Using an optocoupler triggered by a multiplexed LED - Page 1
Forum section: electronics / projects

author: mickc
author_rank: Contributor
timestamp: May 28, 2015, 09:39:02 am
content: Hi,I am trying to convert an aircraft display that uses incandescent 7 segment displays (Wamco KW-104S)I have working 7 segment multiplexed driver for it that runs LED 7 segment displays no problems at all.The incandescent segments obviously don''t like being multiplexed, so they are dim.Question is, can I use an optocoupler on each segment to drive the incandescent segements? Obviously i would need a fairly slow one, as if it was fast it would give the same results as being directly connected.The Wamco displays use 4v and draw 15mA per segment.The original circuit uses 3x UCN5832A 32-Bit serial shift registers with one output for each segment.Cheers!

author: Ian.M
author_rank: Super Contributor
timestamp: May 28, 2015, 10:32:21 am
content: Why would you even want to do that? As long as your multiplexing frequency is high enough that the filaments aren't being stressed by continuous thermal cycling, simply increase the display supply voltage until the RMS voltage across each segment is the same as its nominal DC rating. A factor of sqrt(N) where N is the number of digits would be correct.

author: mickc
author_rank: Contributor
timestamp: May 28, 2015, 10:40:16 am
content: I forgot to mention, the incandescent displays need to be dimmable, so that option wont work unfortunately.

author: Ian.M
author_rank: Super Contributor
timestamp: May 28, 2015, 11:03:24 am
content: Why not?How are you dimming it at the moment?If its really such a problem, don't multiplex! Serial in parallel out shift registers are so cheap that using one per digit isn't really a problem.

author: SeanB
author_rank: Super Contributor
timestamp: May 28, 2015, 07:07:21 pm
content: IIRC from doing that exact same conversion the filament displays are dimmed by strobing the outputs of the shift register drivers ( the one I worked on used CD4050 buffers to drive the display segments) to dim the display. The LED displays are easy to add, just use a series resistor per segment of around 470R, and the displays will be bright enough, and will dim with the rest of the display. If too bright you simply add a diode in the common cathode lead, to give a 0.6V reduction in drive, and thus a drop in brightness ( was easier than replacing nearly 500 small 1/8W resistors, as the displays we got were really good at low current, and too bright compared to the regular ones) to get in the same ballpark. If you are still too bright simply add a neutral density filter behind the glass front over the displays, which both dims them and increases contrast a lot.

author: Ian.M
author_rank: Super Contributor
timestamp: May 28, 2015, 11:11:48 pm
content: Mick's problem is the other way round. He is converting TO incandescent segment displays (ex aircraft).

author: SeanB
author_rank: Super Contributor
timestamp: May 29, 2015, 08:02:38 pm
content: If going that way simply use directly from the low side switch, with a variable DC supply to power the lamps, probably variable between 3 to 5V for the dimming. If they are driven by the shift register there will be no multiplexing, and you will find the major limitation of the filament displays is the low brightness, no way to compensate, as there is a very direct inverse correlation between brightness and lifetime.

author: mickc
author_rank: Contributor
timestamp: June 10, 2015, 05:05:27 am
content: Thanks guys. I am fairly limited in options, I need to keep the multiplexing circuit as it is the only way the simulation software will talk to the hardware.I think the only way around this is to have some sort of switching that will detect if a segment is "On" more often that it is "off" and then operate a transistor to activate that incandescent segment.Basically, if the LED segment is visibly on, the incandescent segment needs to be switched on. This will be a bit of a process with 79 individual segments!

author: Ian.M
author_rank: Super Contributor
timestamp: June 10, 2015, 07:43:46 am
content: Quote from: miccI should say so, especially if you need to keep it dimmable. It would be easier to point a camera at the LED display, do some OCR on something like a Pi then drive ten 8 bit shift registers to directly control the incandescent segments with a DAC controlling a regulator for dimming.I still don't see what the problem is with beefing up the multiplexing to provide a pulse waveform that's just over 4V RMS at full brightness, which will light the filaments normally if the multiplexing frequency is high enough.N.B. If multiplexed, each digit needs a Schottky diode in series with each filament to prevent current flow through series chains of filaments that are supposed to be dark.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 10, 2015, 09:01:28 am
content: Why do you need the original incandescents lit? Isn't that a horrendous waste of time and parts for no benefit and reduced reliability?Tim

author: mickc
author_rank: Contributor
timestamp: June 10, 2015, 09:50:04 am
content: Quote from: T3sl4co1l on June 10, 2015, 09:01:28 amI thought so too, but to get the proper simulator certificationm all displays must replicate the original aircraft in shape, color, brightness etc.