Led dimmer with 555 timer

Hello!

I’ve bought this dimmer (atrached) for my video led light project. But when I turn it on I see black lines on the monitor. I’ve checked the frequency which is 748Hz. This is what causes the lines. I know the frequency can be changed with capacitor or resistors. But looking at this PCB I can’t find which ones. Have anybody ever had this kind of dimmers? Have any ideas how to change the frequency?

R2 and R7 seem to be the timing resistors ,clustered around the 555 called U1. I can't tell what
chip U2 is from that photo.

I presume this is back light dimmer for an LCD video screen?

Lines on which monitor ?

I have one of those but consigned it to the junk box when it failed to perform after about 10 minutes on a 2 mtr strip of whit LED...

if it really is a 555 based unit then there are schematics galore for 555 based dimmers and similar so working out what you need to change should be reasonably easy as it is in most (not all) cases down to two singular items.

Usually a resistor between pin 3 and between pins 2 and 6 (usually tied together) the capacitor / s are usually connected to pin (2 and 6 tied) to ground or 5 and ground.

MarkT:
R2 and R7 seem to be the timing resistors ,clustered around the 555 called U1. I can't tell what
chip U2 is from that photo.

I presume this is back light dimmer for an LCD video screen?

No. It is a dimmer for the led strip.

Here are the photos of the thing:https://we.tl/t-WqWxGzIWQw

U2 is an LM358 chip

ballscrewbob:
Lines on which monitor ?

I have one of those but consigned it to the junk box when it failed to perform after about 10 minutes on a 2 mtr strip of whit LED...

if it really is a 555 based unit then there are schematics galore for 555 based dimmers and similar so working out what you need to change should be reasonably easy as it is in most (not all) cases down to two singular items.

Usually a resistor between pin 3 and between pins 2 and 6 (usually tied together) the capacitor / s are usually connected to pin (2 and 6 tied) to ground or 5 and ground.

That's what I've found on Internet, but can't find it on the scheme. Also changing the resistance may cause high current to the 555 timer, may not it?

Tofer:
No. It is a dimmer for the led strip.

Here are the photos of the thing:https://we.tl/t-WqWxGzIWQw

U2 is an LM358 chip

That's what I've found on Internet, but can't find it on the scheme. Also changing the resistance may cause high current to the 555 timer, may not it?

Well if you replace 1k with 10 ohms perhaps - but that's not an issue here is it?

MarkT:
Well if you replace 1k with 10 ohms perhaps - but that's not an issue here is it?

Now there are two resistors - 1k and 10k. I think I need some thing about 20kHz. It means I have to change 1k to 100 ohms and 10k to 1k. It changes the current a lot. Another way is a cap replacement.
The thing is that I can't find the cap. The cap also influences the frequency. And it doesn't change the current.

I’ve found out today the cap and the resistor. It is a 10k resistor. The cap I can’t measure. Changing the resistance down to 600 ohms I achieved frequency 6.82kHz which is still low. I’m afraid to go lower cause I think it may increase a current inside the timer. Are there any formulas to calculate the cap?

Just a few :wink:

Yeah! Already found it. Thanks!

I wonder why does the manufacturer produce dimmers working in such a low frequency?

Tofer:
I wonder why does the manufacturer produce dimmers working in such a low frequency?

They do this for two reasons:

  1. At low frequencies there is little or no Radio Frequency energy generated. This reduces the possibility of creating problems for other electronic devices (your video notwithstanding)

  2. Each time the driver switches from either on to off or off to on some heat is generated. The faster you switch the more heat. So for an inexpensive device slower is better.

For the human eye you only need to get above 150 Hz or so. Most devices work at slightly higher frequencies as a safety margin for flickering. And going from 150 to 400 Hz or so doesn't make much difference to # 1 &2 above.