In the laboratory of Digiital circuits, we conduct two separate exercises dedicated to latches and Schmitt triggers.
But I am deeply convinced that there is no principal difference between these two groups of circuits since each latch can be made act as a Schmitt trigger and v.v., each Schmitt trigger can be made act as a latch.
For example, in 2008, we did this "magic" with one of my student groups:
https://en.wikibooks.org/wiki/Circuit_Idea/Group_68a#Lab_4:_Op-amp_trigger_circuits_with_positive_feedback
I think it would be interesting to discuss here what is common and what is different between these circuits... and what needs to be done to turn one into another...
Both the circuits, Schmitt trigger and Flip flop have positive feedback. A bistable multivibrator circuit can be converted to a Schmitt trigger.
Hi Nikolay,
Thank you for your participation. With your comment you have started a very interesting discussion about the input properties of these circuits. Here are my insights...
I argue that both circuits are practically the same circuit with hysteresis; only the way we drive it is different.
We can drive them both through one input by two opposite signals and through two inputs by two equal signals.
Regardless of the way we apply the input signals, we can distinguish three states (combinations) of the input signals - two active and one neutral (rest), which correspond to the three regions of the characteristics. The active states correspond to the outer parts of the curve and the neutral state corresponds to the middle part... and this is the place where the circuit has memory.
So, if we want to use it as a memory cell (latch), we can drive it through one input (at one circuit point) by two active input signals - high S (above the high threshold) and low R (below the low threshold), and one neutral signal (between the two thresholds, usually in the middle).
Now we just need to specify what a latch it is - RS or D (as in the magnetic implementation in the attached movie and picture):
https://photos.app.goo.gl/RRqX7bvpqKfZkTcn1
With the same success we can drive this memory cell (latch) at two different circuit points (inputs S and R) by applying an active input signal with high level (above the high threshold). The neutral state here is when both input signals are inactive (inside the hysteresis cycle, between the two thresholds).
A magnetic implementation of this idea is shown in the attached movie and picture:
https://photos.app.goo.gl/mDM4k3HaYuDCa2DL2
BTW the "philosophy" above how we can control this type of memory cell is closely related to the question if it is possible to make an asynchronous D flip-flop:
https://www.researchgate.net/post/Is_there_an_asynchronous_D_flip-flop
Dear Aparna,
It is so nice to see that there are "circuit philosophers" who appreciate eternal circuit ideas... rather than tinkering their mortal specific implementations:)
In the beginning of this lab, I say to my students that (1) hysteresis is the primary great phenomenon here... and (2) it is implemented by means of another great phenomenon - the positive feedback... to obtain first (3), the famous Schmitt trigger.. and then (4) to convert it to the memorizing latch... After, we group latches in registers (5)... and then registers - in SRAMs (6)... to make computers (7).
So, if there was no hysteresis and positive feedback, there would be no Schmitt triggers ... latches... flip-flops... registers... SRAM... computers... internet... ResearchGate... :) That is why it is worth doing this...
But after this my exciting tirade, they look at me cunningly and ask, "Why then, after these ideas are so great, there are so few participants in these questions?"
And what should I answer them then:)?
Or what gives us more benefits - money, career, power... :)? Because "bare ideas" give us nothing:)?
Still, "Are the Schmitt trigger and RS latch the same devices"?
While looking for the answer to this interesting question, we can see another common (now topological) idea in them - they both consist of two connected in a loop amplifying stages. Only in the RS latch they are both inverting, while in the Schmitt trigger both are non-inverting...
We can understand the need of cascading two inverting stages in the case of an RS latch - to make a non-inverting amplifier... to apply a positive feedback:
https://www.researchgate.net/post/How_do_we_present_in_an_attractive_way_the_basic_idea_behind_the_most_elementary_memory_cell?
But why the hell two non-inverting amplifiers are cascaded in the Schmitt trigger... since already there is a non-inverting amplifier?
@Dear Rana,
Schmitt trigger and RS latch are the same device - an amplifier with "self-reinforcing" positive feedback ... but it is driven differently and used for different purposes under different names in the two cases. Here is the "philosophy" behind this device.
In the Schmitt trigger, we use a circuit input that we control with two oppositely changing values ("up" and "down") of one input signal. In the RS latch, we use two circuit inputs which we control with unidirectionally changing values of two input signals.
In the Schmitt trigger, we forcefully hold the input signal at one of its two active states (levels) but are mainly interested in the transition. In the RS latch, we return the input signal back to its passive state and release the memory cell.
If we return the input signal of a Schmitt trigger to its passive (middle) level, it will become a latch (memory cell).
Schmitt trigger has one input, RS latch has obviously two inputs R and S. Both however can preserve one bit of memory.