7-segment to analog meter convertor

Discussion in 'General Electronics Chat' started by jack abraham, Jun 11, 2016.

1. jack abraham Thread Starter New Member

Aug 17, 2010
7
0
I have device and i want to convert value displayed in 3-digit 7-segment in analog meter. i only have assess to 3-digit-7 segment not to any other part of the circuit.
in other word i need to know how to read back value displayed in 7-segment

2. Techno Tronix Member

Jan 10, 2015
140
10
There is probably an analog voltage generated somewhere in your device that could be used to drive an analog meter. Or you can try with LED display digital voltmeter.

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3. ScottWang Moderator

Aug 23, 2012
4,936
777
You have to do the reverse engineering as :
0. Display 0 to 0000 -- 0 (a,b,c,d,e,f) to 0000, if a,b,c,d,e,f segments led all 1(high) then output 0000

1. Display 1 to 0001 -- 0 (b,c) to 0001, if b,c segments led all 1(high) then output 0001

2. Display 2 to 0010 -- 0 (a,b,c,d,e,f) to 0010, if a,b,d,e,g segments led all 1(high) then output 0010

3. Display 3 to 0011 -- 0 (a,b,c,d,e,f) to 0011, if a,b,c,d,g segments led all 1(high) then output 0011

4. Display 4 to 0100 -- 0 (a,b,c,d,e,f) to 0100, if b,c,f,g segments led all 1(high) then output 0100

5. Display 5 to 0101 -- 0 (a,b,c,d,e,f) to 0101, if a,c,d,f,g segments led all 1(high) then output 0101

6. Display 6 to 0110 -- 0 (a,b,c,d,e,f) to 0110, if a,c,d,e,f,g segments led all 1(high) then output 0110

7. Display 7 to 0111 -- 0 (a,b,c,d,e,f) to 0111, if a,b,c segments led all 1(high) then output 0111

8. Display 8 to 1000 -- 0 (a,b,c,d,e,f) to 1000, if a,b,c,d,e,f,g segments led all 1(high) then output 1000

9. Display 9 to 1001 -- 0 (a,b,c,d,e,f) to 1001, if a,b,c,d,f,g segments led all 1(high) then output 1001

Above just for one digital, you need to do three digital.

For three independent digital 7-segments led display:
Using three 8 bits eeprom to decode three digital 7-segments leds to three binary decimal values as 0000~1001.

For three scan digital 7-segments led:
Using one 8 bits eeprom to decode three digital 7-segments led to a three combined binary decimal values, and using three 74HC574 -- CMOS Logic Octal D-Type Flip-Flop to decode the three combined binary decimal values as three independent binary decimal values as 0000~1001.

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4. MrAl Distinguished Member

Jun 17, 2014
2,578
523

Hi,

Fairly hard to do in most cases because usually the digits are multiplexed so their digits or segments are updated in a time frame where they are not 'on' all the time.

You did not say if this is LED or LCD. If it is LCD, forget it. If it is LED, you have a chance if you really want to build a circuit that can detect timing, or else use a microcontroller to detect the multiplexing signals. Still not an easy task even with a 7 segment LED display.

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5. prof328 New Member

Apr 15, 2016
10
20
Probably not possible, I built a device using eight seven segment displays controlled by a Maxim 7219. This chip multiplexes the displays at 800Hz so only one segment of any of the displays is energized at any moment in time. The refresh rate is way above what the human eye can detect and so the display looks like the numbers are solid but they are not.
All of the 'a' segments are connected together, as with the 'b' etc. so eight connections to the 7219 and one connection 'the common' is made from each display back to the 7219 another eight connections to the 7219.

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6. hp1729 Well-Known Member

Nov 23, 2015
2,106
235
Sounds like a place for a PROM and a DAC. But, assuming the display is multiplexed, that sounds like a timing nightmare. Practically, I think your answer is, "no". Yes, as suggested, latch the data on digit selection signals.

What you would need might look like this, if you could find the parts.

Multiplexed or not multiplexed, that is the question.
LED or LCD?

Last edited: Jun 11, 2016
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7. jack abraham Thread Starter New Member

Aug 17, 2010
7
0
It is LED Not LCD

3-digit 7-segment Multiplexed And LED common cathode

Last edited by a moderator: Jun 11, 2016
8. MrAl Distinguished Member

Jun 17, 2014
2,578
523

Hi,

Ok cool. Now, what color are the LED's?

it matters because RED might only be 1.7v to 2.2v whereas blue could be as high as 3.4v per segment. This makes a difference on the logic used to detect the segment pulses. Regular 5v logic wont be able to pick up 1.7v pulses but may be able to pick up 3v pulses. So you might need logic level translators too.

If you are serious about this, you are probably better off using a microcontroller, but i would check the signals with a scope first to see if this is doable.

9. dannyf Well-Known Member

Sep 13, 2015
2,196
417
it is easier to do with led vs. lcd.

the simplest route would be through a mcu: read back the value and then generate an analog output from it. Logic gates possible as well.

10. hp1729 Well-Known Member

Nov 23, 2015
2,106
235
Well the design is close then. Just take the other two 7447s out of the suggested design and run the display lines together. Three inverters, three latches, three PROMs and a DAC and you've got it.

11. GopherT AAC Fanatic!

Nov 23, 2012
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4,094
It would be easier to start with an instrumentation amplifier and build a new meter from scratch.

12. benta Member

Dec 7, 2015
101
24
There's one more difficulty apart from multiplexing: leading zero blanking.
If the display has this as well, it gets really funny.

Benta.

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13. GopherT AAC Fanatic!

Nov 23, 2012
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Honestly, we are good at solving problems but this seems like the OP is asking us to solve his solution. What is the real problem?
What is wrong with a digital readout.
What accuracy is needed?
What range of input voltages are needed?
Does the current meter have automatic range adjustment?

14. tracecom AAC Fanatic!

Apr 16, 2010
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Seems like it would be possible to place a light sensor (LED) in contact with each segment of each digit, connect each sensor to a uC input, read the LEDs that are producing voltage, decode the results, and output the results on a DAC.

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15. GopherT AAC Fanatic!

Nov 23, 2012
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If the strategy is complicated, it will take a complicated solution like this.

We may as well consider a vision system to read the LEDs and create a DAC output to drive the analog meter. I guess we could have a second vision system to confirm that the analog meter is reading the filtered DAC signal correctly.

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16. ian field Distinguished Member

Oct 27, 2012
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It would be an absolute doddle with decoder/drivers fed by a chain of BCD counters - just run a binary counter alongside the BCD chain and use standard DAC resistor ladders.

If BCD is available, it may be possible to convert back to binary - ready made decoder chips have probably long since been lost in the mists of time. Converting from 7-seg is also possible. There may be some "glue logic" decoder diagrams in the databooks on archive.org.

A multiplexed display will require latches between the outputs and the decode back circuit.

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17. tracecom AAC Fanatic!

Apr 16, 2010
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My post was not a joke. Given the dimensions and spacing of the displays, it would be easy to design a PCB to hold the sensor LEDs in the proper locations, and then place the assembly in contact with the display. No electrical connections to the display circuitry would be required. The whole project could be done with less than \$15 in components.

18. GopherT AAC Fanatic!

Nov 23, 2012
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I didn't mean for it to be a joke - the project request is kind of a joke. there are likely a half-dozen ways to get to the OP's end goal but, without him/her explaining the real starting situation and the real goal, it is hard to come up with alternatives.

19. jack abraham Thread Starter New Member

Aug 17, 2010
7
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The color is RED

20. jack abraham Thread Starter New Member

Aug 17, 2010
7
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I thought i can do it with pic16f877a y example(i have it). but i need other method from logic ic not uc