Hi everyone,
I’m planning to build the v7 manual microscope but the illumination power source has got me stumped. I have bought the illumination kit from taulab, so I have the illumination PCB.
The assembly instructions say “The illumination PCB needs to be connected to a constant current power supply (20-50mA). Alternatively a resistor can be added in the cable, as in the [LED workaround], and then connected to Ground and 5V.”
The manual microscope does not use either Sangaboard or Raspberry Pi, so I don’t have anything to plug the PCB in. What power source should I use here? I can imagine one of the following possibilities:
- I actually do need a sangaboard, it is just missing from the list of materials needed
- I need to buy a battery and a battery clip, and solder a suitable resistance between it and the PCB (but I don’t have any experience doing that so I would appreciate more precise instructions :))
- I can use the webcam to power the PCB.
Thanks for your help!
Have you got the illumination PCB?
The Sangaboard and PCB moved forward outside the main OpenFlexure project, and the Manual version got left behind. There was a former version of the PCB that included the current control and so was easier to power.
You do not need a Sangaboard, but you do need a resistor. There are instructions for soldering to a bare LED in the main instructions for the LED workaround. The equivalent page for the OpenFlexure Delta Stage has photographs. If you have the PCB you don’t need to deal with the legs of the LED, you just need to cut one of the illumination wires, either red or black, and solder the resistor into the middle.
If you don’t want to solder, you can use an extra wire with female connectors: One leg of the resistor will go into the female connector of, say, the red wire of your existing illumination cable, the other leg will go into the connector on the new wire. Then you plug the other end of your new wire to the PCB, together with the black wire from the existing illumination cable. (I hope that makes some sense).
Once you have a resistor in the cable by either of those methods, you then just need 5V power. The simplest way to do that is to get a USB breakout board, something like this. You will want one with the header already soldered if you are not able to solder things yourself.
Then as a final touch, use a USB Y-splitter cable, or a small USB hub, to feed the illumination USB breakout and the camera USB cable to a single port on your computer.
@j.stirling has designed a new version of the base with cable tie mounting points and a cable exit that makes this all neater. That is currently available from the draft instructions that you can access through the view app button on Draft: Add a removable plate to the no-pi stand for wiring. (!426) · Merge requests · OpenFlexure / openflexure-microscope · GitLab.
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Thank you, this is very helpful, I think I’ll try my luck with soldering. So just to recap:
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I am going to take the illumination cable and cut one of the wires (no matter which), remove some isolation from both ends of the cut cable, and solder them together with a resistor in the middle. I found this article here which says I should be using a 30-100 Ohm resistor (README.md · master · Filip Ayazi / OFM Led · GitLab). Any suggestions on whether I should be going for 30 or 100 Ohms? I don’t want the light to be too bright or faint if I solder the resistor in place (would it make sense to have a variable resistor or is that overkill?).
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Then I need a 5V power source. If I don’t manage to get a breakout board, could I also make something using a few batteries, like this one: https://www.jaycar.co.nz/3-x-aa-side-by-side-flat-battery-holder/p/PH9274. Do I need to use a different resistor in that case?
PS: I guess to add to my last question: Apart from the 5V specification, what is a bound on the current that can be supplied?
By default Sangaboard runs the LED around 30 mA. This is the datasheet for the LED on the board https://www.farnell.com/datasheets/3048030.pdf , according to the datasheet it will drop 2.75V on the LED so 75Ohm should give you 30mA. That should be bright enough for most uses. I wouldn’t go past 100mA on the LED due to heating, it is enclosed in the plastic housing so it doesn’t have much cooling.
You can use any voltage larger than the LED forward voltage just adjust the resistance (and be aware of the heating on the resistor.
For short periods, 2 AA batteries giving 3V might let you get away without a resistor but check the heating on the LED as that will likely operate around 110mA (and as it heats up current can increase, so this is generally not recommended).
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Thanks! So I’ll assume the following: I need a current of roughly 2.9-3.3V (which is the minimum-maximum range of the specified forward current). To avoid overheating I need to limit the current to something lower than 100mA, and I’ll try to go for something like 30mA.
If I take a battery holder with 3 AA battery slots, I’ll end up with roughly 3.6-4.5V which is probably fine, and I’ll need a resistor of roughly 72-150 Ohms (accounting for the uncertainty in voltage, and allowing a range of 30-50mA)
If I only have 2 AA batteries, I’ll have roughly 2.4-3V, which is not consistently above the forward voltage, especially if the batteries have been used for a while. I’ll asume that would mean the LED would just shut off, so would I be safer off going for 3 AA batteries?
(Sorry for the many questions, I haven’t really done any electronics in a long time)
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Yes, you would want 3 batteries, and a resistor at the low end of that range (the resistor only has the battery voltage minus the forward voltage of the LED)
It is not very critical. The LED will operate over a range of current, and the camera has adjustable gain and exposure time. If your samples are not unusually dark it will just work.
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Just to wrap this up (in case someone finds this thread in the future). Using 3AA batteries and a 100 Ohms resistor, soldered together as described above, did indeed work and makes the PCB light up as intended.
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