LED display multiplexing

Thread Starter

ci139

Joined Jul 11, 2016
1,696
Simple question :

The d/s of the LED-s often specify pulsed drive duty ratio of 1/10 -- ? Does it mean i can multiplex 10 digit LED matrix with no readout impact ?
 

Irving

Joined Jan 30, 2016
2,058
Can you post the datasheet in question. You can multiplex any LED but of course their brightness will depend on the on-off ratio. It is therefore quite common to drive muliplexed LEDs harder to compensate, as long as they don't overheat.
 

Papabravo

Joined Feb 24, 2006
16,772
it's almost a rule d/s independent ... a random pick https://www.cree.com/led-components/media/documents/C503B-BCS-BCN-GCS-GCN-1094.pdf Pg.2 (Top of the page Note1)

the question is simply : ? would the 1/10 duty 10 digit display flicker too heavy to be readable
(i can test it out quick - just wanted to know if there is a point to try such test ...)
When operating at the extremes you have some flexibility in adjusting duty cycle and forward current limit. For example if the normal continuous forward current for an LED is 20 mA, you may be able to drive 100 mA through it with a 20% duty cycle, for a similar brightness effect. Much like the definition of horsepower (550 ft-lbs./sec), not all combinations of forward current and duty cycle are possible. Some experimentation will be required since brightness tends to be highly subjective.
 

Thread Starter

ci139

Joined Jul 11, 2016
1,696
Thanks to everyone for their input !

// i need to build a simple frequency counter for the couple of tests/projects -- got some historic soviet 7seg things (common cathode) (img) ...
// i don't want to bend their pins much before the final mount ...
 

Reloadron

Joined Jan 15, 2015
6,058
During the 80s Radio Shack published a notebook written by Forrest M Mims III, actually several notebooks. One of the popular circuits was a 6 digit frequency counter using 7 segment LED displays. You may want to Google around but I did find this circuit The popular chip at the time was the MC14553 3 digit BCD counter. The process was referred to as strobing the 7 segment displays which amounted to multiplexing. Nice part is two counters are easily cascaded making a six digit counter and all the workis done on the chip. Motorola was MC14553 and the common CMOS name was 4553. A 4511 was used as the display driver. A circuit for the Latch, Reset and Count functions was easily made using a common CD4011 with a CD4013 for the time base. I forget the F2 (upper frequency limit) of the counter was. I have no idea if the chip is still in production but would think similar are out there.

Ron
 

Thread Starter

ci139

Joined Jul 11, 2016
1,696
well - thanks for pointing out - but my intent is to use a minimal display driver circuitry ...

-- i just realized after posting the thread that i can't MUX by digits as the max current for the one 7-seg digit is likely 60mA·120% (maybe ×200% 140% ?)
-- i got the circuits to drive each digit from stationary BCD 2 7SEG decoder ... but that is exactly what i try to avoid here (use too much circuits for nothing)
АЛС320Б,В
I.FW 12(10) mA , ≤60°C(≤70°C)
I.FW.P 60mA , t.ON ≤ 1ms , D ≤ \(\frac1{12}\)
I.FW.TOT 84mA (all segments Σ-med)
U.FW 3V , U.REV 5V
P.FW 10·(10mA·3V) ? 300mW ? (T<35°C) // ! not confirmed !
P.FW.DYN 300mW/50·(T-35°C) [mW] // ! not confirmed !


-- i guess i have to walk through segments or MUX by segments ... if such doesn't work out not too complex i have options left
  • frequency to voltage converter (displayed by DMM/DVM - i only need 2 or 3 digit accuracy at this time)
  • buffered simple binary output of the counter stages
  • VFD or LCD - both of the least takes more specialized circuitry to be introduced ... which i try to avoid at this point

will do math.-cal modelling/-analyses of the MUX-ing schemes . . . then decide
no deadlines - i just have to confirm couple of thing before starting to compile some other stuff
 
Last edited:

Reloadron

Joined Jan 15, 2015
6,058
When you MUX a 7 segment display you turn ON or Off the display common anode or cathode. You do not MUX the segments. You normally use a transistor on the common cathode or anode to turn the display on or off. That is clearly shown in the link I provided. An NPN 2N2222 will handle 800 mA. This link has some examples.

Ron
 
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