Delta v.1 Build Questions

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#1

Firstly, I just want to say how amazing the Openflexure project is and how inspiring it is to see really great design so dramatically increase accessibility to a technology.

I’ve built a motorized microscope and am enjoying it immensely, and am now in the process of building a Delta v.1 with reflection illumination. I apologize if this is an overly simple question (I’m a biologist, not an engineer :slightly_smiling_face:), but I noticed the instructions call for powering the star LED with a separate power supply rather than through the Raspberry Pi GPIO pins, is it possible to power the star LED this way?

Thank you!

#2

The Pi GPIO pins can only supply quite a small current. This is enough for a small LED, but if you get it wrong you break the Pi. The star LEDs . As you already have a motorised version you have a 5V supply that you can easily access.

#3

Thanks @jakwin!

I think the normal reason that LEDs are in the star package is that they are high power and may need a heatsink in some applications. Due to the high power of the LEDs they may damage the pi.

I think the point comes back to something we are seeing more and more: We need some good standardised electronics. It would be great to see a later version of the Sangaboard have the capacity to power the pi and have the flexibility to drive other higher power peripherals. However, baby steps, we first need to find a way to produce Sangaboards so they are readily available.

#4

That’s great you’re enjoying it!

Agreeing with the above, some of the star LEDS can draw up to 1.5A and require a forward voltage >3V, which the Raspberry Pi GPIO pins can not support. I recommend you use a variable power supply and start by keeping the currrent low as at full power they get very bright and hot.

#5

I suppose the issue is many people don’t have easy access to a variable PSU. Do you normally use it in current control or voltage control? Do you know what the approximate voltage and current are? This may allow us to suggest a suitable low cost potion for powering them at the drawback of not being controllable?

Are there cheap boards for LED power control? I assume we need more than just a variable resistor as we will cook it. Perhaps something to get @kaspar’s input on.

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#6

This is extremely helpful, thank you very much for explaining and providing guidance. The prospect of having the electronics standardized around the Sangaboard would be really attractive, particularly for individuals like myself who are excited to implement the scope but are not necessarily electronically minded or have a variable PSU on-hand. But I will say as it stands the instructions, paired with the forum, did make the process and scope very accessible to print and assemble.

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#7

Yes agree that access to a PSU is not easy, but for now I think it’s the safest way. I normally fix the voltage to their spec and adjust the current to get the correct brightness for the reflection illumination. These are the specs for the four colours:

UV:
image
Blue:
image
Green:
image
Red:
image

I agree a lower cost solution would be great. I haven’t done much research to that myself, but a driver board will also require a switch (physical or software), to limit exposure to the sample.

#8

Are the voltages you write here the max voltage set into the PSU and then you control the current? Or are they the resulting voltages once you get the desired current?

#9

Those are the specs from the documentation. I normally run the power supply in constant current mode, I initalise the voltage to the spec (3.3V) and then just adjust the current to the required brightness. For example when I do that for the blue LED a sensible value for the current is about 0.1A, and the voltage is then 2.78V.

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#10

Great thanks!

I think to do this on the cheap we would need MOSFET circuits which we could then control via a filtered PWM output on the Sangaboard (Would need to expose one in a later version). Not sure how much the remaining ripple in the filtered PWM would affect things, the other option would be a little I2C board that did all the DAC and had the FET on it, worth checking Adafruit for such boards.

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#11

Constant current LED drivers should be much easier to find on breakout boards. They are systems that are included in all LED torches. Generic constant current regulators also ought to be easier to find. I think the problem is that they are generally just built into the main item.
Something like this is great, giving adjustable voltage and/or current and with a meter incorporated. Basically it is a 5A variable PSU. But not available from sensible sources: Zerone 5A 75W Constant Current: Amazon.co.uk: Electronics
or the same without the meter: 6PCS Lithium Battery Charger Module Board,XL4015 5A DC: Amazon.co.uk: Electronics

For low current LEDs the CL2 series CL2N3-G Microchip, Led Driver, 1 Output, Constant Current | Farnell are awesome. They are fixed current, 20 or 25mA, but they just go in line with no external components. They can be paralleled for higher current - although 1A would be going a bit far!