Detecting 7-Segment LED's With Phototransistors

You have to worry that the "data" doesn't update during the "exposure" -- regardless of whether your exposure is via a camera or something watching segments (optically or electrically).

E.g., if two successive values are 39.9 and 40.0, do you see these? Or, 30.0, 30.9, 39.0, 49.0, 49.9, 40.9, etc.? The OP hasn't indicated what the display (data) update rate is. Nor whether the display is multiplexed, etc.

[All things that can be *measured* but nothing that can be settled upon a priori]

Your character recognition software can either resolve the +/- LSD ambiguities or vote for the best solution based on any bias in successive measurements. Or just flag a bad reading. As I said in an earlier post, temperature usually doesn't change very fast in most applications.

But to support your one point, the OP has not provided much information as to the goals for this task. I certainly would not begin any design until that was cleared up.

Nobody remember the NS 74c915? (7seg to bcd decoder, (may be available in china )

delo

"Roger Monroe" ha scritto nel messaggio news: snipped-for-privacy@googlegroups.com... Hi All, I'm tasked with logging into a PLC the temperature of a waterbath over time. Well, not exactly. The waterbath has a three character display that shows its temperature. Three 7-segment LED's would show, for example, 45.7 I have a PLC that I have to log what the display shows. I cannot access the back of the display board to pickup any output. So my idea was to use several phototransistors (I've seen tiny ones) each strategically positioned on a segment of the displayed characters. I bought two styles from Midwest Surplus (no markings) and tested them with my desk lamp as the light source and it was cool to watch them turn on and off (first time with these things). But when I tested them at the display of the waterbath they wouldn't turn on. I tried a waterbath with a green display and one with a red display but none of my phototransistors would work. Digikey lists wavelengths as a parameter in their search engine for phototransistors. How would I know which wavelength to buy and am I even on the right track? One guy suggested, instead of photodiodes, a webcam run through some OCR software, then the ascii transmitted to the PLC. Now THAT was thinking outside the box. Not really do-able in my situation. Any suggestions? All help is appreciated, Bart

even 98.8 you need to 'debounce' each segment. (or digit, or reading)

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On a sunny day (11 Nov 2013 08:27:38 GMT) it happened Jasen Betts wrote in :

There is more to it... I was lookin at my little radio controlled clock just now, and it says ||.|| (that is the date) and it very soon, in an hour or so, will be 11:11 (hours), so this unique ||.|| ||:|| seems simple to decode.. (the year |||| we missed that opportunity) BUT How big are the numbers, below a certain size you NEED a camera (no space for photo cells).

And I noticed to make the display more readabe it looks as if they use the left two segment places for 'one' in the month, and the right 2 in the day of month, or was it the other way around? This is LCD of course, but we need an universal solution. I stay with my analog video camara solution (with one PIC) it will even work on an LCD.

Exactly (though if the "native" font doesn't put tails on

9's, you might be able to recognize *those* anomalies -- but it wouldn't help if the tail failed to "persist").

"Reading" being the hard one. The display can tell you when a digit is being displayed. But, there are no guarantees that the "set of digits" are updated synchronously with the data update. Nor where in that data update cycle the data is presented to the "output" (e.g., if the digits are muxed left to right, one would *hope* the data is updated just prior to the presentation of the leftmost digit. But, it may happen just *after* the leftmost digit. And, maybe at a different point the NEXT time it is updated).

In addition to the lack of dead-time between segment updates, some displays actually "leak light" between segments.

Nowadays, I wouldn't *assume* any particular multiplexing scheme (*or* static drive) -- whether the design *looks* like it uses dedicated hardware *or* a (possibly buried) MCU/sequencer. E.g., DPM's intended to drive LCD's or LEDs. Too many "app notes" are now "this is how you are EXPECTED to use this" (and that's how we get away with NOT filling in lots of parameters in the datasheet).

program a micro for it.

Well yes and no. Look up a 7447. And thanks to it, it is usually digits that are scanned rather than segments.

?-)

Since there are at least two digits you know that there's mux'ing going on. It should be easy enough to extract the mux'ing rate, then look for two successive reading matches? ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
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I love to cook with wine.     Sometimes I even put it in the food.

So capture a video with a high frame-rate camera, then slow it down and watch it. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
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I love to cook with wine.     Sometimes I even put it in the food.

Den søndag den 10. november 2013 09.03.16 UTC+1 skrev John Fields:

ays...

In this case choosing a micro over handfuls of gates and discretes isn't limiting to one path it is looking at both paths and choosing the one with the open gate instead of the one where you have to crawl through mud and over a barbed wire fence

-Lasse

You think everything can be done with a micro but, as everyone knows, it's all analog when you get down to the nitty-gritty >:-} ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

doesn't

Actually the scan rate is probably much higher than the update rate.

?-)

I've done "Charlieplexing" (segment by segment scan) with a 3-digit display, but it's got limitations. Usually it's digits, but the order is not fixed.

To get an idea of the scanning, move the display back and forth and you can usually see a scanned LED display "break up" (the scan frequency is usually < 1kHz.

There are static drive DVM chips though, e.g. the venerable 3.5 digit

71x7, long of tooth.

Best regards,

--sp

Den mandag den 11. november 2013 22.55.58 UTC+1 skrev Jim Thompson:

lds

deways...

one with the open gate instead of the one where you have to crawl through m ud

I am well aware and I never said everything can be done with a micro, just that is this case a micro is the simplest fastest cheapest most capable ans wer

had the main objective not been to solve the problems, but a challenge to d o it only with components made before 1980 then it would have been different

-Lasse

Can you show us how you would do it with a micro... two digits only to make it "easy" for you >:-} ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

Den mandag den 11. november 2013 23.40.58 UTC+1 skrev Jim Thompson:

Fields

hat

sideways...

he one with the open gate instead of the one where you have to crawl throug h mud

st that is this case a micro is the simplest fastest cheapest most capable answer

o do

nt

come up with a way to read the segments, LDRs/photodiodes etc. you'll need that regardless of how you want to implement it

connect to IOs, sample at regular intervals, when ever segments are stable (to account for possible muliplexing) convert segments to numbers using a l ookup table or how ever you want to do it, and spit it out through a serial port, a DAC, binary on a set of pins or what ever you need

-Lasse

Sounds like more time to program the micro than to build a hardware version ;-) ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

Den tirsdag den 12. november 2013 00.17.16 UTC+1 skrev Jim Thompson:

n

s:

hn Fields

s that

are

ook sideways...

g the one with the open gate instead of the one where you have to crawl thr ough mud

s,

just that is this case a micro is the simplest fastest cheapest most capab le answer

e to do

erent

ed

le

a lookup table or how ever you want to do it, and spit it out through a ser ial port, a DAC, binary on a set of pins or what ever you need

if it was 1980 and you had to erase an eprom and ship to someone with a pr ogrammer then maybe ;)

-Lasse