition. The 3 1/2 digits should illuminate "-" as verification. I think some digital displays have these input functions built in already, but my AD2021 doesn't appear to have it.

    Can someone with more experience than myself provide some instructions on how to illuminate all the digits to accomplish this test? I have some ideas of my own based on the Boolean logic gate symbol diagram in figure 2, but I don't want to go trying random things for fear of breaking the display. Perhaps connecting terminals L, 7, 12, and P in parallel to the signal common of the power supply would illuminate all digits on the display?

    Here's the datasheet:

    AD2021 Datasheet
  2. BobK


    Jan 5, 2010
    As far as I can tell, this module is a voltmeter with an analog input. There is no way to control the segments except by putting an analog voltage on the input. You could make all the segments except the decimal points light up by putting -1.888 Volts on the input.

  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    The datasheet is of no help. If you're referring to figure 2: that's a serial to parallel converter for the serial data output of the AD2021 and is of no use for testing the internal display.

    If you're willing to dig into the device there may be a digital method (besides Bob's idea). Typically the internal display will be multiplexed (see this detailed explanation). FInd out whether it is a common anode or common cathode circuit. Add series resistors from each segment to a common "test" wire. These resistors should have approx. twice the resistance of the original resistors in the AD2021. If your display is common cathode type, connect the "test" wire via a pushbutton to Vcc (e.g. 5V). If the type is common anode, tie the pushbutton to GND. Since the digits are usually scanned continuously, all segments should light up when the pushbutton is pressed.
    Disadvantage: "off" segments may lightup faintly because they "see" some current due to the test resistors. If that matters, add additional diodes in series with each test resistor (and before the pushbutton). Chose the polarity such that for a common cathode type display the anodes of the series diodes show towards the pushputton. For a common anode type display the diodes' cathodes should point towards the pushbutton. You may have to lower the resistance of the test resitors when using a series diode to compensate for the voltage drop across the diode.

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