# Basic Digital logic circuit wrong sequnce flasher

#### acurry023

Joined Oct 26, 2015
10
I am designing a circuit that requires a flashing LED when performing a wrong sequence. The correct sequence has 2 possibilities. The circuit has 8 switches that initialize D flip flops. The correct sequences are as follows; SW1, SW2, SW3, SW4, SW5 or SW6, SW7, SW8, SW1, SW2, SW3, SW4, SW5. The flashing LED is to be powered by a 555 timer which I already have configured. However for the life of me I cannot figure out how to get the timer to trigger off a wrong sequence. Can anyone help out with this? The requirement to complete the flasher is to use basic logic flip flops or some combination of and/or/nand/nor/xor gates.

I am using multisim for the basic design and then building the circuit with actual hardware.

#### absf

Joined Dec 29, 2010
1,968
Can't understand what you are describing. Show us how the flip flops and 555 and switches are connected...

Allen

#### acurry023

Joined Oct 26, 2015
10

#### acurry023

Joined Oct 26, 2015
10
The timer on the right is the one that needs to trigger the wrong sequence. If everything is done right I can turn on switch 5 and the LED attached to the output will turn on. If done wrong it will not. The right sequence(s) is what I listed above, basically if I were to start the sequence with switch 2 then go to SW1, SW3, SW4, SW5 the LED connected to that timer should flash alerting me to a wrong sequence. I hope that helps. Thank you.

#### absf

Joined Dec 29, 2010
1,968
You can use the RST pin of the 555 to control flashing the LED3. When RST is high the LED should flash and vice versa.

Try putting a NOT gate between output of U9A and RST of 555 and see if it works.

Allen

#### acurry023

Joined Oct 26, 2015
10
In doing that the 555 still would need something powering the VCC. I tried running the output to the VCC of the 555 using the NOT gate and the light flashed until the right sequence is put in but it needs to be the opposite function. Any suggestions for that?

Thank you for the help Allen

#### acurry023

Joined Oct 26, 2015
10
I should say that the LED needs to stay off until the wrong sequence is put in then needs to flash to indicate the wrong sequence

#### absf

Joined Dec 29, 2010
1,968
I did it this way as attached...

Did you not separate out the Rst from the Vcc pin of 555?

Last edited:

#### acurry023

Joined Oct 26, 2015
10
Initially you were right I didn't not have it separated out from the VCC. I hooked it up the way you showed there and its the same as before it flashes until the correct sequence is input then it turns off. The project requires the opposite unfortunately, it should stay off until the wrong sequence is put in then turn on. Maybe I need to change the logic before then to get the desired output?

#### absf

Joined Dec 29, 2010
1,968
OK, since this is homework, I think I have given enough clues for you to continue tracking as where to get this logic signal.

#### acurry023

Joined Oct 26, 2015
10
Yes thank you again it was very helpful I appreciate your time

#### WBahn

Joined Mar 31, 2012
29,184
This is a VERY bad design from a few standpoints, mostly because of the use of gated clocks throughout. Even if it works in MultiSim, it will almost certainly be nothing but a royal headache in an actual implementation.

#### acurry023

Joined Oct 26, 2015
10
We are running into that problem now. I had a feeling that would be an issue during hardware implementation. The project requires 8 different states to be shown by using switches. Would it be better to use them as the D input of the flip flops as opposed to the clock input for physical inplementation? I know this isn't a forum for giving answers but any suggestions are helpful

#### acurry023

Joined Oct 26, 2015
10

#### absf

Joined Dec 29, 2010
1,968
You can google for "combination lock circuits using flip flops" on the interweb and get ideas from there.

Are you allowed to use only d type flip flops?

Allen