schematic problem

Discussion in 'Embedded Systems and Microcontrollers' started by rayda, May 2, 2011.

  1. rayda

    Thread Starter New Member

    Apr 23, 2011
    i m trying to draw a circuit that supposed to display the current temperature in the 7-segment, and the light of the 7-segment will dimmer or brighter according to the detected light intensity by the LDR. The switch is for the use to change the temperature unit from celsius to fahreheit or vice versa.
    However, my circuit seems to have something wrong that it does not run as i expect.
    Can anyone tell me what is wrong with my circuit?
    i m using Proteus to draw it by the way.

    Thank you very much.
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  2. guitarguy12387

    Active Member

    Apr 10, 2008
    Can you explain the behavior you are seeing and why you think it is wrong?

    A few things to check:

    -Often SSDs are active LOW
    -You may needed current limiting resistors on the LEDs.... check datasheet
    -Observe SSD inputs on a scope/DMM and see what you get.
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    Yes definitely add a series limit resistor into each ABCDEFG and DP line. You need that to set the current into each LED at about 4 times the max brightness, as you are multiplexing 4 displays so each is on a max of 1/4th the time.

    LED brightness for a PWM control as a percent of current is linear, meaning if you want 50% brightness you keep it on 50% of the time. You have a bit more work to do as you are also multiplexing so you need to somehow synch the Q1 Q2 Q3 Q4 segment signals with the Q5 PWM signal.

    Something bad happened to your schematic, the left side of U1 has multiple pins tied together and I do not believe you intended that.

    Your circuit has a good amount of complexity so I would not try to get it all to run at once. One way would be to get the display to show just a number, then the PWM for brightness, then the LDR control input, and finally the temperature input.