My learning day ..

Discussion in 'General Electronics Chat' started by happs, Mar 5, 2005.

  1. happs

    Thread Starter Member

    Feb 27, 2005
    24
    0
    Hey :)

    Well I thought I would say something about my day in front of the breadboard.
    I wanted to learn some about counting circuits so I built a fast astable circuit with a 555 timer, worked fine; connected it to a 4060 Binary Counter, which also worked great; and from the 4060 to my newly delivered 7 Segment Decoder IC (74LS47, or somethin) .. didn't work QUITE as well! Heheh...

    Wired my 7 Segment LED display to the decoder, flipped the switch ... nothing! Ahhhh, I checked ALL my wires, all the connections and wired up LED's in different places to make sure the currents and the pulses were all working correctly; which they were. Decided that maybe the signals from the 4060 Binary counter to the 7 segment decoder were not strong enough and wired up some transistors to the outputs of the Binary counter to boost the signal ... still NO joy!
    Took the LED display OFF, wired up an LED to one of the outputs of the Display Driver (G node) -- still no joy!!!!
    After HOURS of playin around and looking stuff up; decided to take the transistors off the outputs from the 4060 Binary Counter, wire it directly (as first attempt) to the display driver and managed to get an LED (not the 7 seg display) to flash as if it was one of the led's in the display.

    Why did the transistors stop the signal, does anyone know ?
    Can't do a diagram yet ..
    Example:

    Bit 4 from 4060 was connected to Base;
    Positive terminal on breadboard to Collector
    Emitter to input on 7 segment Display Driver

    ???

    Anyway, so I have one LED flashing as if it was one inside the 7 seg display -- this is just to test the circuit was actually WORKING! heheheh -- this took me all day.
    RIGHT ... now ... after much head scratching I also discovered what Anode and Cathode means!! ... And my 7 segment display has a common CATHODE, not anode as it should be! DEARY ME! hehehe.

    Now I have the thing pretty much working (although I can't work out why removing the transistors helped) exept it is just one led and not the 7 seg display ... so ... because my display is a common cathode I cant use it :( ... is there any way to get around this ? I mean, reverse the polarity ?? I know that there isn't a way to make the LED's in the display work the other way round, but is there a way to convert the signal from the display driver so it works with a common cathode 7 segment display ? (I have a common anode ordered, shoudl be here on monday, but I can't wait .. I need to check this stuff out! heheh)

    ANYWAY

    Eventful, head scratching, eureka day.

    -Happs

    EDIT: the right word is probably .. invert.
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Don't worry, you should still be able to recover from the common anode common cathode mix-up.

    You should be able to use a PNP buffer stage to get back on track. See the diagram below.

    Just tie your common cathode terminal to ground and place one of these stages in line with each of the seven segments. I set the stage up to deliver about 10 milliamps to the each of teh segments.

    Your experiment with transistor boost is hard to diagnose without knowing exactly how you hooked them up.

    Hope this helps you out.
     
  3. happs

    Thread Starter Member

    Feb 27, 2005
    24
    0
    Hey, thanks for the diagram!! It worked! heheh.
    I now have it counting from 0 to 9 and through to 15 (tho the last 6 are strange symbols because it is a 4060 I'm using). I am really happy! hehhe. thankyou, it is much appreciated. I'll draw a diagram for the transistor setup later on, need to go and spend some time with the missus. :)

    - Alex

    Oh .. and ...
    It didn't seem to work with the 1K resistor from the base to the V+ (3v in my circuit), so I took that out and it works fine. Is this a problem ? It seems to work fine.
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Good thinking. You did the right thing by removing the 1K resistor to address the mal-function. The 4.7K resistor between the 74LS47 and the base of the PNP transistor was sized on the basis of a 5V power source. Since you are using a 3V power source then the 4.7K would be too much resistance and there would not be sufficient base current to saturate the transistor. By eliminating the 1K resistor, you permitted all of the current to reach the base resulting in plenty of base current.

    The 1K resistor serves as a "base return" resistor. The purpose of a base return resistor is to pull current back out of the base when the drive from the open-collector driver is removed. This forces the transistor to shutoff much quicker than it would without the base return resistor. A base return resistor is more critical when the rate at which the transistor is being switched becomes critical. In your application the base return resistor is not a must since switching speed for your circuit is not that critical.

    Congratulation on getting your circuit up and running. You have glimpsed just the tip of a very fun iceberg.

    Keep learning and having fun and remember we are all here to help make your experimenting as much fun as possible.
     
  5. happs

    Thread Starter Member

    Feb 27, 2005
    24
    0
    Hey hgmjr :)

    I have attached the transistor set up I used from the 4060 Binary counter to the 74LS47 7 segment decoder. This set up didn't work for some reason, not that it really matters now as the circuit is working, but I was just wondering why ?
    Having actually drawn it out (got some neat software :)) I think maybe I should of made a connection from the emitter to the -V as well as to the decoder ?


    - Alex.

    [attachmentid=452]
     
  6. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Happs,

    After viewing the transistor stage in your post, it is clear to me why it failed to work. Your instinct is correct. The missing element is a resistor from the emitter of the NPN transistor to ground or -V.

    The transistor configuration you came close to implementing was a "common collector", also referred to as an "emiitter follower". Emitter followers are usually used to buffer a signal from a signal source with a high empedance to an input with low impedance like a speaker. This is because it acts like a current amplifier. It has a voltage gain of one.

    It has two things to look out for. The first is that it introduces a voltage level shift of 0.7V (Vbe) between the signal applied to the base and the output signal at its emitter. The second thing is that this 0.7 volts is a function of temperature and so it will change by small amounts as the device temperature changes.

    Since you are using a 3 volt supply source, a shift of 0.7 volts is a significant portion of you voltage source.

    One thing while I am thinking of it, the 74LS47 you are using is intended for use with a 5 volt power source. It may get cranky at 3 volts so be on the lookout for that.

    Good Luck.
     
  7. happs

    Thread Starter Member

    Feb 27, 2005
    24
    0
    Hiya hgmjr :)

    Thanks for the help there -- I kinda wondered about the emitter to -V when I was actually drawing it out. :) I have ordered a voltage regulator from an online store that should be here for tuesday, I think it will allow me to drop a 9v battery to 5v. Not sure tho, but I am gonna find out! heheh.
    Since you're doing a great job in helping me out, I just wanna say thankyou :) You've been great and it is much appreciated :)

    - Happs
     
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