binary to bcd converter query

Discussion in 'General Electronics Chat' started by hunterage2000, Jan 8, 2013.

  1. hunterage2000

    Thread Starter Active Member

    May 2, 2010
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    Hello all,

    Can anyone tell me if theres an alternative to the 74185 binary to bcd converter or how I can make one? I cant find one anywhere in the UK.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Unfortunately, I don't think there is any. Binary to BCD conversion is not trivial as you may know. Read this for more.
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    As the 74185 is pretty much obsolete even if you found some (findchips.com turned up 10 pieces total) you would be better served using something else.

    What is the application? It would probably be better to redesign the circuit as to not need this at all.
     
  4. KnRele

    New Member

    Jan 7, 2013
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    The 74185 must have been obsolete for decades. There might be some available from NTE or similar in case you have a broken one that needs replacement in some equipment, but I don't think there is any but new old stock available.

    An alternative might be to put the conversion-table into one or more EPROMs: use the address-lines as inputs and the data-lines for output. Yet another option might be to use some decoders such as the '138 and a number of gates to create a «BCD encoder» from them, essentially fabricating a ROM from SSI chips.

    Or program a microcontroller such as one of the PICs with the desired lookup-table. These are some ideas.
     
  5. hunterage2000

    Thread Starter Active Member

    May 2, 2010
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    its to be used for the output of an ADC0804, I have used 5 bits out of 8 for 32 levels i.e 0-31C. Basically the circuit is:

    Thermistor 4 res bridge circuit > AD623 Inst amp > ADC0804 > bin-to-bcd > 2x bcd-to-7seg

    I have no MP experience.
     
  6. t06afre

    AAC Fanatic!

    May 11, 2009
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    If you have some PROMS lying around maybe you can use that to mimic the function
     
  7. hunterage2000

    Thread Starter Active Member

    May 2, 2010
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    yeah I have the truth table but its big, I looked at PLA's but not sure on how they are programmed. Is PROM's similiar to PLA's?
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    My first thought would be to replace the bin-to-bcd and the bcd-to-7seg blocks with either a microcontroller (MCU) or a PROM. If you have an MCU with 14 available I/O pins, then you can use 5 for input, 7 for the LED segments, and 2 to turn on the desired display. If you used one common-anode display and one common-cathode display you could do this with just 1 pin to select the display.

    If you don't know anything about MCUs, this would be a good little project to get your feet wet with.

    Another reasonable alternative is to use a PLD (mostly CPLDs these days). FPGAs have taken over much of this kind of work, but I don't know what the smallest FPGA you can get these days is, but it is probably extreme overkill. This is the kind of thing that PLDs were intended for, so you might look at that path.

    If you REALLY want to stick with discrete logic, then you could certainly do so. You have seven circuits, each with five inputs and one output. You can determine the SOP or POS forms for each circuit and may even find some terms in common that would allow you to reduce your gate count. But I can almost guarnatee that, once you start down this path, you will eventually conclude that you don't want to keep going on it.
     
  9. t06afre

    AAC Fanatic!

    May 11, 2009
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    Is it not a quite common IC that contain an ADC and a display driver output ICL7107 and ICL7135 if I remember correct
     
  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Iwould do it with a microcontroller but if you want to keep it old school, you could use a clock signal to count up (into two 4-bit binary counters) until the 8-bit number matches number in your 8 bit adc (test for a match with 8 NOR gates with all outputs to one 8-input AND gate). Use a NAND Gate to feed the pulse and stop when your 8-input AND goes high.

    At the same time you count into the binary counters, also capture the pulse train with a pair of cd4026 counter/7 seg drivers in series. The CD 4026 can directly out put to seven seg display.

    It will only work if your adc values Change slowly in comparison to your pulse train.
     
    Last edited: Jan 9, 2013
  11. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Or, you could skip the whole ADC and display circuitry and simply add a $10 digital panel meter and directly read the voltage from your amplifier. It won't be 0-31 but it might be usable.
     
  12. Anthony_Harder

    New Member

    Nov 26, 2007
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    i was in the same situation looking for that binary to Bcd converter, i was making a digital clock, when i realize how hard was to find it, i made the converter as a code inside the main 16f84a microcontroller of the system, it works great
     
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