# NOR Gate S-R Latch help

Discussion in 'General Electronics Chat' started by JacobEdward, Oct 2, 2015.

1. ### JacobEdward Thread Starter New Member

Apr 29, 2015
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0
Here's my attempt:

I'm following this configuration:

I'm trying to recreate a NOR Gate S-R Latch with discrete components instead of using an IC (I don't have the IC and I'd also like to prove these little black box's aren't just magic)... The configuration of the NOT gate part of the NOR gate has it so that the output is actually being driven by the input, with the output only being driven LOW when there is a positive voltage on the middle gate lead... this is problematic because, the LED's I'm using will be fried if I allow any more current through, but the output of the two NOT gates need to be strong enough to drive the corresponding gate but not strong enough to fry the LED... when I drive the corresponding gate with the alternate push button instead of the output of the alternate NOT gate, the NOR gate operates as expected... looking for ideas of how to solve this issue...

Another question I have is if this "hysteresis" phenomenon can be used to "remember" a voltage level with an op amp buffer configuration and a DAC? Have the DAC set a voltage on the buffer, then use a couple of transistors to switch the input to the buffer from the DAC to itself, thereby allowing the DAC to set a specified voltage on some other buffer op amp.

2. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
Just use a series resistor to limit the current through the LED. For example, say your LED's rated voltage is 2V, and it's nominal current is 10mA. If you're power supply is 5V, then the resistor is calculated thusly:

R = (Vcc - Vled)/Iled = (5-2)/.01 = 300.
I just can't visualize it.

3. ### crutschow Expert

Mar 14, 2008
13,006
3,232
You need to post the schematic of your circuit.

4. ### WBahn Moderator

Mar 31, 2012
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4,792
Don't expect people to reverse engineer your circuit based on a picture of it. Provide a schematic.

The amount of current needed to flip the state should be minimal -- you don't need to overdrive anything if you are truly implementing two NOR gates. But we can't tell because you haven't provided a schematic.

In theory you could create a sample-and-hold circuit the way you describe, but you will have issues with drift.

5. ### AnalogKid Distinguished Member

Aug 1, 2013
4,535
1,251
As vast and complex as the field of electronic circuits can be, there are only two simple steps to communicate your ideas, ask questions, join a discussion, etc.

1. Draw a schematic.
2. See #1.

ak

6. ### MikeML AAC Fanatic!

Oct 2, 2009
5,450
1,066
Usually the act of drawing out a schematic from a bread-board will make the error glaringly obvious.

7. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,957
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The simplest transistor based S-R latch looks like this

8. ### JacobEdward Thread Starter New Member

Apr 29, 2015
23
0
Thanks! That's a different design from what I was trying, but I'm definitely going to try it!

Sorry everybody else, been wrapped up with life stuff, going to make a schematic asap, ttfn

9. ### WBahn Moderator

Mar 31, 2012
17,743
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Please avoid text-speak. Limit yourself to abbreviations that are WIDELY known outside the space of people that only have thumbs.

10. ### crutschow Expert

Mar 14, 2008
13,006
3,232
Amen to that.

11. ### AnalogKid Distinguished Member

Aug 1, 2013
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Correction: att

ak

12. ### JacobEdward Thread Starter New Member

Apr 29, 2015
23
0
I have the specific circuit you shared in front of me (I used the potentiometer's to get the 10k resistors) and it's not working... I've verified the transistors are performing as they should in different configurations... Here's a clip of what I'm seeing:

13. ### WBahn Moderator

Mar 31, 2012
17,743
4,792
It's very difficult to troubleshoot a video, particularly when it is almost impossible to back out what the actual circuit is (as distinguished from what it was supposed to be).

The first thing I would suggest you do is take out a blank sheet of paper and sketch out the circuitry as actually wired. That will probably reveal the problem. If it doesn't, measure the voltages at all of the relevant nodes (which are the four nodes connected to the 10 kΩ resistors) under the four key operating points -- while the left button is being pressed, after the left button has been released, while the right button is being depressed, and after the right button has been released. Tabulate and post those 16 values and we will have a lot more to work with.

14. ### absf Senior Member

Dec 29, 2010
1,492
372
I sim the circuit on proteus and it works fine.

When the circuit switches on, both LED are lighted. When one button was pressed, one LED would go off while the other one stayed on. Pressing the other button would flip the LEDs.

Allen

15. ### JacobEdward Thread Starter New Member

Apr 29, 2015
23
0
LOL, just more confirmation that simulations and theory do not give you real knowledge of physical processes, experimenting in the physical world gives you real knowledge... could you build this circuit on a breadboard and show me what it does?

16. ### dl324 Distinguished Member

Mar 30, 2015
3,242
619
Analyzing this circuit without breadboarding or simulating is pretty straightforward.

Closing the left switch forces Q2 off. When Q2 is off, current is injected into the base of Q1 through D1, R1, and R3; causing it and D2 to turn on. When the left switch is released, Q1 will force Q2 to remain off.

Closing the right switch forces Q1 off. When Q1 is off, current is injected into the base of Q2 through D2, R2, and R4; causing it and D1 to turn on. When the right switch is released, Q2 will force Q1 to remain off.

The minimum beta of the transistors determines maximum values for R3 and R4.

absf likes this.
17. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,387
1,605
Not very likely anyone will do this. Not for someone not willing to draw a schematic of the work they are working on.

Electronics is a complicated art, and knowledge is transmitted thru three fundamental documents:

The schematic to show what devices are used and how they interconnect.

The layout drawing to show how the parts were placed.

The bill of material (BOM) to show what parts were used.

Given these three documents anyone else can understand what you did to build another. For a simple thing like yours a pencil sketch schematic including part numbers or values will suffice, along with one still picture of the breadboard.

Skipping steps like you did will lead to disaster, as you are currently experiencing.

(BTW, one of the very first circuits I built were just like this flip flop you are making. It is as good a starting point as any.)

Sinus23 and absf like this.
18. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
I can't see any wiring mistakes, though I don't see all the wires clearly.

Could you have the switches wired for "normally closed?" I ask because the LED's light when you press the left switch.

Have you verified the potentiometers are in fact set to 10K?

19. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,957
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Well I build it on the breadboard in 2001, and YES this circuit work as expected.
As for the 10K resistor. You can use larger resistor values up to 100K.
What type of a transistor you've used ? Also modern LED are very "current sensitive" devices. They can light quite bright, even if the current is lower than 1mA. So in order to rule out this case please add resistors (1kΩ....10kΩ) in parallel with the LED.
And I try to build this circuit tomorrow

EDIT
I decided to build this circuit now, and here you have a results.
I use 2xBC337 and 2x10kΩ and 4x1kΩ

• ###### 4.JPG
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Last edited: Oct 20, 2015
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20. ### JacobEdward Thread Starter New Member

Apr 29, 2015
23
0
I understand the theory of what is suppose to be happening (so that isn't the problem), but my empirical observations are showing something different... will do more tests today however, thanks for helping!