# 555LED fading circuit simulation problem.

Discussion in 'Homework Help' started by justinjunghkim, Dec 24, 2014.

1. ### justinjunghkim Thread Starter New Member

Dec 23, 2014
7
0
HI guys.

I have a problem to simulate a simple 555 circuit with OrCad tool. Output of 555 is closet to zero( see red scope) I checked my circuit multiple times, and I didn't see a problem. What went wrong? I know the 555 that I used in the schematic works properly, because I used same one to build a square wave output, and it worked.

As you see in my circuit I didn't use a LED, because all LEDs that I tried to simulate with said " ERROR -- Less than 2 connections at node N062761" whenever I ran the simulation. In my library there are so many different type of LEDs but none of them work. I want to use LED with FV=3V. What should I use?

Question Summary:
What's wrong with my circuit?
What component of LED should I use for the simulation?

Merry Christmas!

Justin

File size:
83.1 KB
Views:
38

Apr 5, 2008
15,534
2,300
3. ### justinjunghkim Thread Starter New Member

Dec 23, 2014
7
0

I tried not connecting pin7, but it still didn't work. There was green circle at the pin 7 after running, and it showed me a message: ERROR -- Incorrect number of interface nodes for X_X1.
I read article that you linked, and I also found project I need. There is 555 fading led circuit with calculation detail. It's just what I need. However all projects that I found seem don't have a problem running the simulation.
I know the circuit works, because I built it and it worked, but I don't understand why simulation didn't work.

4. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,316
1,109

Neither of the diagrams work in the PDF work, the pictorial or the schematic.

Did you build the hysteretic oscillator from the link that Bertus provided? It does work.

5. ### justinjunghkim Thread Starter New Member

Dec 23, 2014
7
0
JoeJester,

The problem that I have now is simulating Orcad. If you see the schematic from the link that I provided or Bertus provided above, pin 5 and 7 are not connected to anything. I searched different schematics for the same purpose of circuit, but every single circuit that I found doesn't connect pin 5 and 7 to anything.
When I simulated, the orcad showed me an error message : ERROR - Incorrect number of interface nodes for X_X1 (555circuit). Also green disks appeared on the nodes of pin 5 and 7. I also connected both pin 5 and 7 to the ground. At this time it didn't show me an error message, but output of 555 is really low. I will attached file of my simulation and its result.

Thank you.

File size:
66.2 KB
Views:
24
6. ### MrCarlos Active Member

Jan 2, 2010
397
132
Hello justinjunghkim

The circuit you show in your post #5 does not work in the OrCad simulation for several reasons.

The voltage between terminals B-E in the transistor is too low for the Trigger and Threshold inputs in the 555.
So not oscillate.

If your OrCad simulator have meters, add a voltmeter to measure the voltage of the battery. Must be 9V.
Also measures the base current of the transistor.

The value of current limiting resistors for LED’s should be calculated according to the electrical characteristics of these.
(Vcc - VF) / IF = Rx.
It is said that when a current, IF, flowing through the LED is a voltage drop VF in it

Compare Bill_Marsden’s circuit against your design, you will see that there are several things to improve on yours.

File size:
490.5 KB
Views:
24
7. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,316
1,109
You might want to consider changing R1 and C1 to 1k ohm and 4.7 millifarads. This will increase the drive current to the two transistors.

I just put jumper pins on 5 and 7 so I didn't get an error. They are titled NCa and NCb meaning No Connection.

One other point, the RC time constant is about 4.4 seconds, so you definitely need to look at more than 10 seconds ... I would run the simulation time for 60 seconds or 30 seconds. Also tell the simulation to start with zero initial values.

File size:
26.8 KB
Views:
43
Last edited: Dec 27, 2014
8. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,316
1,109
Mr Carlos,

Bill's design is two flashing LEDs. The design the OP has is for fading LEDs. It would be a lot easier if they were just flashing LEDs.

9. ### WBahn Moderator

Mar 31, 2012
17,450
4,701
Can you post the actual netlist? That will let us determine exactly which node N062761 is and to see what it is connected to.

10. ### MrCarlos Active Member

Jan 2, 2010
397
132
Hello JoeJester

Yes I know,
What I try to point out to the OP is that the voltage between base and emitter of the transistor lets not work at 555.

That's all.

11. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,316
1,109
His complaints of errors could be just the two open connections. The OP kept insisting that he had a working model but never showed us a picture of how that "working model" was connected.

Hopefully he will garner enough to get his stimulation working.

12. ### justinjunghkim Thread Starter New Member

Dec 23, 2014
7
0
Thank you for great help!

I was working on whole day and I finally figured it out what happened to my circuit.
There was two major problems for my simulation.
First problem was my value of R2. There is the resistance range of R2 that I can use and it's totally depends on R1. I think R1 changes gain of transistors. I don't know equations to support my thought. However in the linked below explains how the gain changes range of output resistance.
In the calculation the person assumed that the gain is 100. He calculated resistance range as follow.

$R_{min} = \frac{U_{R,max}}{i_{R,max}} = \frac{6 V}{10 mA} = 600 \Omega$

$R_{max} = \frac{U_{R,min}}{i_{R,min}} = \frac{3 V}{0.4 mA} = 7500 \Omega$

I manually changed resistance to find my max resistance R2 given R1 is 150. I found out after R2 = 13k, the output goes to the rail.

Second problem was pin 5 must go through a capacitor before it connect to the ground. I found out from one of random figures of different type of 555 circuit schematics for Orcad simulation. I still don't understand why, because I don't need it for the real design.

I attached a my new circuit and simulation.

I would appreciate if someone explain detail role of R1 and why I need capacitor at pin 5.

Thank you.

File size:
66.1 KB
Views:
20
13. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,316
1,109
Now that you got it "working" you can remove that capacitor from pin 5 and see if it continues to work. the same applies to pin 7. You might get a wire not connected error because you haven't placed a something on those to terminate the wire (a jumper ... both having separate names)

I see you changed both R1 and R2.

R2 and C2 form the Charge and Discharge time of the 555. Without R1, when the transistor turned on, it would short the 9 V supply out, interrupting the timing cycle.

Your original circuit in post 1 is nothing like your final working circuit. I'll be sure to get more clarity from you the next time you pose a question.

Dec 23, 2014
7
0
JoeJester,