latch with inverter isn't latching

ericgibbs

Joined Jan 29, 2010
10,768
Hi 48,
Consider when you apply the High Input pulse, U2 goes High, which applies a High on the input of U1 [ via the 4k7].
When the input pulse goes low, the pulse source which has only low internal 200R resistance, pulls U1 input Low, so U2 out goes low.!

Where did you get the circuit.?

E

BTW: You have the Input pulse powering the Inverters.!
 
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ericgibbs

Joined Jan 29, 2010
10,768
hi 48,
On the web circuit it uses a mechanical push switch which goes open circuit when not pressed.
Also the junction of the 220k and 47n is linked back to the switch.?

Redo your asc circuit, using the Switch model, which is operated by the Vp,and it should work.
Do you follow OK.?
E

EDIT:
Added the switch.
 

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ericgibbs

Joined Jan 29, 2010
10,768
Works excellent, but this will not work with some high going pulse instead?
hi,
The problem is the LTSpice voltage low driving source impedance, when the pulse falls to 0V, its the same as the mechanical switch being pushed again.

Exactly what do you plan to do with the circuit, there may be an alternative method.

E
 

Alec_t

Joined Sep 17, 2013
11,640
The simple post #11 circuit doesn't have a reset arrangement, so can't toggle on successive inputs.
Can you use a CD4013-based circuit instead?
 

ericgibbs

Joined Jan 29, 2010
10,768
The simple post #11 circuit doesn't have a reset arrangement, so can't toggle on successive inputs.
Wanted to power a led with a latch when a pulse comes into the latch. The latch would drive some low side transistor to power the led.

hi alec,
As far as I can understand the TS, he is not asking for a Reset function, just a latch On.
E
 

Alec_t

Joined Sep 17, 2013
11,640
The latch circuit shown is presumably starting up in the latched 'on' state. If you don't want that, you can use a .ic command to set node a to an appropriate starting voltage.
 

ericgibbs

Joined Jan 29, 2010
10,768
hi p48,
For the LED driver, do you want to toggle the LED On for one Vp input pulse and Off for a following Vp input pulse and so on.? ie: On, Off, On ......
E
 

AnalogKid

Joined Aug 1, 2013
8,590
As far as I can understand the TS, he is not asking for a Reset function, just a latch On.
Without a reliable power-on-reset, that has limited usefulness IMnsHO. It might be that the TS actually wants an electronic toggle FF (alternate-action circuit). If only there were some way to find out... Hmmm ...

pager48 -

How do you want the circuit to function?
When it is first powered up, do you want the output on or off?
Let's assume it powers up in the OFF state. When a signal pulse comes in, the circuit toggles to the ON state. OK, then what?
How do you turn the output OFF again? Do you want it to go off when the next signal arrives? That is, toggle between ON and OFF states with each input pulse?
Or just stay on until power is removed?
Or something different?

ak

just noticed that eric asked the central question. Oh well, the electrons are already dead.
 

ericgibbs

Joined Jan 29, 2010
10,768
post 18.
For the LED driver, do you want to toggle the LED On for one Vp input pulse and Off for a following Vp input pulse and so on.? ie: On, Off, On ......
E
hi AK,
If you are intent on quoting me, at least use the latest one I posted, post #18. in an effort to find out what the TS is really asking, rather an out dated earlier post.

E
 

ericgibbs

Joined Jan 29, 2010
10,768
hi AK,
Thanks for the reply.

The TS has already made it clear that he wants the latch to be Set when he sends it a voltage pulse.
The output will be used to drive an LED to indicate power on.
Initially the TS was trying to use a voltage pulse to Set a latch he had found the web.

Unfortunately the design of that circuit required a mechanical push switch and it would not work with a input voltage pulse.

I posted a simple latch which could Set with a positive going pulse.

@Alec_t raised a good point which had not be addressed, ie: does the TS want to have the the ability to Rest the latch with a further/next input pulse.

This is why made post #8, to check the Set/Reset latch requirement.

If you read thru all the posts, the TS basic queries were answered by the posts.

As you know 90% of the job, is trying to find out what a TS is trying to do.:rolleyes:

Eric
 
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