Hi,

I have a circuit I built using this as a reference: http://www.ohmslawcalculator.com/555-astable-calculator

Where R1 is 4.3K, R2 is 5.1K and the capacitor is 100nF. The circuit works, but after a few on-off cycles, it starts to flicker when it's on, every two cycles. Here is a quick video showing the issue (you will see me touching caps, but they have nothing to do with this part of the circuit).

As you can see in the attached schematic (the flashing part of things is using the A side of the 556 timer), I am using a transistor to be able to drive a load bigger than the 556 allows (LEDs). I tried adding a 1K resistor in series with the base, a 10K pull down, both, but nothing worked.

I am learning and would greatly appreciate your help.

Regards,

frtrkap

 

hi Francis,
Welcome to AAC.
I see the LED's flickering, what is 'net0' on RST_B.?
How is J1 linked.?
E

 

Hi,

Net 0 is VCC. J1 is a 6 pin male header. I use a jumper (link) to connect 1-2 for the breathing mode, 3-4 for flashing or 5-6 for solid on.

Thank you

 

hi,
You have the transistor configured as an Emitter Follower, so if a Base resistor is used , its usually a low value approx 50R.

Does the flickering occur in all 3 Modes.?
E

EDIT:
Clipped and converted your image, its easier to work with.

 

Hi,

The flickering only occurs in flashing mode. To be fair, "solid on", while being a mode, completely bypasses the circuit, feeding VCC to the LEDs directly.

I will try 50R.

Regards,

 

Hi,

50R in series with the base produces the same issue: solid on.

Regards,

 

Do you have values for R3, C2 and C3.?

 

Eric,

R2 is 4.3K
R3 is 5.1K
C2 is 100nF
C3 is 10nF

Regards,

 

Try placing a 10μF-100μF electrolytic capacitor from Vcc to GND.

 

There is no power supply decoupling.

 

hi,
I do not see any capacitive decoupling on the 12V power rail.?
I would add 100nF and a 220uF cap from +12V to 0V, have the 100nF near the 556.
If you don't have those exact values use the closest you have.
E

 

Hi,

I'll look around my caps to see what I can dig up. Question: I tought decoupling was only for noisy power sources? I tried powering it with a 12V battery and it did the same thing.

Am I mistaken thinking that? (most probably)

Regards,

 

Hi,

The decoupling cap I put (33uF, 50V) across 12V and ground worked!

BUT, it rendered my "breathing" circuit non-functional. I think the added capacitance is what causes it to remain on... Any advice?

Regards,

 

Hi,
What is a 'breathing circuit', how does it appear on the LED's ??
E
EDIT:
I don't see how adding power supply decoupling would effect the 556 function????

 

Eric,

Something like this:

Regards,

 

hi,
OK, see that.

Your circuit is not the same as the video circuit which uses two transistors.
How are you trying to achieve the current saw tooth wave form by using the second half of the 556.?
E

 

Hi,

I'll look around my caps to see what I can dig up. Question: I tought decoupling was only for noisy power sources? I tried powering it with a 12V battery and it did the same thing.

Am I mistaken thinking that? (most probably)

Regards,

This is a common omission and mistake. Decoupling capacitors are absolutely essential for 99.9% of electronic circuits DC or AC and independent of the type of power supply. Why? Because it is the chips themselves that introduce noise on to the power supply, not just from the power supply. Every time a gate or amp changes state, noise is introduced into the power supply lines because the device is demanding a sudden surge of current which the supply lines are not able to supply. That is where the decoupling capacitors come into play. When strategically placed, decoupling capacitors are able to supply that immediate demand to the chip.

 

hi,
OK, see that.

Your circuit is not the same as the video circuit which uses two transistors.
How are you trying to achieve the current saw tooth wave form by using the second half of the 556.?
E

Actually, I found the circuit somewhere and copied it... I don't have a real understanding of it...

 

OK,
So for 'breathing' the LED's would slowly repeatedly brighten and dim.

In the last video the circuit used 2 transistors to give the required current gain, perhaps you could adapt that method.?

 

hi F,
Confirm the value of C1 on Trig_B pin, 100nF seems a low value.???
E