industrial digital thermometer

In the above circuit, what is the reason of connecting transistor to 7-segment display?

What's the difference between the two display with transistors and other two with only resistors?

Also please explain why LED is used after regulator, whether it is simply indication(5V) to the microcontroller, or it is used to reduce the power dissipation

  • \$\begingroup\$ related: The [discontinued by now] SAA1064 7-segment LED character driver used a similar scheme with external transistors. \$\endgroup\$ – Nick Alexeev Feb 25 at 17:18

The one display that is connected through the transistor has two segments that are controlled by a single pin - since it's the 'b' and 'c' segments I'd assume that it's generating the '1' for a 3 1/2 digit display i.e. "1999", so the current needed to drive the two segments is more than a single Arduino pin can provide.

  • \$\begingroup\$ also please explain why Led is used after regulator, whether it is simply indication(5v) to microcontroller or to reduce power dissipation from regulator \$\endgroup\$ – Priya Feb 26 at 15:04
  • \$\begingroup\$ LED 1 is marker "POWER", it just indicates that the system is live. Not sure what value that adds if the display is always on, is there anything in the code to blank the display? \$\endgroup\$ – Phil G Feb 26 at 17:58

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