Hello Guys,
i've recently put my hands on an old laptop screen which LCD was broken, and used the LED backlight to make a Light Panel (my girlfriend is into drawing, i've made it so she can use it to draw using transparency....)
anyway, now i'm trying to dim it using a NE555P, but whatever i'm trying, it's not working... most of the time i'm getting an "On/Off rotary switch-like" reaction from potentiometer...
i haven't used any MOSFET since i've left the circuit board on the backlight, nor any diodes, i'll upload my circuit in the next post.
My question here is only; can you guys give me any advice, or a circuit i can make ?
Here is the circuit;
i've tried value going from 20Hz to 500Hz
i'm not much into electronic so...
here is the link if you need it:
http://www.falstad.com/circuit/circuitjs.html?cct=$+1+0.000005+4.818269829109882+56+5+50%0A165+544+240+560+240+0+6%0AR+608+208+608+192+0+0+40+6+0+0+0.5%0AO+672+304+720+304+0%0Aw+544+272+512+272+0%0Ar+512+272+512+208+0+220%0Aw+512+208+608+208+0%0Aw+512+368+544+368+0%0Ac+512+368+512+416+0+0.0000047+2.675258692838685%0Ag+512+464+512+480+0%0Aw+512+336+512+368+0%0Aw+512+336+544+336+0%0Ar+512+272+512+336+0+220%0A174+576+464+640+464+0+100000+0.6188+PWM+CV%0Aw+608+448+608+400+0%0Aw+640+464+768+464+0%0Aw+768+464+768+208+0%0Aw+768+208+608+208+0%0Aw+512+464+512+416+0%0Aw+576+464+512+464+0%0Ao+2+32+0+38+10+0.00009765625+0+-1+output%0A
that is not the correct way to do a PWM using a 555. if you want to use pin 5 to do PWM, then you set up up the 555 as a monostable, and feed it external pulses into pin 2 using another 555 or whatever.
if you wanna use a single 555, then you follow the standard way of doing it, using 2 diodes and a pot on pin 2. google it.
thanks for replying,
So i've tried a few other circuits folowing these schemactis;
http://www.instructables.com/files/orig/F27/G3TA/FOHTTUY8/F27G3TAFOHTTUY8.jpg
and an another one using two 555 and none of them actualy worked....
i realy dont know what to do here.
maybe there's something going on with the circuit board?
btw, sorry for the poor english
just tried using an Arduino to generate a square wave, and modulate its duty cycle using a pot, still not working...
i'm realy lost here.
>>920757
show us what you have op take a picture. this is an image board after all.
if you are turning the control circuit on and off it might be unhappy with the voltage and detecting a brown out or unstable voltage.
most of the time there is a control for the light level, find it.
there is a PWMIN pad on the circuit board, that's where i'm doing all the magic...
i've just managed to make it work, using 12V instead of 5... that's curious...
i'll take picture when i'll be back from work
it seems like a huge waste to use a $30 computer to do the job of a 50-cent chip. circuits like this (http://www.dprg.org/tutorials/2005-11a/) do work, but you're probably doing something wrong. one possibility is that your circuit is working but that the way the LEDs are being driven is wrong. i'd hook up to pin 3 a single red LED in series with a resistor (220 ohms for 5V or 560 ohms at 12V), and see if the pot can dim it. if so, the PWM part is working.
Ok so, i've managed to make it work using only one 555 on my breadboard, an now i've soldered everything on the PCB it's not working anymore...
here's a picture of my PCB, can you guys check what's wrong?
>>922218
check your traces on the backside, or take a picture of it. Honestly, protoboard debugging is a lot easier on your end since you can immediately flip the board around and trace the connections. Go through each component and make sure they're connected to only what they're supposed to be connected to.
>>922218
Can't see the traces, but as a beginner, the failproof method for checking is:
1. Draw the schematic on a piece of paper
2. Check each and every wire and trace them over your schematic once it's checked
Stop when everything is checked, hopefully you will have found your error. If everything seems good, maybe you have a bad solder joint somewhere?
>>922218
I only see one resistor there. 555 Astable circuit should have two. Why is the output out on the right instead of next to 555 pin 3?
>>922224
i used this sketch here:
http://www.dprg.org/tutorials/2005-11a/
here's the backside of the PCB, i've already checked for shorts, there is none.
here's a Fritzing of the circuit:
the way the circuit is designed, it really needs an output transistor because pin 7 can only sink current, (i.e carry current from pin 7 to ground) while sourcing current (from plus to pin 7) comes through a quite large resistor so you wont light an LED very much.
so, you either connect the LED and series resistor between + and pin 7, or connect it to pin 3 (instead of 7) and ground.
as to why it's not working, i cant tell from the photos; the Fritzing looks fine.
>>922265
it's working well on the breadboard!
Duty cycle goes from 5% to 95% and frequency is around 144hz...
the PCB version is the problem, and i dont know why.
thanks for replying anyway
>>922265
For the transisor, i think there is one included in the circuit board from the backlight screen...
i've glued the pad where the PWMin is with hot glue since it was SMD sized and the solder tend to break easily.
i realy think the problem is on the PCB
>>922226
Check for short circuits between adjacent tracks, and check that your breaks actually break.
