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I'm designing a driver to interface four 7-segment digits (8V, 20 mA per segment) with an AS1115 driver (5V supply, constant current outputs). Following an internet source, I have the circuit shown below. I'd understand how to pick the resistors if the chip were a voltage source. However, I'm an electronics novice and not terribly comfortable with current sources. How do I pick the resistor values?

Schematic

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    \$\begingroup\$ Current source = no resistors needed but also no choice but to match the LED current to the driver. If you drive two at the time, you will best case have half the brightness on both. Also, CC drivers may or may not like having the LEDs switched in and out. Check the datasheet for the driver. AS1115 seems to be made for the very purpose so probably no issues. \$\endgroup\$
    – winny
    Aug 14 at 19:44
  • \$\begingroup\$ I can't drive the LEDs directly because the IC is a 5V part and the LEDs want 8V. \$\endgroup\$ Aug 14 at 19:48
  • \$\begingroup\$ Oh, that will be more tricky. You will be sending 20 mA of base current. Hmm. Do you have an 8+ V rail available? \$\endgroup\$
    – winny
    Aug 14 at 19:50
  • \$\begingroup\$ I'm using a 5V-9V DC-DC converter. \$\endgroup\$ Aug 14 at 19:51
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    \$\begingroup\$ This might be an easier way: embedded-lab.com/blog/… \$\endgroup\$
    – Mattman944
    Aug 14 at 21:59

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