Problem getting the full power of Excitation LED

[jwghim]

Hi all,

I am able to get the Ca-images of neurons with the miniscope.
But I am having a difficulty in getting the right LED power.
I measured the current flowing the LED for the excitation LED.
At low intensity level (about up to 40%), the current increased linearly.
But after a certain intensity, the LED intensity or the current didn’t increased any longer.
Three other miniscopes showed the similar results.
I also tested with other PCs but they didn't worked properly, either.

I thought there was voltage drop somewhere in the circuit of the CMOS.
3.3V and 1.8V had no problem.
But I have found voltage drops in 5V as the LED intensity increased.
And there was no more increase in the LED current with no more voltage drop.

Here are the conditions and results that I got.
-Length of Cooner coaxial cable = 80 cm
-DAQ board V2.01
-CMOS PCB V3.0

I'd appreciated it if anyone has any ideas on what to check. Thanks!

[Daniel Aharoni]

Hi jwghim,
Thanks for the detailed question. The current version of the CMOS PCB maxes out at about 40% LED intensity. This is due to a combination of factors but generally you shouldn't need intensities over 40%. For example, when imaging GCaMP6f in hippocampal CA1, we generally don't exceed 5% LED intensity and often times run at 2%.

I have designed a new LED driver circuit that can reach higher intensities and will be incorporated into a future version of the scope.

[jwghim]

Hi Daniel,

Thanks for your reply. I'm glad to hear that you're working on a new LED driver.

[Lee Hyeonju]

Hi
I have questions about LED power.

My labs miniscopes have low LED power and I want to know the voltage of LED.
(Light intensity at the end of 2mm-diameter GRIN lens is 0.05mW)

How can I measure the voltage of LED? I want to know the right way.

I tried to measure voltage on SMA pcb between inner and outer soldering part and it was 4.9V
and I also measured on LED pcb between + and - pannel and it was 2.45V

I saw somewhere(in discussion board) that LED voltage must be upper than 5V

1) Where should I to check the LED voltage?

and 2) How I can improve it?
We just order DC adaptor, but this adaptor cannot turn on the DAQ pcb. it is 24V 5A adaptor and I don't know why it was happened.

There is no one who can advise these simple problems.. please help!

[Daniel Aharoni]

Hi Lee,
You want to measure current not the voltage drop across the LED. The LED should run at around 2.5V to 3V but that necessarily tell you how much power the LED is consuming. Instead you need to measure the current flowing through the LED by placing a multimeter in parallel with the LED. Most likely your LED is damaged and the issue you are seeing will be resolved by soldering on a new LED. The other source of the issue could be that you did not short the 2 half circle pads on the bottom of the CMOS PCB.

[Lee Hyeonju]

Thank you Daniel.

According your advise, i measured current through the LED and it was 0.6mA
Is that mean that LED was damaged?
But when I tried to measure resistance of LED with multimeter, LED was shine brightly.
And when i connect cmos to DAQ, LED really shiny first but immediately be weak. Is this phenomenon is usual?

How about problem of LED pcb?

Actually I bought LED and SMA pcb from local fabrication company and all of LED have problem.

+
by the way, "placing a multimeter in parallel with LED" is right connection for measure current? not series connection?

[Daniel Aharoni]

Hi Lee,
The current through the LED should range from about 0.2mA to over 40mA as you adjust the excitation power throught the DAQ software. If the current stays at 0.6mA it is likely that the LED is damaged or that there is a small short somewhere along the LED wires.

Placing a multimeter in parallel with the LED will not measure current. You need to place the multimeter in series with the LED to do so.

[Lee Hyeonju]

Hi Daniel!

I found the problem.
I don't know why, but length of my coax cable was problem.
When I make it short(2m->1m) my miniscopes work.

Thank you for your consideration