Detecting 7-Segment LED's With Phototransistors

If a Webcam or even Labviews machine vision is out of the question, you could try the same color LED to sense the segment. It will be close to the wavelength and would produce a small voltage. How ambient lighting will effect it I'm not sure. The webcam idea starts to look better because software is flexible and it's been done before.

Cheers

The phototransistors will respond to the LED segments, but the current will be low. The LED segments may also be scanned - or may not - so you could look at the phototransistor current on an oscilloscope and see if anything is going on.

Phototransistors don't exactly turn on and off, they just conduct more as the light increases. You might get microamps, or even nanoamps, from an LED segment. Most of them have similar spectral responses. [1]

You'd need a lot of photosensors, 21, or possibly less if you think through all the patterns. Messy.

Why not add a new temperature sensor in the water bath? Connect it to a cheap USB data acquisition thing.

[1] I was playing with a large-area photodiode this afternoon, 13 mm^2 active area. Huge, actually. Office room light gave me roughly 200 nA. To get 200 uA, I had to hold it about 6" below a bright halogen under-counter lamp thing.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

Somewhere on the web there's a table of visible light colors and wavelengths. I think red starts getting visible around 700 or 750nm. I can't remember what the wavelength of green is. Google is your friend.

Order your phototransistors by wavelength and sensitivity -- you want nice sensitive ones. Then use bigger and bigger collector resistors until you start seeing a response. This ain't rocket science.

--
Tim Wescott 
Control system and signal processing consulting 
www.wescottdesign.com

Name the PLC, does it have a serial port ?

hamilton

r 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

ss the back of the display board to pickup any output.

each strategically positioned on a segment of the displayed characters.

m 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 di splay of the waterbath they wouldn't turn on.

ut none of my phototransistors would work. Digikey lists wavelengths as a p arameter in their search engine for phototransistors. How would I know whic h wavelength to buy and am I even on the right track?

OCR software, then the ascii transmitted to the PLC. Now THAT was thinking outside the box. Not really do-able in my situation.

On hand, I've got several Horner PLC's, models XLe and XL4, both with seria l communication available for applications. I've also got several Automatio n Direct's "CLICK" family of PLC's, also with serial communication availabl e in the ladder software. All are a joy to program, Bart

Why can't you add a temperature probe and log it directly? (Or, are you trying to characterize the display's performance?)

You probably don't have to use 7 detectors for each digit. Examine the "segment mapping" for each displayed digit value. You have 7 signals encoding only 10 values. In theory, you only need 4 bits for this (though you might not be able to pick them off the display directly!)

Build a Karnaugh map for each and see what they have in common.

Lots more light from your desk lamp as well as spectral content. Also, desklamp gives you a higher duty cycle (I suspect the LED displays are multiplexed so each "lit" segment is really "off"

70% of the time!)

Try using an LED (of the same color) as a photodetector -- with a fair bit of gain!

You also still have to deal with the possibility that the display is multiplexed -- beating against the frame rate of the camera.

Does the water bath/indicator have a 4-20mA output or any other means of getting at the "signal" it is displaying?

Basically you are dead in the water unless either it is either in pitch dark or a multiplexed display and you can use frequency of the wanted signal to detect the segment on or off against strong ambient light.

It would be orders of magnitude easier to put another independent thermocouple into the thing being controlled.

--
Regards, 
Martin Brown

--
Instead of phototransistors, you might want to try LDRs.

ime. Well, not exactly. The waterbath has a three character display that sh ows its temperature. Three 7-segment LED's would show, for example, 45.7

the back of the display board to pickup any output.

h strategically positioned on a segment of the displayed characters.

ith 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 displ ay of the waterbath they wouldn't turn on.

none of my phototransistors would work. Digikey lists wavelengths as a para meter in their search engine for phototransistors. How would I know which w avelength to buy and am I even on the right track?

R software, then the ascii transmitted to the PLC. Now THAT was thinking ou tside the box. Not really do-able in my situation.

I have an external thermometer to log the temperature of the water and I ha ve an AC current transducer on the power cord (a kind of "clamp ammeter") t o log the amperage, revealing when the heater is on or off. I need to log t he display so I have a record of what the waterbath is saying the temperatu re is. I cannot crack open the waterbath's case to access the electronics i nside (well I could but that opens up a can of worms, "Observer Effect"). I have 12 available inputs on the PLC so I'm working on a way to logic out the three 7-segment displays to a 4 bit binary number, doable (AND, NAND, etc.).

- This is a great group! Bart

I'm liking the web-cam idea- it's perhaps a lot easier to implement than you think. There are a very finite number of possible readouts, so make up template images with each possible readout (maybe you can record the temperature while it's ramping, otherwise tedious hand editing). Then to do a reading compute sum of least squared difference between the captured image and the many template images (sum the squared difference of each pixel between captured image and template image) to find the one which most closely matches (the one with the least difference). There will clearly be a lot of simple preprocessing you can do: mask out the background, convert image to binary to distinguish between on and off..

Long ago I did something just like this: I captured output to VFD and simulated keypresses on a credit card terminal to automate data entry of medicaid cards to verify entitlement for patients at a methadone clinic. I certainly did take the machine apart :-) Used Xilinx XC2064 FPGA. My consulting fee was much less than the cost of the software package which did the same thing.

--
/*  jhallen@world.std.com AB1GO */                        /* Joseph H. Allen */ 
int a[1817];main(z,p,q,r){for(p=80;q+p-80;p-=2*a[p])for(z=9;z--;)q=3&(r=time(0) 
+r*57)/7,q=q?q-1?q-2?1-p%79?-1:0:p%79-77?1:0:p158?-79:0,q?!a[p+q*2 
]?a[p+=a[p+=q]=q]=q:0:0;for(;q++-1817;)printf(q%79?"%c":"%c\n"," #"[!a[q-1]]);}

a f b g e c d h

_ _ _ _ _ _ _ _ | | | _| _||_||_ |_ ||_||_| |_| ||_ _| | _||_| ||_| _|

you need g to tell 0 and 8 apart you need b to tell 6 abd 8 apart you nned e to tell 9 and 8 apart you need f to tell 9 and 3 apart you need either a or d to tell 9 and 4 apart you need either a or f to tell 1 and 7 apart it looks like 5 is enough. a,b,e,f,g

--
For a good time: install ntp 

--- news://freenews.netfront.net/ - complaints: news@netfront.net ---

He may be able to improve on this. I.e., is the leftmost digit 0-9? Or, some smaller range? Likewise, can the rightmost digit assume all values? Or, just every 0.2 degrees, etc.? Can he switch the indicator to degrees C/F to change the range of options he has to deal with?

Can "blank" appear (e.g., leading zeroes)? Or, "display off"?

Note, also, that some "fonts" might remove the tails on 6 and or 9. Others might add a serif to the 7... It depends on whether there is a discrete "seven segment decoder" driving the display segments, or a bit of software, or some outputs from an integrating ADC that has been coaxed into serving this role, or...

(Also depends on what the segments actually look like; there are lots of "artistic" versions of the stock "7 straight line segments")

And, is it possible for a segment/driver to fail? (is this a short term "experiment" or a long term "monitoring process"?)

On a sunny day (Thu, 07 Nov 2013 00:37:05 -0700) it happened Don Y wrote in :

Been thinking about your suggestions, and now for something shocking: I think one can do this with a PIC 18F14K22 and a cheap analog PAL or NTSC camera module. No character recognition software, no big processor.

Long Time Ago, In A Place Not Far From Here, Jan Panteltje Designed And Wrote MVP

In Those Days Evil Forces Added Bad Impulses During The Vertical Interval Originally Designed To Protect The Empire The System Was Used Against Its Own People MVP Was The Rebels Answer And Claimed Back Their Own Territory.

Now if you look at the contents of the tgz you will find a very slow 8 pin PIC doing analog video processing.

The way I did that is use one of the PIC comparators to slice the video so to make a sync separator, and then detect the vertical interval and count lines. Now the plot PIC has 2 comparators, the other one can also be used to slice the video signal at say at 50%, so you can tell light from dark areas. IF, and that is a requirement, the camera can be fixed relative to the display (some bracket perhaps), then, as 18F has internal 16 MHz oscillator and 4 x PLL, so 64 MHz clock, 16 M instructions / second, it would be easy to count a few lines in every frame, and in a line look for 'segment on' or segment off' The rest is just a few lines asm, to 'decode' the 7 segments back to say a serial RS232 stream with numbers and CRLF for example one new reading per second, We need to watch out for flipping digits, scanning, so one needs to take the result of a few frames, and if nothing changes between frames use that as result, maybe even average over some frames to defeat scanning. Any changes would point to flipping digits. Could be an interesting project. I am not volunteering, but hey, here is the source to get you going if you like video and asm.

Greetings Luke Skywater

ime. Well, not exactly. The waterbath has a three character display that sh ows its temperature. Three 7-segment LED's would show, for example, 45.7

the back of the display board to pickup any output.

h strategically positioned on a segment of the displayed characters.

ith 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 displ ay of the waterbath they wouldn't turn on.

none of my phototransistors would work. Digikey lists wavelengths as a para meter in their search engine for phototransistors. How would I know which w avelength to buy and am I even on the right track?

R software, then the ascii transmitted to the PLC. Now THAT was thinking ou tside the box. Not really do-able in my situation.

So why can't you do the web cam/ OCR idea? If not that I'd much rather hac k open the display and get access to the electrical signals, rather than tr ying the photo-transistor idea... that seems 'squishy'. (to use a technica l term.)

George H.

I think the camera is a good idea, You can use not a webcam but a digital out camera like this:

(Or one of her sisters with more resolution...)

Instead of putting sensors on leds segments, You find the segmants in some position in a string of byte, obviously the display and the camera system must be rigid

bye delo

On Wed, 6 Nov 2013 05:06:21 -0800 (PST), Roger Monroe wrote:

--- Here's an encoder for a single digit:

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SYMATTR InstName A2 SYMBOL Digital\\inv 64 208 R0 WINDOW 3 8 104 Invisible 2 SYMATTR Value trise 50n tfall 50n vhigh 5v SYMATTR InstName A3 SYMBOL Digital\\inv 64 304 R0 WINDOW 3 8 104 Invisible 2 SYMATTR Value trise 50n tfall 50n vhigh 5v SYMATTR InstName A4 SYMBOL Digital\\inv 64 400 R0 WINDOW 3 8 104 Invisible 2 SYMATTR Value trise 50n tfall 50n vhigh 5v SYMATTR InstName A5 SYMBOL Digital\\inv 64 496 R0 WINDOW 3 8 104 Invisible 2 SYMATTR Value trise 50n tfall 50n vhigh 5v SYMATTR InstName A6 SYMBOL Digital\\inv 64 592 R0 WINDOW 3 8 104 Invisible 2 SYMATTR Value trise 50n tfall 50n vhigh 5v SYMATTR InstName A7 SYMBOL voltage -448 704 R0 WINDOW 3 20 97 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR Value 5 SYMATTR InstName V1 SYMBOL voltage -368 704 R0 WINDOW 3 22 99 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR Value 5 SYMATTR InstName V2 SYMBOL voltage -288 704 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V3 SYMATTR Value 5 SYMBOL voltage -208 704 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V4 SYMATTR Value 0 SYMBOL voltage -128 704 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V5 SYMATTR Value 0 SYMBOL voltage -48 704 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V6 SYMATTR Value 5 SYMBOL voltage 32 704 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V7 SYMATTR Value 5 SYMBOL Digital\\and 256 736 R90 WINDOW 0 -6 51 VRight 2 WINDOW 3 16 52 Invisible 2 SYMATTR InstName A8 SYMATTR Value trise 50n tfall 50n vhigh 5v SYMBOL Digital\\and 192 816 M90 WINDOW 0 -5 76 VLeft 2 WINDOW 3 16 112 Invisible 2 SYMATTR InstName A9 SYMATTR Value trise 50n tfall 50n vhigh 5v SYMBOL Digital\\and 448 736 R90 WINDOW 0 -6 51 VRight 2 WINDOW 3 16 52 Invisible 2 SYMATTR InstName A11 SYMATTR Value trise 50n tfall 50n vhigh 5v SYMBOL Digital\\and 384 816 M90 WINDOW 0 -5 76 VLeft 2 WINDOW 3 16 112 Invisible 2 SYMATTR InstName A12 SYMATTR Value trise 50n tfall 50n vhigh 5v SYMBOL Digital\\and 640 736 R90 WINDOW 0 -6 51 VRight 2 WINDOW 3 16 52 Invisible 2 SYMATTR InstName A14 SYMATTR Value trise 50n tfall 50n vhigh 5v SYMBOL Digital\\and 576 816 M90 WINDOW 0 -5 76 VLeft 2 WINDOW 3 16 112 Invisible 2 SYMATTR InstName A15 SYMATTR Value trise 50n tfall 50n vhigh 5v SYMBOL Digital\\and 832 736 R90 WINDOW 0 -6 51 VRight 2 WINDOW 3 16 52 Invisible 2 SYMATTR InstName A17 SYMATTR Value trise 50n tfall 50n vhigh 5v SYMBOL Digital\\and 768 816 M90 WINDOW 0 -5 76 VLeft 2 WINDOW 3 16 112 Invisible 2 SYMATTR InstName A18 SYMATTR Value trise 50n tfall 50n vhigh 5v SYMBOL Digital\\and 1024 736 R90 WINDOW 0 -6 51 VRight 2 WINDOW 3 16 52 Invisible 2 SYMATTR InstName A20 SYMATTR Value trise 50n tfall 50n vhigh 5v SYMBOL Digital\\and 960 816 M90 WINDOW 0 -5 76 VLeft 2 WINDOW 3 16 112 Invisible 2 SYMATTR InstName A21 SYMATTR Value trise 50n tfall 50n vhigh 5v SYMBOL Digital\\and 1216 736 R90 WINDOW 0 -6 51 VRight 2 WINDOW 3 16 52 Invisible 2 SYMATTR InstName A23 SYMATTR Value trise 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SYMATTR InstName A33 SYMATTR Value trise 50n tfall 50n vhigh 5v SYMBOL diode 224 944 R0 WINDOW 0 21 64 Left 2 WINDOW 3 -18 73 VLeft 2 SYMATTR InstName D8 SYMATTR Value 1N4148 SYMBOL res 192 1024 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 10K SYMBOL res 192 1120 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 10K SYMBOL res 192 1216 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 10K SYMBOL res 192 1312 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 10K SYMBOL diode 288 944 R0 WINDOW 0 -20 1 Left 2 WINDOW 3 52 74 VLeft 2 SYMATTR InstName D9 SYMATTR Value 1N4148 SYMBOL diode 416 944 R0 WINDOW 0 21 64 Left 2 WINDOW 3 -18 73 VLeft 2 SYMATTR InstName D1 SYMATTR Value 1N4148 SYMBOL diode 528 944 R0 WINDOW 0 21 64 Left 2 WINDOW 3 -18 73 VLeft 2 SYMATTR InstName D2 SYMATTR Value 1N4148 SYMBOL diode 800 944 R0 WINDOW 0 21 64 Left 2 WINDOW 3 -18 73 VLeft 2 SYMATTR InstName D3 SYMATTR Value 1N4148 SYMBOL diode 992 944 R0 WINDOW 0 21 64 Left 2 WINDOW 3 -18 73 VLeft 2 SYMATTR InstName D4 SYMATTR Value 1N4148 SYMBOL diode 1184 944 R0 WINDOW 0 21 64 Left 2 WINDOW 3 -18 73 VLeft 2 SYMATTR InstName D5 SYMATTR Value 1N4148 SYMBOL diode 1376 944 R0 WINDOW 0 21 64 Left 2 WINDOW 3 -18 73 VLeft 2 SYMATTR InstName D6 SYMATTR Value 1N4148 SYMBOL diode 1568 944 R0 WINDOW 0 21 64 Left 2 WINDOW 3 -18 73 VLeft 2 SYMATTR InstName D7 SYMATTR Value 1N4148 SYMBOL diode 1760 944 R0 WINDOW 0 21 64 Left 2 WINDOW 3 -18 73 VLeft 2 SYMATTR InstName D10 SYMATTR Value 1N4148 SYMBOL diode 688 944 R0 WINDOW 0 -31 0 Left 2 WINDOW 3 52 74 VLeft 2 SYMATTR InstName D11 SYMATTR Value 1N4148 SYMBOL diode 864 944 R0 WINDOW 0 -28 1 Left 2 WINDOW 3 52 74 VLeft 2 SYMATTR InstName D12 SYMATTR Value 1N4148 SYMBOL diode 1056 944 R0 WINDOW 0 -32 2 Left 2 WINDOW 3 52 74 VLeft 2 SYMATTR InstName D13 SYMATTR Value 1N4148 SYMBOL diode 1440 944 R0 WINDOW 0 -30 1 Left 2 WINDOW 3 52 74 VLeft 2 SYMATTR InstName D14 SYMATTR Value 1N4148 SYMBOL diode 608 944 R0 WINDOW 0 21 64 Left 2 WINDOW 3 -16 75 VLeft 2 SYMATTR InstName D15 SYMATTR Value 1N4148 SYMBOL Digital\\buf 1888 976 R0 WINDOW 3 8 120 Invisible 2 SYMATTR Value trise 50n tfall 50n vhigh 5v SYMATTR InstName A35 SYMBOL Digital\\buf 1888 1072 R0 WINDOW 3 8 120 Invisible 2 SYMATTR Value trise 50n tfall 50n vhigh 5v SYMATTR InstName A36 SYMBOL Digital\\buf 1888 1168 R0 WINDOW 3 8 120 Invisible 2 SYMATTR Value trise 50n tfall 50n vhigh 5v SYMATTR InstName A37 SYMBOL Digital\\buf 1888 1264 R0 WINDOW 3 8 120 Invisible 2 SYMATTR Value trise 50n tfall 50n vhigh 5v SYMATTR InstName A38 TEXT 256 880 Left 3 ;9 TEXT 448 880 Left 3 ;8 TEXT 640 880 Left 3 ;7 TEXT 832 880 Left 3 ;6 TEXT 1024 880 Left 3 ;5 TEXT 1216 880 Left 3 ;4 TEXT 1408 880 Left 3 ;3 TEXT 1600 880 Left 3 ;2 TEXT 1792 880 Left 3 ;1 TEXT -456 760 Left 2 ;a TEXT -376 760 Left 2 ;b TEXT -296 760 Left 2 ;c TEXT -216 760 Left 2 ;d TEXT -136 760 Left 2 ;e TEXT -56 760 Left 2 ;f TEXT 24 760 Left 2 ;g TEXT -400 880 Left 2 !.tran 20 startup uic TEXT -464 1424 Left 3 ;7-SEGMENT TO BCD ENCODER TEXT -464 1472 Left 3 ;JOHN FIELDS 07 NOV 2013 TEXT -416 968 Left 3 ;DEC TEXT -288 928 Left 3 ;SEGMENT TEXT -400 1024 Left 3 ;01 TEXT -304 1024 Left 3 ;0 1 1 0 0 0 0 TEXT -304 968 Left 3 ;a b c d e f g TEXT -400 1064 Left 3 ;02 TEXT -304 1064 Left 3 ;1 1 0 1 1 0 1 TEXT -400 1104 Left 3 ;03 TEXT -304 1104 Left 3 ;1 1 1 1 0 0 1 TEXT -400 1144 Left 3 ;04 TEXT -304 1144 Left 3 ;0 1 1 0 0 0 1 TEXT -400 1184 Left 3 ;05 TEXT -304 1184 Left 3 ;1 0 1 1 0 1 1 TEXT -400 1224 Left 3 ;06 TEXT -304 1224 Left 3 ;1 0 0 1 1 1 1 TEXT -400 1264 Left 3 ;07 TEXT -304 1264 Left 3 ;1 1 1 0 0 0 0 TEXT -400 1304 Left 3 ;08 TEXT -304 1304 Left 3 ;1 1 1 1 1 1 1 TEXT -400 1344 Left 3 ;09 TEXT -304 1344 Left 3 ;1 1 1 0 0 1 1 TEXT 1992 1344 Left 2 ;MSB

If you muxed in the segments you could save a lot of hardware.

can you do that?

Won't there be an issue if the segments are sequentially activated, which is, I think, the case with many 7-segment driving schemes... relying on the eye's persistence response? ...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.

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Definitely. 

Just for fun, I've shown a static encode for a single 7-segment 
digit as a baseline. 

If the OP comes back with the actual drive scheme, then an 
appropriate design can follow.

I wonder if a seven segment display could be used as the detector? The red leds should give some photo response to the red light from the display. It should match up pretty closely.