Wednesday, December 21, 2011

Sequential LED Bar Graph Circuit Using IC 4017 Explained

The article describes a simple method of making an incremental LED bar graph by using the IC 4017, which is rather equipped with specifications not suiting the present functions. Let’s learn how we can mod the IC for the operations.Here we study an easy method of making an array of LEDs “chase” to develop into a sequential LED bar graph. The LEDs start from one of the 10 pin outs of the IC and go on switching one after the other until all the LEDs are lit forming an incrementing bar graph. The circuit uses the ordinary IC 4017 for implementing this interesting LED light sequence.

Circuit Description

The main component of this sequential LED driver circuit is the popular Johnson’s Decade Counter IC 4017. As we all know, the normal functioning of the IC involves sequential shifting of its outputs 1 to 11, in response to a clock signal applied at its pin #14.
The outputs become high in sequence such that the previous output becomes low immediately as the “high” position “leaps” through the assigned pi-outs.
If LEDs are connected to the outputs, the above sequence would produce an effect of an illuminated “dot” jumping from start to finish and repeating the sequence.
Though the effect looks interesting, fails to bewitch the folks simply because the illuminations produced are very low. This is because, only one LED or lamp glows at any instant while sequencing, not enough to make the system very eye-catching.
However the sequencing factor of the IC cannot be ignored as it’s one complex function that cannot be achieved a single IC and the chip must be credited for this attribute.
So, what can we do to improve the above feature such that the engaged lights become more attractive and the sequencing feature is also exploited at the same time?
One idea would be to stop the former LEDs in the sequence from shutting down while the array is sequencing. It means now as the illuminating sequence begins, the LEDs light up one after the other to form an illuminated “bar,” until the whole array is lit up. Once the whole sequence ends, the entire “bar” is shut off and the cycle repeats all over again.
However since it won’t be possible to do any modification inside the chip, probably doing this through external amendment is the option left.
To keep the LEDs hold their illuminations even with the sequencing logic going low, we would require some kind latching arrangement with the LEDs for implementing the trick.
As we all know an SCR is one device which latches up its output pin outs when its gate is triggered. The function is available only with DC supplies though, and here the circuit being operated with a DC, becomes perfectly suitable for the above application.
Referring to the figure we see that all the output pin outs of the IC are configured to the gates of the corresponding SCRs, and the LED are connected across the positive and the anodes of the scr.
When the IC outputs start generating the shifting pulses, the SCRs close one after the other, illuminating the LEDs in sequence and latch the illuminations in the incrementing order until the last LED is lit. After this the whole array switches OFF. The switch-off feature of the LED chain is implemented by T3 and is introduced exactly for this function.
T3 being a PNP transistor, remains switched ON as long as the output at pin #11 is low. Pin #11 being the last pin out in the whole sequence remains at logic low until the sequence concludes over it, making it also go high.
As soon as pin #11 becomes high, the base of T3 is inhibited from conduction, switching off the power to the LEDs and the SCR. The SCR latch breaks, shutting off the whole array and the sequence gets initiated again from LED 1 at pin #3.
The shifting or the sequencing of the outputs is directly depended on the frequency of the input clocks, applied at pin #14 of the IC.
Any astable multivibrator may be used for sourcing the clocks. Here we have used the common transistor type of AMV, which is perhaps the most simple to build and configure.
C1 and C2 may be varied for getting different clock pulses that would in turn decide the forming rate of the LED bar.
Alternatively you may add VR1 and VR2 in series with R2 and R3 for directly varying the display rates as desired.
The capacitor at the base of T3 is placed so that the transistor switches after a while, and allows the last LED at pin #11 to light up completely before the whole “bar” gets shut off.
Resistors R5 to R15 are included to restrict the current to the SCR and also to stop the IC from getting unnecessarily heated up.
The LED bar graph generator circuit may be operated right from a supply range of 5 volts to 15 volts DC.
If the supply is selected 12 volts, 4 LEDs can be accommodated with a series limiting resistor (not shown in the diagram, but is required).

Parts List for the proposed sequential LED bar graph generator circuit

R2, R3 = 10K,
VR1, VR2 = 47K,
Remaining all resistors are = 1K,
C1, C2, C3 = 10uF, 25V
All SCRs are = BT169,
IC1 = 4017,
All LEDs = As per choice




6 comments:

  1. this is exactly what i would like to make but i am confused on how adding the cap at the base of t3 keeps it from shutting off immediately. im assuming it charges through the ic pin 11 while in LOW state (sinking right?) but when pin 11 goes high, how does the cap stop the signal momentarily? i guess im not understanding how/where/when it discharges.

    oh and you labeled the cap im referring to as C1 but i think its supposed to be C3 right?

    Thanks

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    Replies
    1. It is always a little confusing to simulate a PNP device as is here.
      As long as pin11 is low the capacitor is able to store some charge inside it, as soon as pin11 goes high, the stored charge inside the capacitor restricts pin11 high from entering the base of PNP by maintaining the required -0.6V level for some moment of time until finally the capacitor discharges and allows the potential to rise well above -0.6V at the base switching OFF the device.

      Delete
    2. Hi i wrote on another post , but this i the one and i have some questions if it possile to replay:D
      1 what are those to led's that apper, and led with are in tail light are those 2 on each pin (can i put 3 or 4)
      2 can i put the whire that arrive from relee of turn signal( how interfeear with circuit, does it work ok?)

      and i am not sure i understant "If the supply is selected 12 volts, 4 LEDs can be accommodated with a series limiting resistor (not shown in the diagram, but is required)." if a have 12 V on IN , i must have 4 LED on a pic of IC4017 insted of 2

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    3. 1) those are series limiting resistors with each LED strings.
      2) yes you can take the supply from the turn signal relay for powering the circuit

      yes 4 LEDs (5mm) can be accommodated with 12V supply.

      you may also like to refer to the following design:

      http://homemadecircuitsandschematics.blogspot.in/2013/03/sequential-bar-graph-turn-light.html

      Delete
  2. at second circuit ( http://1.bp.blogspot.com/-Wq7rO28xDBM/UUwbAIKzoKI/AAAAAAAADvQ/5cG8ONiXZAI/s1600/modified+car+turn+signal+indicator+circuit.png ) is ok if i replace 111234 sequence LED with 222222, and rezistor of 470 ohm on each

    ReplyDelete
    Replies
    1. I did not understand what you meant by: "is ok if i replace 111234 sequence LED with 222222"

      Delete

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