Help: Driving a 5 inches 7 segment display

Discussion in 'The Projects Forum' started by abo0badr, Apr 25, 2013.

  1. abo0badr

    Thread Starter New Member

    Jan 27, 2012
    12
    0
    Good morning guys,
    I had finished a small project for car racing, in that Project I have a timer displaying the running time for each lap, I am currently using the small 7-segment displays which is powered up by 5 volts only. This was for testing purposed only (or lets say a miniature only).

    Now I have to convert everything into semi-industrial type of equipment. So I have decided to use a 5 inches or 6 inches common anode 7-segment display to display the time.
    My only problem is how am I going to do it as I am a newbie to bigger 7-segment displays.

    I am already using a 74LS47 (BCD-To-7Segment Decoder) and a simple 2n2222A to drive each multiplexed display using 5volts.

    I already new that the bigger 7 segment displays requires around 12 to 15 volts (based on 4 to 6 inches displays).

    My question is what should be the replacement parts for the BCD-To-7Segment Decoder and the 2n2222A Transistor?
    Any help will be much appreciated. Thank you​
     
  2. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    Are you really multiplexing with the 74LS47? Are you using a microcontroller to do this or something else? A schematic would be a great help.

    I suggest looking at the ULN2003 or ULN2803. These can be driven by the 74LS47 and provide up to 50V, 500mA to a 7-segment display.

    Are you planning to make your own displays from LEDs or will you buy them premade?
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,995
    745
    Are you using common cathode for the small display, but want to use common anode for the larger display?

    A diagram would help us further.

    if so you need to feed the outputs of the 74ls47 into an inverter ic like the cd4069,
    http://eeshop.unl.edu/pdf/CD4069UBC.pdf

    and the use the output from that to feed a N-mosfet or npn transistor with a 10K drive resistor, this will give you the higher current to drive the big display.

    Also replace the 2n2222A for a Tip41

    http://www.drixsemi.com/TIP41C.PDF

    otherwise use a uln2804 http://www.datasheetcatalog.org/datasheets/90/366828_DS.pdf
     
    Last edited: Apr 25, 2013
    elec_mech likes this.
  4. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    Doh, I didn't realize the 74LS47 outputs were active low. I'd then suggest using a CD4511 or CD4543 with a ULN2004 or ULN2804. These can be powered from up to 15VDC. Of course, this will depend on what is generating the BCD signals . . .
     
  5. abo0badr

    Thread Starter New Member

    Jan 27, 2012
    12
    0

    Thanks for the reply dude. Yes they are active low. is there anything wrong in multiplexing using 7447? I am using a PIC16F877A as the microcontroller.


    here is the schematic of my design.

    [​IMG]


    but this is an old design, I already have added 2 extra segments so that should be 6 digits. And this design is doubled, for 2 Cars in the race. Already done it also, so all in all 12 digits, for each car the timer is 2 milliseconds digits, 2 seconds digits, and 2 minutes digits. so each car has a 6 digits timer.

    all these is already running in very good condition, don't mind the buttons I have functions for that. it's clearly that the design is good. but my problem is how am I gonna change those small segments into 5 inches? what am I going to replace as the drivers and a bcd to 7segment decoder?

    do you say that i should replace th 74LS47 with CD4511 or CD4543?? and replacing the 2N2222A with ULN2004 or ULN2804??

    Thanks dude.
     
  6. abo0badr

    Thread Starter New Member

    Jan 27, 2012
    12
    0
    Don' mind the 2N3904 transistors, already replaced them with 2N2222A
     
  7. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    Okay, the first question now is what are the voltage and current requirements of these 5" digits you're referring to? Are you going to buy pre-made or make your own? In either case, what is the forward voltage and current required to power each segment? This will better dictate what your options are.

    Based solely on your schematic, you appear to have plenty of spare I/O pins on the PIC. You could save on some parts by decoding the BCD to 7-segment directly on the PIC. This would eliminate the need for a 74LS47 or similar. You'd then need 7 output pins instead of 4.

    You could then connect a ULN2003 directly between the PIC and the display along with either transistors as you mentioned or a source driver IC which is similar to ULN2003 except it sources current instead of sinking it.

    The transistor selected will be determined by the current draw of 7 segments. If each segment draws 20mA, then you're looking at 140mA. Now, there is a good chance you'll need to increase the current going to each segment since you're talking about multiplexing, what, 12 digits? Tentatively, the 2N2222 is probably okay, but you may want to consider a larger transistor as Dodgydave suggested.

    If you want to make as few changes as possible, you could put a TTL inverter between the 74LS47 and the ULN2003. This adds extra parts, but the choice is yours.

    Another thought is to look at using something like the 74LS06/07. Looking at the datasheet, it can sink up to 30V at 40mA. In theory, you could use this to connect directly between the 74LS47 and the 7-segment display if the current requirement isn't too high. You may find a better IC with some looking around.
     
  8. abo0badr

    Thread Starter New Member

    Jan 27, 2012
    12
    0
    Well, What I found on the local store has a part number HSH-50012G, But I could not get any datasheet on the web. There are other 4 inch segments on the web which requires small voltages like min 1.8v and max 2.4 with a Reverse Voltage VR of 5V, and a Peak Forward Current IPF (Duty 1/10 @1KHZ) of 150mA. And also I have found another website that have a datasheet of another 5inches segment which voltage is 12volts and a current of 25mA maybe. I'm not sure what voltage and current the HSH-50012G could require but I'm going to ask the shop maybe they know about it.

    Again, I said that is an old design, now I have 12 digits already, because again I told you this design was double for 2 cars, I mean the pins and the segments. I'm good with one micro controller. the only problem I am worrying about is the replacements to be made for a bigger 7-segment display, that is all my concern.

    So I will probably using a ULN2003? and I did not get your point when you say "except it sources current instead of sinking it" What does it mean? and how I'm going to make it?

    And what is the use of the TTL Inverter in the design? Why am I going to use it? By the way, the Segments are Common Anode, they Light by an Active Low signal, And my 74LS47 is an Active Low signal also.

    Another last thing (I hope I did not say too much, sorry for the inconvenience) What is the use of a 74LS06/07? or you're telling me it could replace the ULN2003?? Thanks.
     
  9. abo0badr

    Thread Starter New Member

    Jan 27, 2012
    12
    0
    I just confirmed that the 5 inches 7-segment I will be buying operates using 15v and 15-20mA current.
     
  10. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    Okay, I think the easiest thing to do is replace the 74LS47 with a CD4511. This will operate with your PIC and outputs high signals instead of low ones. You can then connect the output of the CD4511 to a ULN2003. The ULN2003 is then connected to the cathodes of your segments.

    For the anode side, you might get away with a source driver such as this. It can be connected between the PIC and the common anode of each digit. The caveat is it can only source up to 500mA per channel. At 20mA, you'd need 140mA if all 7 segments are lit which is no problem. But if you have to pulse a higher current, say a 150mA per segment, then you'll need 1.05A and this IC won't cut it.

    Do note that you'll lose ~1V across the ULN2003 and ~1-1.5V across the source driver IC. This means if you need 15V across each segment, you'll need at least an 18V supply.

    If this won't work on the anode side, look for a PNP transistor capable of handling double the expected peak current required by a digit (7 x peak current of single segment to account for worst-case scenario). If you just use a BJT or similar, expect ~1V drop across the transistor. Alternately, look for a logic-level controlled P-channel MOSFET. Next to no voltage drop.
     
  11. abo0badr

    Thread Starter New Member

    Jan 27, 2012
    12
    0
    Thank you so much for the wonderful comment and information.
    Sorry I did not see your comment, Was away for a summer vacation.

    By the way I tried Buying the 5 inch 7-segment display but has a problem turning it on, Already tested it with direct power of 15 volts, and it is good. but when I tried to create a simple count up program using a microcontroller to count from 0 to 9, The display is not turning on.

    The design of the circuit is the same as the above image, but I replaced the 2N3904 with a High Voltage High Current Transistor TIP41C, and the collector voltage to 15 volts, I tried using a tester to measure the emitter voltage going to the 7-segment display when HIGH/5V is supplied to the Base of the TIP41C. and it was not supplying 15Volts to the segment, rather it only supplies 5 Volts.

    What is the problem? is the current not enough to switch the TIP41C???
    or what Shall I do?

    By the way, the data sheet of the segment says it needs 15volts 20 to 25mA to turn on the segment.

    Your Design is much precise but my problem is as of the moment I don't Have those chips, but it is easy to order them. The only Big problem is the Entire electronics shops here and in the capital city (where I order my chips) do not have a 5 inch common cathode 7-segment display. As I notice your design needs a common cathode segment right? They only have Common Anode. And I prefer not to use an inverter to invert the output, it may apply a delay or something?

    What do you Prefer with my chips?

    As of the moment I have a Darlington Array ULN2003A , a TIP41C, a 15volts supply, and a couple of resistors :D
     
  12. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    You're using the transistor as a common collector as shown in your schematic. This type of configuration is fine for using a small current to control a larger current, but not ideal for using a small voltage to control a larger voltage.

    To do this, you need to use a common emitter configuration.

    Since you're using the transistor to control Vcc going to the display, you need to source current (as opposed to sink current). To do this, you will need to use a PNP transistor, not an NPN. The emitter of the PNP goes to Vcc (15VDC) and the collector goes to the common anode of a digit.

    That means you'll need to provide up to 25mA x 7 = 175mA through the PNP transistor, so select one capable of 400mA or more to be safe. A PN2907 or similar should work fine.

    Note you'll need to drive each PNP transistor with a low signal from the PIC instead of a high signal. Thus, to turn on a digit with the PNP, send a low signal to that specific digit and a high signal to the other three.

    No, this is for a common anode digit, so you're good. The ULN2003 sinks current, so it works well for a common anode display - you want to sink current from each cathode lead of the display.

    You may need to increase the voltage going to the display. Again, there will be ~1V drop across the PNP transistor and ~1V drop across the ULN2003, plus you'll want to continue to use resistors on the cathode leads to limit current to each segment. I'd suggest an 18VDC power supply.
     
  13. abo0badr

    Thread Starter New Member

    Jan 27, 2012
    12
    0
    Thank you so much. I will try it this sunday and will be back to you. Thank you for not quitting with me :D

    You provided me with Ideas I never knew about. like the common emitter config.
     
  14. abo0badr

    Thread Starter New Member

    Jan 27, 2012
    12
    0
    I already did what you said. But I tried doing a simple switching circuit with just the PN2907, a 15volts source, and a 5volts source. I connected the Emitter to Vcc 15 volts, the Base to a ground to test if it is switching, and tested the collector output. With a tester I get around 3.4volts at the collector pin. and my LM7815 (15volts voltage regulator) gets a little bit hot. is there any solution?

    or please kindly post a schematic or something to be sure with. thanks a lot
     
  15. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    I've tried testing a couple of transistor circuits with undesirable results. I've learned the hard way a PNP cannot be fully shut off with a base voltage lower than the emitter in common emitter mode. However, I think I found a solution, check out this link. It describes using an NPN connected to a uC which in turn controls a PNP connected to the load. This should address your needs. Again, you could use a sourcing IC (similar to the 2003 but sources current instead of sinking it), but it'll cost less to use a couple of transistors for each of your common anode connections.
     
Loading...