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In my application, I should control 4 display composed of 7 segments. I want to use a classic method with 4 BJT and segments all driven by a Micro. An update rate will show up each value on the powered display.

Particularly is that I'd like to have the possibility to interchange between Common anode screen and Common Cathode.

Is there an optimized or right way to do that?

Test I've done: - Two BJT, single gate, single resistor; - Single BJT with inverting lines using jumpers;

I'll post some images in some hours, but despite my schematics. How do you advise to proceed?

EDIT: I've performed this two kind of test: enter image description here enter image description here

Here's my BJT tests, power supply is a single buck regulator that regulates form 24v to 5v. There's a resistor on each port that goes from my Micro (PIC16F15354) on a single segment bus.

In the Test 1, all were fine, I've tested both CA and CC displays without problems, One were brighter than other because a voltage drop on BJT when used with LEDs between emitter and ground. I measure a voltage of 5v on my gate.

In the Test 2, no leds lights on, I've tried to measure voltage and I see 0.8v on each BJT gate.

What is wrong?

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    \$\begingroup\$ Please add schematics. I'm not aware of any reason you would want to have that functionality, so I don't think there is any canonical way of doing it. \$\endgroup\$ – Hearth Jun 16 at 21:21
  • \$\begingroup\$ When you say you want to interchange between common anode/cathode, what do you mean? You'd like the circuit to support plug and play of either one? \$\endgroup\$ – Simeon R Jun 16 at 21:22
  • \$\begingroup\$ I'll add schematics in some hours. (unfortunately actually I'm busy) Yes, short of a plug and play. \$\endgroup\$ – Singed Jun 16 at 22:07
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Typically your micro will have push-pull outputs, so the segment drivers are taken care of (hardware-wise) for free (though some microcontrollers may be better at sinking current than sourcing).

The commons have to handle 7 or 8 times as much current but with a 25% duty cycle.

If you want the PCB to be adaptable you can put pads for both PNP (common anode) or NPN ( common cathode) and just populate the appropriate one.

If you need it to work changing only the display you could stack npn and pnp voltage followers and leave out the base resistor, however the extra voltage drop compared to a saturated transistor means that it might be more appropriate with a 5V supply than a 3.3V.

In any case, the firmware has to change.

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