74121 Monostable not working properly

Thread Starter

danicdb

Joined Dec 4, 2020
11
Hello there!

I have a traffic light system assignment to do and so far I think it is done, at least on paper :D I used state machine logics to get it done.
The thing is my timer circuit does not work properly and it is mainly because I cannot get the 74121 to work as I want it. I am trying to get different timings for the different states in the state machine. I checked the 74121 datasheet many times and I cannot see what connection I am doing wrong. As I checked, while connecting A1 & A2 straight to ground and clock signal to B with its RC configuration to Cx & Rx/Cx inputs, I should get a pulse according to the RC configuration, however I do not. What I get is an active pulse with glitches (see picture).
I have tried many times with different clock signals input and it does not change. I thought using a 777 as monostable but i would rather to get the timers with no switch need. I also tried to simulate it in multisim and proteus with similar responses.

Could you guys give me same light on this, please?

Please, find enclosed the transient analysis (4s transient.png) and the design (timing short.rar) for a 4s timer with its proper RC configuration (I used the .7*R*C formula to get the active pulse width....t=.7*560e3*10e-6=3.92s-->almost 4s)

PS: Sorry for any basic mistake I could be making.
 

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Thread Starter

danicdb

Joined Dec 4, 2020
11
Thanks for the warm welcome Dennis!

Welcome to AAC!

Post the schematic so we don't have to extract something from your .rar file.
Sorry about it but as it does not allow me to upload multisim files, I thought it could be in a rar.

I have just done it in pspice, however I do not know if it does work as I am newbie in pspice.
But basically those are the components from my circuit.
 

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dl324

Joined Mar 30, 2015
12,791
The timing resistor is larger than the recommended maximum of 40k. Try decreasing the resistance and increasing C.

I question whether Multisim is capable of simulating the one shot correctly, but following limitations in the datasheet can't hurt.
 

crutschow

Joined Mar 14, 2008
26,996
I checked the 74121 datasheet many times and I cannot see what connection I am doing wrong.
You need to look at the data sheet not just for the connections, but also for the limits of the components connected.
Using a timing resistor more than 10 times the recommended value is not the way to get the circuit to operate correctly.
 

Thread Starter

danicdb

Joined Dec 4, 2020
11
The timing resistor is larger than the recommended maximum of 40k. Try decreasing the resistance and increasing C.

I question whether Multisim is capable of simulating the one shot correctly, but following limitations in the datasheet can't hurt.
¬¬!
Sorry about it. You are right, my RC was above the recomendation.
I got the right configuration now with comercial values for R and C as 22k and 220u, respectively, and it should give me a 3.388s pulse. However what I get is an constant active pulse with glitches every 200ms (aprox).
Tried in mutlisim and in proteus with similar results (wrong ones), See attached pictures
Keep not knowing if the circuit really works in pspice.
 

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dl324

Joined Mar 30, 2015
12,791
I got the right configuration now with comercial values for R and C as 22k and 220u, respectively, and it should give me a 3.388s pulse.
I breadboarded the circuit with 24.3k and 220uF and estimated the pulse to be around 4 seconds.
 

Thread Starter

danicdb

Joined Dec 4, 2020
11
I breadboarded the circuit with 24.3k and 220uF and estimated the pulse to be around 4 seconds.
Great! What a relief :)
Funny thing that neither of both softwares are able to simulate this device properly. And actually, I will have to modificate the circuit because I need to "prove" the whole circuit works as it should be. I think I will simulate the 74121 as a function gen and a switch as I only need active pulses of a specific length in specific moments.

Thanks for your help guys and specially to Dennis ;)
 

dl324

Joined Mar 30, 2015
12,791
Funny thing that neither of both softwares are able to simulate this device properly.
That's the thing about simulators. You need to know when it's giving inaccurate results and invest enough time to learn how to run the simulator. That's my failing. If I simulate a circuit and don't get the expected results, I breadboard it and move on. Aside from using logic simulators to avoid the expense of buying components and the tedium of breadboarding, I like to design analog circuits with the simulator between my ears and breadboard to prove it works.
 

Thread Starter

danicdb

Joined Dec 4, 2020
11
That's the thing about simulators. You need to know when it's giving inaccurate results and invest enough time to learn how to run the simulator. That's my failing. If I simulate a circuit and don't get the expected results, I breadboard it and move on. Aside from using logic simulators to avoid the expense of buying components and the tedium of breadboarding, I like to design analog circuits with the simulator between my ears and breadboard to prove it works.
I have spent almost 2 weeks with this issue, looking for any similar problem through forums and websites and I could not find anything regarding a problem within the simulation. That is why I decided to write in this forum. If I had had the components at home, I would have breadboarded it and as you wrote, move on. I think I will go for it next time if it happens something similar ;)

Show the trigger pulses.
I think there is something wrong with your simulation setup, not with the simulator.
You may be right.
I am using multisim as the simulation software and this picture is the oscilloscope screen. Hope it brings the info you are looking for, as I am quite newbie with this. :) Otherwise, please let me know.
 

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Thread Starter

danicdb

Joined Dec 4, 2020
11
Show us a screen with the trigger pulse(s) and the output of the one shot.
The following pictures show the trigger input pulse in 10kHz (I uploaded 2 pictures, "trigger input 10kHz.PNG" which is synchronous with the output. And "trigger input 10kHz ZOOM.PNG" which is the same signal however with some zoom to be able to apreciate the pulse width) and the output signal that I get from the circuit (uploaded too as "Output.PNG").

I simulate 10 seconds and it should be a 2 pulses of almost 4 seconds each in the output signal, but as you can see guys I am getting an active pulse with glitches every 200ms.
 

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dl324

Joined Mar 30, 2015
12,791
The following pictures show the trigger input pulse in 10kHz
Try changing your trigger frequency to be slower than the one shot period. The datasheet says it's non-retriggerable, but it might not be well behaved enough to not glitch the output.

To check the behavior in the simulator, you can put a probe on the node with the timing R and C. What I observed was a voltage of about -4V when the OS was triggered and rising (seemed linear) with a voltage reversal when it times out.
 

Thread Starter

danicdb

Joined Dec 4, 2020
11
Try changing your trigger frequency to be slower than the one shot period. The datasheet says it's non-retriggerable, but it might not be well behaved enough to not glitch the output.

To check the behavior in the simulator, you can put a probe on the node with the timing R and C. What I observed was a voltage of about -4V when the OS was triggered and rising (seemed linear) with a voltage reversal when it times out.
Right, I adjusted the trigger frequency to 5Hz (which is almost the period of the output signal got before of 200ms) and I got the signal shown in picture "trigger input 5Hz plus RC node.PNG". Actually, in the picture I am able to se as they are not glitches. It seems like a train of pulses where its pulse width increases until to reach a bit more than 200ms and then starts over a new train of pulses. I am not able to understand why it behaves like that but it seems related to the tau (RC relationship) as it follows it. However, it should not, because as I understood from the datasheet, with the RC configuration it should give a specific output but it does not.
I also uploaded the simulation with 4Hz as the trigger frequency and it seems that it does not get the same output with the same voltage in RC node.
 

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dl324

Joined Mar 30, 2015
12,791
Right, I adjusted the trigger frequency to 5Hz (which is almost the period of the output signal got before of 200ms)
If the one shot period is 3.4s, you need the trigger signal to be slower than 0.29Hz. 5Hz gives you trigger pulses every 200ms and that isn't what you want.
 
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