NOR Gate S-R Latch help

WBahn

Joined Mar 31, 2012
29,979
You're absolutely right, as it turns out, I was using mosfets, not BJT's
Be sure to learn this most valuable lesson.

Consider what I suggested in Post #13: "The first thing I would suggest you do is take out a blank sheet of paper and sketch out the circuitry as actually wired."

We tend to assume that what we did we did correctly. Don't let that assumption get you trapped. Once you hit a stumping point, very quickly throw all of your assumptions out the window and check everything as it was actually done, not as it was supposed to be done. Don't assume that a resistor is 1 kΩ just because it is supposed to be a 1 kΩ resistor -- check it (either visually via the markings or via measurement). Don't assume that a transistor is a 2n3904 just because it's supposed to be a 2n3904 -- check it.
 

dl324

Joined Mar 30, 2015
16,846
You're absolutely right, as it turns out, I was using mosfets, not BJT's (I didn't realize they were mosfets from the original NOR Gate SR Latch)... but I've replaced them with the proper 2N3904's this time...
I wired up the circuit using 2N7000 N channel MOSFETs and verified that it works. Took me about a couple minutes.

Substituted NPN transistors for the MOSFETs and confirmed that that also works.

I used 1K current limit resistors and 100K resistors. Beta for 2N3904 may be too low for 100K resistors, but 10-50K should work; though you'll have the "off" LED dimly lit without the 1K parallel resistor shown earlier. I used a house marked NPN that has a beta 2-3X higher than 2N3904.
 

Thread Starter

JacobEdward

Joined Apr 29, 2015
23
The values for nodes C and D don't make sense. They should either be around 0.7V or 0V.

I assume the circuit still isn't working. I haven't and won't watch your videos. I'm retired and have a lot of discretionary time, but I won't waste it watching videos.

There are only 2 transistors; if you understand how it works, it shouldn't take more than a few minutes to wire or troubleshoot.
The videos are more for my benefit, just as a means to document my learning for later on, thank you so much for your advice, it really helps! I'm still unsure what I should do however, I suppose I can do yet another test to see if all the components aren't malfunctioning, but I'm pretty sure they're not...
 

Thread Starter

JacobEdward

Joined Apr 29, 2015
23
I wired up the circuit using 2N7000 N channel MOSFETs and verified that it works. Took me about a couple minutes.

Substituted NPN transistors for the MOSFETs and confirmed that that also works.

I used 1K current limit resistors and 100K resistors. Beta for 2N3904 may be too low for 100K resistors, but 10-50K should work; though you'll have the "off" LED dimly lit without the 1K parallel resistor shown earlier. I used a house marked NPN that has a beta 2-3X higher than 2N3904.
Could it have been the voltage level? I know 6.4 V is a little high in the acceptable range... I'll switch back to the 2N7000 mosfets just to be sure we're using the same transistor.
 

Thread Starter

JacobEdward

Joined Apr 29, 2015
23
Be sure to learn this most valuable lesson.

Consider what I suggested in Post #13: "The first thing I would suggest you do is take out a blank sheet of paper and sketch out the circuitry as actually wired."

We tend to assume that what we did we did correctly. Don't let that assumption get you trapped. Once you hit a stumping point, very quickly throw all of your assumptions out the window and check everything as it was actually done, not as it was supposed to be done. Don't assume that a resistor is 1 kΩ just because it is supposed to be a 1 kΩ resistor -- check it (either visually via the markings or via measurement). Don't assume that a transistor is a 2n3904 just because it's supposed to be a 2n3904 -- check it.
Alright, will be making the drawing later today, thanks for the help
 

dl324

Joined Mar 30, 2015
16,846
Could it have been the voltage level? I know 6.4 V is a little high in the acceptable range... I'll switch back to the 2N7000 mosfets just to be sure we're using the same transistor.
For some reason, I thought at least one of the two schematics posted used a 9V supply, so that's what I used. A lower voltage would make low beta more problematic.

I'll try 5V and get back to you. But, at most, it would only require the 100K resistor to be made smaller for the BJT case and make no difference for the MOSFET version.
 

dl324

Joined Mar 30, 2015
16,846
it's this specific "hysteresis" latch/memory phenomenon which is the trouble...
There is no hysteresis in this flip flop. A switch press turns off one transistor and turns on the other. If you try to turn off a transistor that's already off, nothing happens.

Do as WBahn suggested and draw a schematic of your circuit as wired. Don't make assumptions about the component values you used. For whatever isn't working, take voltage measurements and address anything that doesn't make sense.
 

Jony130

Joined Feb 17, 2009
5,487
I've successfully built the NOT, AND, OR, NAND, and NOR gates with all the components I'm currently using... it's this specific "hysteresis" latch/memory phenomenon which is the trouble...
This circuit do not have any "hysteresis". But this circuit have a positive feedback instead .
And tell me what resistors values do you have now ??
 

Brownout

Joined Jan 10, 2012
2,390
I've successfully built the NOT, AND, OR, NAND, and NOR gates with all the components I'm currently using... it's this specific "hysteresis" latch/memory phenomenon which is the trouble...
The problem, however, is that in your circuit, composed of two cross-coupled inverters, neither of the simple inverters are acting correctly. If you can get one to work, then replicate it with a 2nd version and cross-couple them, then you're done. Since others have successfully breadboarded your circuit, it is clear the design is valid, but some error on your particular build is preventing your from working. By backing up and building and then validating the stages in a step-by-step fashion, you'll most likely find your error.
 
Top