# Running LED - all LED turn ON when reach the last LED?

Discussion in 'Electronics Homework Help' started by mod, Jul 21, 2016.

1. ### mod

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Apr 30, 2016
i have a schematic of running led (image on attached file),
on that project, LED will light on from 1st led, one by one ,until it reach the last led, and repeated.
My homework is, make all 16 LED's Light on (together) when it reach the last led. how can i make it possible?

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2. ### Harald KappModeratorModerator

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Nov 17, 2011
The 74154 is not suited for this without additional circuitry. The 74154 has mutually exclusive outputs, meaning that only one output can be low at any time.

Hints:
1. How can you control the LEDs by a second circuit (connected to the LEDs' cathodes) without destroying the 74154?
There are several possibilities using e.g. diodes or logic gates.
2. What is the 74193 showing on its output pins when the last count is reached (where you want all LEDs to be on)?
Can you use one of the output signals of the 74193 to achieve "All LEDs on" by activating the above second circuit?
These are rhetorical question. Of course this is possible. It's up to you to find out how,

3. ### mod

10
0
Apr 30, 2016
this is actually the real schematic circuit from my teacher (i got this from my friend), the function is same, nothing different with the first image. only adding ic 7400.
can i make my question possible, with make manipulation on ic 7400?

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4. ### Sunnysky

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121
Jul 15, 2016
looks like a single LED chaser back and forth. Kitt car era.

U3-C & D make an SR flipflop active low input while their outputs enable U3-A&B to steer input negative pulses which get inverted to count up or down.

Last edited: Jul 21, 2016
5. ### hevans1944Hop - AC8NS

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Jun 21, 2012
To get any of the sixteen LEDs to light you have to connect the LED cathode to common. However, the 74154 has sixteen totem-pole outputs, so you cannot just connect the LEDs to common through fifteen transistors when you get to the last state. That would short to logic common all fifteen of the 74154 outputs that are high! Totem-pole TTL outputs do not like to be shorted to common.

You will have to cut the connections between the LED cathodes and the 74154 outputs and insert some logic. I will leave this as an exercise for the student since this is a homework problem. Hint: seven small-scale integrated circuit logic packages are required.

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Feb 21, 2016
Clue: Diodes

7. ### grumps

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Jul 11, 2015
speed up ? the counter / driver , above flicker rate :

( just kidding ... maybe >: )

Sunnysky likes this.
8. ### hevans1944Hop - AC8NS

4,547
2,122
Jun 21, 2012
Also note there is only ONE current-limiting resistor for all sixteen LEDs in the original schematic. That's enough if only one LED will be illuminated at any given time, but if you try to turn on all sixteen at once... poof! So, besides opening the connections to the cathodes of each LED to insert some logic, you also need to insert a current-limiting resistor in series with each cathode and connect it to the output of that logic. The R1 resistor should then be removed and replaced with a jumper wire because its function will be taken over by the sixteen separate current-limiting resistors.

9. ### Sunnysky

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Jul 15, 2016
You can make them all appear to be on at once by scanning fast without flicker but it would be dimmer since duty cycle is 1/16. But if you can switch a smaller series R during this fast scan, you can run at 15% rated If with ultrabright LEDs then run at 150% or absolute max for experimental purposes.