I need help with a push button...

[raghuprabhu]

I am a newbie in electronics.

I need a circuit (which I can use as a subcircuit) in Logisim 3.8.0

When the pushbutton is clicked ONCE, the LED should come on and STAY ON. Here when the clock sticks over, the LED turns off off. I want the LED to stay on!

When I click the push button the LED should tun off and not before!

Using the following circuit, I have to click the push button and hold it down for a couple of cycles of the clock for the LED to come on.

Similarly when the LED is on, I have to click the push button and hold it down for a couple of cycles. How can I make it instantaneous please?

Thank you in advance.

 

[ZipZapOuch]

You'll need to connect the reset to ground. If it needs to be useful with a push button, let me know. If your LED switches when you RELEASE the button instead of when you press the button, swap the 10k resistor and switch.

 

[raghuprabhu]

Thanks ZipZapOuch,

I am building a circuit for a stop watch interface.

When I click the push button 'Star / Stop", the LED "Clock Started" should light up.

It is happening in my case but after a delay and I have to hold the push button down for a long time.

When I click the "Start / Stop" again the LED should shut down instantly instead of me holding the push button for a long time. (2-3 seconds)

How do I get rid of this delay?

The following is inside the start subcircuit.

Thanks

 

[ZipZapOuch]

You need to adjust the button to be either logic high or logic low in the unpressed state and the opposite in the pressed state. You need to ether connect as shown in my schematic with "pull up" resistors OR you need to possibly right-click to set the simulator to know what the button does (I'm assuming your complaint is about the simulator and not a real-life circuit). If it is a real life circuit, you definitely need the pull-up resistor and insure that the button is actually connecting the chip's pin to 0V or logic high voltage (1).

 

[raghuprabhu]

Thanks ZipZapOuch

 

[rjenkinsgb]

Note that with a real-world switch, the contacts will bounce, opening and closing a few times within a millisecond or two.

If you are trying to design a practical circuit that you can later build, you have to allow for that.

One simple approach is to use a D type flip flop, with the /Q signal connected back to D by a series resistor, and with a small capacitor to ground from D.

Something like a 220K and 0.1uF should do with a CMOS D type; that delays the signal at D a few milliseconds so switch bounce does not cause it to keep toggling, but another press after that delay effect will cause another toggle.

The switch and its pullup or pulldown still connect to clock input.

 

[danadak]

Do not tie directly to CMOS inputs large caps :

CMOS Logic input capacitor maximum value - Logic forum - Logic - TI E2E support forums​

Hello experts, I got general question of maximum input capacitor value for CMOS logic (e.g. HC, AHC, HCT family) input pin. Sometimes, we add capacitor at

e2e.ti.com

https://toshiba.semicon-storage.com/info/application_note_en_20210131_AKX00111.pdf?did=63525


Regards, Dana.

 

[rjenkinsgb]

Do not tie directly to CMOS inputs large caps :

That should read:

Do not tie directly to CMOS inputs large caps, in circuits that have other loads on the same supply that can cause an unusually fast supply voltage drop.

It also says LARGE caps; 0.1uF is not large in that context.

 

[danadak]

Regards, Dana.

 

[rjenkinsgb]

Statements such as you quote, without full context to which conditions they apply, are meaningless noise.
They are dependant on the logic series or device type, and the power conditions.

eg. Some TI data sheets permit a maximum of 1uF connected directly to a CD4000B series input.

And, any limiting requirement is totally dependant on the rate of fall of the supply voltage. The rate of fall must be fast enough that the capacitor discharge current exceeds the permitted input protection current, for there to be risk of damage.

Not all inputs can work with external resistors anyway, in which case external bypass diodes are appropriate.

 

[danadak]

The quote is from the Toshiba Ap note I posted links to earlier. Let Toshiba know they are meaningless.

Statements such as you quote, without full context to which conditions they apply, are meaningless noise.
They are dependant on the logic series or device type, and the power conditions.

Really, none of us knew there was a difference in device geometries, process, power supply
affects, protection network used internal, speed, why that is a revelation worth shouting to
the world and a Nobel prize.....


Regards, Dana.

 

[ZipZapOuch]

Thanks ZipZapOuch

Did you get it to work? Was it a real circuit or a simulation?