3 digit 7 segment help? - Page 1
Forum section: electronics / beginners

author: algorithm
author_rank: Regular Contributor
timestamp: March 10, 2021, 03:20:25 am
content: So im perplexed by how the 7 segment display works with digits. I want to power the 7 segment with batteries, manually and statically wiring what segments. But grounding each respective digit results in the same segment on all 3 lighting. Also some pins for segments are used by 2 digits. I know normally the display is visually tricking the user by rapidly cycling. Does anybody know of the best way i would go about wiring each segment seperately off battery without 3 separate battery compartments lol? Maybe a virtual ground? Any ideas are appreciated.

author: retiredfeline
author_rank: Frequent Contributor
timestamp: March 10, 2021, 03:32:29 am
content: Those displays are intended to be multiplexed to save pins. If you can't manage that you'll have to get a non-multiplexed display.Assuming you are trying to display 3 static digits, if you think about it in information theoretic terms, you are trying to send 21 bits of information into 10 pins. The channel has to be enlarged and this is done by time multiplexing.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 10, 2021, 05:04:33 pm
content: This should make sense of it.https://fsimdeck.wordpress.com/electronics3/led-7-segment-multiplexing/Why: massive power saving, simpler wiring/ PCB, easy dimming, lower cost.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 10, 2021, 06:07:38 pm
content: For modern LED the power savings are a myth. Most LEDs are relatively linear in output power versus ouput power. It deviates unsually to lower output at low (for modern LED often < 1 µA) and high (e.g more than rated current) current. The main point is easier wiring and less pins at the chip/µC - so cost saving.It may have been an issue with the very old LEDs in the 1970s and the LEDs are not regularly tested at very low current - so the intensity may scatter more at a low current than with multiplexed segments.

author: emece67
author_rank: Frequent Contributor
timestamp: March 10, 2021, 07:06:21 pm
content: .

author: Terry Bites
author_rank: Super Contributor
timestamp: March 10, 2021, 09:02:41 pm
content: Explain myth- have LEDs got less efficient?

author: tooki
author_rank: Super Contributor
timestamp: March 11, 2021, 11:45:25 am
content: Quote from: Terry Bites on March 10, 2021, 09:02:41 pmLEDs have become vastly more efficient. LEDs’ efficiency is lower at higher currents (i.e. doubling the current doesn’t quite double the light output), at least above a certain point. (That’s why lighting LEDs get so hot: they’re usually being run well beyond their range of highest efficiency.) So in theory, having an LED display where each LED (when on) was driven at a constant low current would be more efficient than multiplexing it at a higher current for less time. But I suspect the current needed by all the extra control electronics would more than negate any savings in the LEDs.In terms of raw efficiency, modern (emerald) green and white LEDs are insane. With traditional green LEDs used as indicators (or display segments) you really needed the 20mA per LED. With a modern green one, 1mA is more than enough. I often run them at less than that!!

author: MikeK
author_rank: Super Contributor
timestamp: March 12, 2021, 12:49:12 am
content: I had to test the "myth":I have a clock module with a 4-digit 7-segment display. I tested two methods: 1) Turn on all segments in each digit and multiplex the digits and 2) Turn on each segment and multiplex the segments and digits (turn on seg a, digit 1...seg g, digit 1...segment a, digit 4...segment g, digit 4). I had it switch between the two methods and could not see any difference in brightness, and no flickering.Using my Korad KA3005P power supply set to 5V it showed 23mA with method 1 and 21mA with method 2. Not a huge difference, but not nothing either. And messing with the duty cycle and refresh rate could bring both of those numbers down.

author: rdl
author_rank: Super Contributor
timestamp: March 12, 2021, 03:10:40 am
content: LEDs have gotten vastly more efficient. Thirty years ago it took 10-20 mA for reasonable visibility. There are some LEDs today that will leave serious afterimages at less than 1 mA. I have some old Panasonic 7-segment displays that barely light at 10 mA, while some newer ones I have from China made good reading lights at under 6 mA . Using 4 of them in a clock I had to crank the current way down at night.Here is photo of an older green LED next to a modern InGaN green LED. The newer LED is running at only a tiny fraction as much current as the older LED (320uA vs. 15 mA). In this photo the InGaN LED is actually badly over-exposed (blown out). If the exposure had been correct the older LED (actually all of the other LEDs) would have barely been visible.

author: MikeK
author_rank: Super Contributor
timestamp: March 12, 2021, 01:24:28 pm
content: Nobody's doubting that modern LEDs are more efficient.

author: brucehoult
author_rank: Super Contributor
timestamp: March 13, 2021, 12:56:48 am
content: Quote from: algorithm on March 10, 2021, 03:20:25 amYou've got no choice but to somehow send Vcc and GND each to three different wires in sequence, with each GND connected to one digit and each Vcc connected to the segments you want to light for that digit -- with diodes from the Vccs to the segments to prevent cross-leakage.But it's just as easy to use 10 outputs and do it properly. With a small chip such as a $1 8 pin ATTiny85 with only 5-6 outputs (5 if you don't want to complicate reprogramming it) you could use three outputs for the digits and two outputs to drive a '595 shift register.