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# LE

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#### viswa

##### New Member
This is the problem statement...

There are 3 led s ... On a single button press, First led shd glow, for 500 msec... Jus after 500 msec 1st led shd be off and second shd glow for 500 msec... In the middle the third led shd start glowing after 250 msec of button press , for 500 msec and shd be off after that...
1st - 0 to 500 msec
2nd - 500 msec to 1 sec
3rd - 250 msec to 750 msec

Pl suggest me a way to build this circuit... without involving microprocessors/controllers... Jus using timer circuit...

Think 555's and how you would connect them to do what you want.

Why not use a microcontroller?

Likely it's a homework problem.

But of course it is.

Why not use a microcontroller?

its that i don want to use microcontroller for this... want to try without using them... so only asked ... suggest some methods pl...

I'm thinking monostable multivibrators and some glue logic.

May not even need much glue logic. Just use one trailing edge of one 555 pulse to trigger the next.

May not even need much glue logic. Just use one trailing edge of one 555 pulse to trigger the next.

But what about the 3rd one, starting @250ms?

I guess if you had 4 one-shots (os0, 0s1, 0s2, 0s3)@250 apiece, and used the following

1. os0 OR os1
2. os2 OR os3
3. os1 OR os2

May not even need much glue logic. Just use one trailing edge of one 555 pulse to trigger the next.

I was thinking of this at first... but can you tell me some easy triggering ways (like a photocoupler)...

But what about the 3rd one, starting @250ms?

I guess if you had 4 one-shots (os0, 0s1, 0s2, 0s3)@250 apiece, and used the following

1. os0 OR os1
2. os2 OR os3
3. os1 OR os2

I can get the meaning somewhat but not fully understandable... Can you Pl explain a bit...

example:
1. os0 OR os1 : means take an OR gate, connect the inputs to one-shot 0 and 1, then connect the led to the OR gate output.
2. etc.
3. etc.

example:
1. os0 OR os1 : means take an OR gate, connect the inputs to one-shot 0 and 1, then connect the led to the OR gate output.
2. etc.
3. etc.

Oh ok ok.... Thank you...

The output of one 555 can trigger two other 555s without an added OR gate.

Carl, the or gates combine the outputs of the one-shots to acheive the desired 'on' times for the LED's.

Another way would be to use a simple 2-digit counter clocked at 250ms. Then use this decode:

1 = NOT q3
2 = q3
3 = (q1 OR q2) AND NOT (q1 AND Q2)

Last edited:
OK BrownOut, I wasn't fully following what you were suggesting.

You could also use 4 one-shots of 500ms, 500ms, 250ms, and 500ms.

The is 1st is triggered by the switch and lights the first LED.

The 2nd is triggered by the falling edge of the 1st one-shot and lights the second LED.

The 3rd is also triggered by the switch and its falling edge triggers the 4th which lights the fourth LED

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