Electronic Lock using 74LS74 DTFF

[marconi]

In your #3, is the note about C7 actually referring to C6? I can see why C6 may be required in a simulation of the circuit to reduce the frequency response of the circuit to avoid high frequency oscillation and also to progressively reduce the impact of high frequency thermal noise of one of the 'state switching' transistors. But I do not see such a low value having much impact using real components where like for like they are not electrically identical as they are when modelled in the simulator.

If you ponder for a moment this - there actually needs to be a minute difference in electrical response between the two halves of the bistable circuit otherwise it would settle happily on a 'knife edge' ie: for exact similarity between both halves of the bistable circuit the transistors would be partially on to the same degree. Real components inevitably provide the minute difference between the two halves.

As a further learning point, one sometimes see capacitors in parallel with R79 and R80 - commutation capacitors is one name for their purpose - which speed up the switching between the stable states of TRleft on & TRright off and TRleft off & TRright on. If you want to learn how they work look at this piece:

Commutating Capacitor or Speed up Capacitor - EEEGUIDE.COM - https://www.eeeguide.com/commutating-capacitor-or-speed-up-capacitor/

 

[_q12x_]

On a funny note... Here is a bit of Romania for you; haha (that dude is not me)

Dragos from Romania_4185993782837515395_n.mp4

www.electriciansforums.net

 

[_q12x_]

In your #3, is the note about C7 actually referring to C6?

Oh shi*/t - yes I didnt see it. It is referring to C6.
You see, the proteus simulator IS auto-renaming a lot. And is good for the most part. But not in my case with my static notes.....
I get a very nasty error -sometimes- in this simulator. Read carefully the timestep number. Is to the power os -17!!!! its enormous. So the simulator is doing some monkey bizniz behind the scenes.

My solution to add this cap there, on that node, was to delay that extreme small number.
It is a simulator thing. Not a real thing !
But I also get your point with :
"between the two halves of the bistable circuit otherwise it would settle happily on a 'knife edge' "
very good observation !

 

[_q12x_]

I made a movie about this project. I hope you like it.
I am not allowed to post my video link here on the forums, because marconi asked me to. The reason was 'swearing' and I can not change it at my age. Heh.
Instead, follow the link or search the name in Youtube and you should find it very Easy.

 

[_q12x_]

Mister @marconi
What is the role of the Set pin (4&10) on the 7474 chip? Besides setting or inverse of a reset.
Is it an input? If it is, then I believe it should be linked to + or -. To avoid random triggering. Right?
I believe I RE-solved the circuit problem and make it even smaller than it was and even better in functionality.

This is the new and improved cct !
All those INPUTS (S) representing the SET pin of the FF, are now linked to + rail, directly. This result comes from testing in reality. I was still having problems on the last (4th) FF on the CLK pin, with that bouncing button. But I believe, NOW when all these inputs are set to High, and not floating anymore, might affect the good functionality of the FFs. I think I am right... what you say?

 

[_q12x_]

Dismiss my #14 post. I discovered another -simpler- solution. That movie I posted is no more.

 

[marconi]

Mister @marconi
What is the role of the Set pin (4&10) on the 7474 chip? Besides setting or inverse of a reset.
Is it an input? If it is, then I believe it should be linked to + or -. To avoid random triggering. Right?
I believe I RE-solved the circuit problem and make it even smaller than it was and even better in functionality.
View attachment 107289
This is the new and improved cct !
All those INPUTS (S) representing the SET pin of the FF, are now linked to + rail, directly. This result comes from testing in reality. I was still having problems on the last (4th) FF on the CLK pin, with that bouncing button. But I believe, NOW when all these inputs are set to High, and not floating anymore, might affect the good functionality of the FFs. I think I am right... what you say?
View attachment 107291

Unused input and control pins should never be left floating to avoid spurious operation of the logic circuits.

 

[_q12x_]

Unused input and control pins should never be left floating to avoid spurious operation of the logic circuits.

Thank you.
Indeed. I remembered it way too late. I did mentioned that Im still learning the logic ICs, right? So the very big mistake I did was to Not convert properly the original cct into my ICs that I have ! But is a lesson for me. I spent 2 days on it. I think....

 

[marconi]

Some switch and push button debouncing circuits:

What is Switch Bouncing and How to prevent it using Debounce Circuit - https://circuitdigest.com/electronic-circuits/what-is-switch-bouncing-and-how-to-prevent-it-using-debounce-circuit

How to Switch De-bounce an Electronic Circuit - https://www.circuitbasics.com/switch-debouncing/#:~:text=Switch%20bouncing%20is%20not%20a,changes%20in%20the%20switch%20signal.

You may need such circuits for all your push buttons.

Also put say a 100mF capacitor across the supply rails - a decoupling capacitor - close to where the supply is attached to the board.

 

[_q12x_]

I have an interesting question: