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I want to control a 4-digit multiplexed 7-Segment + Decimal Point Display (SH5461AS).

This diagram (which allegedly comes from the datasheet of this display) suggests to use 4 external transistors to connect ground and therefore 'activate' one of the digits to control it. multiplexed 4 digit 7-segment display with 4 transistors

However, all the tutorials I have found

don't use transistors but use GPIO's as GND or VCC thereby controlling the display without transistors.

My question is: is this method without transistors safe? (of course with resistors) An electrical engineer friend of mine suggested that by using an GPIO as GND the current might flow through the microprocessor itself therefore potentially creating dangerous current in the MC.

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  • It all depends (on the detail you have omitted). Provided the displays run from 3.3V and draw less than 16mA per segment AND 16 mA per digit it is OK. Again this depends on the code driving it.
    – Milliways
    Dec 29, 2020 at 10:45
  • @Milliways made it clear to me that using it directly would already be close to the maximum allowed currents for the raspberry board. As I plan to also use other components I will go with the safer transistor method to avoid any harm.
    – julius100
    Dec 29, 2020 at 10:54
  • Note that there are 2 methods of multiplexing 7-segment displays. The most common drives all segments in parallel and switches between digits. The other drives all digits in parallel and switches between segments; in this case the current limiting resistor should be in series with the digit (normally cathode). I prefer this (less common) method as it has a lower peak current from the Pi.
    – Milliways
    Dec 29, 2020 at 11:51

1 Answer 1

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After spending ~ 20 min searching for an actual data sheet on the SH5461AS - and not finding one - I'm of the opinion this is JAPCJ (just another piece of Chinese junk). My best advice is chuck it in the bin & find a part by a reputable manufacturer. There are lots of manufacturers that publish spec sheets (even on parts as old as this one) - here's an example. I would encourage you to adopt this position when you buy electronic parts: No spec sheet = No sale.

That said, and climbing down from my soap box, it seems that at least one person may have gotten away with using a GPIO pin direct-connected to pins 6, 8, 9 & 12. In the absence of a spec sheet from a reliable manufacturer, the low-risk approach is to use the transistors.

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