Troubleshooting NE555 astable operation

Thread Starter

srecenvid

Joined May 10, 2018
31
Hello everyone. This is my first post here, but it is not going to be last one for sure :)

I am trying to build astable output circuit with NE555. It should be simple and no problem circuit application. I read manual and selected resistors and capacitors for 600hz output operation. C1=0.33uF R1=1kOhm R2=3k0hm(pot and i can adjust it).

I used Eagle to and this is circuit(pic1) and basic layout(Pic2) i am using.

This is only one part of final pcb, but for testing i soldered only elements needed for timer to work (ne555, 3 resistors and 3 capacitors). (pic3)

When connected to 5v power supply and Oscilloscope connected between Pin3(out) and Pin1(gnd) i get nice 600hz +-10% signal (pict4). Problem starts after 10-20s when all i get is 5v without switching off. (pict5)

When power supply is disconnected back pwm signal is back for next 10-20s and after few sec its only 5v.

I tried different resistor values, and i tried different NE555 ic. but result is still the same. I am using multilayer unipolar (Non-polarised) capacitors.

I have no idea how to solve this problem. So any help is welcome and i am sure there is someone who knows what mistake i have done and how to fix it.
 

danadak

Joined Mar 10, 2018
4,057
What is the voltage on the CV pin when it stops running ?

Why is the scope timebase stopped in the pic, no longer triggering ?

Regards, Dana.
 

dl324

Joined Mar 30, 2015
14,691
Welcome to AAC!

It is preferable to host pictures on this site so they're available for future reference.
I used Eagle to and this is circuit(pic1) and basic layout(Pic2) i am using.
The relevant pictures:
Circuit:
upload_2018-5-10_13-2-55.png
Waveforms:
upload_2018-5-10_13-3-24.png

Why are you making boards for circuits before you've verified that they work?

What is the voltage on the trigger input when the timer stops?
 

Thread Starter

srecenvid

Joined May 10, 2018
31
Welcome to AAC!

It is preferable to host pictures on this site so they're available for future reference.

The relevant pictures:
Circuit:
View attachment 152280
Waveforms:
View attachment 152281

Why are you making boards for circuits before you've verified that they work?

What is the voltage on the trigger input when the timer stops?
Thank you. I will host pictures directly on this site from now on.

I tested most parts of circuit except NE555 timer -.- ...i thought it's simple part and can't cause any problems... i guess i was wrong.
 

danadak

Joined Mar 10, 2018
4,057
I have had problems that were intermittent with cheap pots
before, is the wiper losing contact when pot heats up just
a little ? Or the contacts at end of pot carbon strip poorly
formed....

Regards, Dana.
 

Thread Starter

srecenvid

Joined May 10, 2018
31
Thank you for all help so far,

i changed again capacitor and i also changed now pot with fixed resistor. Output seems stable. Is it normal that frequency is changing over time. When cold started it was 607 hz now after few min it is on 625 and its increasing.

EDIT (after 15 min we are back and we have same problem as before).

No components are heating or at least when i checked all of them i could not say something is getting warm.

also about R7, it is part of basic circuit provided on NE555 manual.
ne555.png
 
Last edited:

MisterBill2

Joined Jan 23, 2018
12,402
Indeed a leaky capacitor for either C1 OR C2 in the circuit will cause a problem. Because the voltage on pin5 is the reference for the comparator a leaky cap there will cause funny problems. You can easily prove or disprove a bad control pot by substituting a fixed resistor. If the problem stops that was it.
 

AnalogKid

Joined Aug 1, 2013
10,096
Problem shows up after continuous operation; problem goes away after a non-operating period. Might be a thermal issue.

As a temporary test, increase R6 from 1K to 10K. This will decrease the output frequency, but also relieve stress on the discharge transistor. If the problem goes away, increase R5 by 10x and decrease C1 by 10x to get back to the original operating range.

ak
 

Thread Starter

srecenvid

Joined May 10, 2018
31
Problem shows up after continuous operation; problem goes away after a non-operating period. Might be a thermal issue.

As a temporary test, increase R6 from 1K to 10K. This will decrease the output frequency, but also relieve stress on the discharge transistor. If the problem goes away, increase R5 by 10x and decrease C1 by 10x to get back to the original operating range.

ak
Thank you for all help again :) really helpful forum so far.
I tried to change R6 as suggested and it did help a bit bit not for long... i just prolonged working time.

I was also checking other threats on this forum about 555 timer and i found some people suggesting electrolytic 1uF capacitor should be used between Pin1(gnd) and pin8(vcc). The same idea is found also on 555 manual (10.6 layout example) 555tim.png

So after i used 1uF capacitor as described i am running PWM signal without stop for last 2 hours. For now it looks ok.
But can someone explain to me why i am getting frequency shift after time? i started with frequency 612hz and now i am up to 620hz.
Small temperature changes? Capacitor charging properties?
 

danadak

Joined Mar 10, 2018
4,057
Yes, a 555 Timer is not a high precision device, coupled with passive
component drift, probably yields over T and V a 2 -5% design.

What is your design goal, should you be thinking about a crystal oscillator
clocked PWM ?

Regards, Dana.
 
Last edited:

Thread Starter

srecenvid

Joined May 10, 2018
31
Yes, a 555 Timer is not a high precision device, coupled with passive
component drift, probably yields over T and V a 2 -5% design.

What is your design goal, should you be thinking about a crystal oscillator
clocked PWM ?

Regards, Dana.
Thank you Dana,

i am trying to use 555 timer output as step signal for stepper board, I will be using constant speed which is adjusted once and it runs always the same. For stopping and starting i will use enable pin on stepper driver + direction signal.
 

Alec_t

Joined Sep 17, 2013
12,894
If your timing capacitor is an X7R type ceramic one then its value will change by about 1% per 6°C temperature change.
 

AnalogKid

Joined Aug 1, 2013
10,096
Yes, a 555 Timer is not a high precision device, coupled with passive component drift, probably yields over T and V a 2 -5% design.
The genius of the 555 design is that its internal accuracy is very good, better than its design simplicity would suggest and way better than other timing parts of its era. The circuit relies on three 5K resistors (hence the name 555). While IC technology in 1970 could not produce an accurate resistor without expensive trimming processes, it could produce extremely accurate resistor *ratios* with no special methods. Magazine articles at the time emphasized that the majority of a 555 circuit's accuracy was dependent on the external components, and that the 555 contributed "less than 1%" to the output error.

An R-C timer circuit of any nature that drifts less than 3% as it warms up is functioning normally. More stability than that will require a ceramic resonator or crystal.

ak
 
Top