With your pot set at max output voltage, the unloaded voltage divider produces only 5V/3= 1.67V and the LEDs get less than 1V each. Whte leds at less than 1V produce no light.

Whatever. Adjust the resistance values to suit your taste.

 

I think the circuit should be like this:

Rather than a series resistor for every LED, put two LEDs in series across the 5 volts, with one resistor for the string of two. less power wasted as heat, both in resistors and in the control circuit.

 

The one I recently put together used three LED's in series, with a 2n7000 & PWM to control them.

 

Post #1 in this thread says white LEDs.
They are 2.8V to 3.6V so from a 5V supply they cannot be in series.

 

Post #1 in this thread says white LEDs.
They are 2.8V to 3.6V so from a 5V supply they cannot be in series.

They will probably not show full brightness, but probably some.

 

LEDs can't be dimmed with 1M resistor in series,

Not directly as you noted but using a darlington transistor configuration will work with the 1M trimmer you mentioned.
This circuit can be assembled on a small perf board.
This worked with the white LEDs I had on hand. Max current at 120ma.
R2 sets the max current limit when the trimmer is at 0 ohms.
R1 sets the minimum brightness level when the trimmer is at 1M.

 

What is the model number of the camera?

Camera is "Neo" and lsusb recognizes it as "0ac8:307b Z-Star Microelectronics Corp. USB 1.1 Webcam". Attached pictures show disassembled webcam and part of it's PCB from which I took the schematic. R17 is on the opposite side connecting RG to VCC.

 

As I have stated earlier I started this thread to make original design work but didn't want to experiment on the PCB. You gave me an idea that i could experiment on breadboard and, when I find values of R16 and R18 and appropriate transistors, solder them to the PCB. I would appreciate if you can suggest me the values of resistors and transistors, but thanks anyway because you answered my first question whether the circuit can be LED dimmer.

 

OK, now we see that the original intent is a repair. That makes a fair difference. If the circuit as shown functioned as needed at one time, but not now, then a failed component or connection must be corrected. If the function was never correct then that is a quite different situation. Adding PWM control would be a very big deal sort of change.. So at this point the question becomes "what does the TS want to be able to do?"

 

what does the TS want to be able to do

What is TS?
If it is thread starter then:
I want to solder some resistors to R18 and R16, some transistors to Q3 and Q4 and use the camera with LEDs illuminating the object I am recording. I should be able to adjust the LED brightness by (external) RV1.

 

OK, sorry about not clarifying TS. Yes, it is Thread Starter. Moore obvious than OP, Original Poster..
The appearance in the photos is that the camera board already has LEDs. The original method of control is not obvious to me. And adding any control function to that circuit board will be a challenge.
Providing an external control of the internal LEDs with an external variable resistor is certainly possible, but much greater flexibility can be had with a separate lighting package. The other benefit would be no chance of camera damage.

 

I should be able to adjust the LED brightness by (external) RV1.

OK so the pc board has been modified and the LEDs are now in series with RV1. Is that correct and do they come ON?
Looking at your schematic in post #1 I don't see R17 on the pc board and 100 ohms would be too high if it's actually connected as shown.

 

Original Circuit with R17 at 10 ohms.
Max current through the LEDs is controlled by the value of R17.

 

Original Circuit with R17 at 10 ohms.

R17 is on the opposite side of the PCB connecting RG to +5V. It is marked 101, I agree with you that it should not be 100 Ohms, I will check it with a meter.
Thank you very much, I will try this ASAP.

 

OK so the pc board has been modified and the LEDs are now in series with RV1. Is that correct and do they come ON?

They do come ON only when RV1 is shorted, even slightest movement of RV1 turn them OFF.

 

They do come ON only when RV1 is shorted, even slightest movement of RV1 turn them OFF.

Which schematic??

 

Which schematic??

 

To dim the LEDs then you need a dimmer circuit.
These are not dimmer circuits:

 

To dim the LEDs then you need a dimmer circuit.
These are not dimmer circuits:

Please read my post #1, the one which started the tread. The dimmer circuit was in the design, but missing in the implementation. Active components were not soldered at all and replaced with jumper wire connecting RV1 to anodes of the LEDs. I think that the value of RV1 (1M) remained from the original design. It is too high to work as a dimmer and, you are correct, it works as a switch. However, when I replace RV1 with 1K pot (right part of your schematic is the right one) it works like a dimmer. R17 is 100 Ohms (101 marking) it is limiting the current (and max LED brightness) to 3-4mA per each diode depending on their voltage drop 3.2 to 2.6V. I will get the real values when I get time to measure them in circuit.
I want to reconstruct the original circuit with 1M RV1. I need the values of R16 and R18 as well as suggestion for the transistors.

 

hi william.
Not a very good dimmer circuit, but this is what LTSpice shows.
Allowing approx 20mA/LED.
E
Equivalent transistors with a similar specification will work.
The LED's should be matched for Vforward