Scanning Laser Microscope With Arduino

Hello!

I'd like to share my latest project, a scanning laser microscope with you.

Some words concerning the principle: The pinciple is quite simple. The laser beam is focused on the object and the reflective light is being measured with a photodiode (in my case a BPW34). To focus right the current through the focusing coil is varied. The position of the object is changed by two Speakers, which are driven with different voltages. The speaker-membrane is moving and therefore the object-table, which is connected with the membrane is moving too. With this way of moving head you can reach movements down to µm.

Step 1: The Cd-drive

A Commercial cd-rom-drive is ideal for our purpose. It combines everything we need, a laser-Diode, a lens with variable Focus and a photodiode. In my case I've changed the build in photo Diode with a lot of Pins against a single BPW34 with just two Output-Pins. Older cd-drives are suitable for the laser microscope because they aren't so miniaturized like modern drives. I use a cd-rom-drive instead of a DVD- or bluray-drive, because in the early days of this technology the pins have been larger and more separated than in modern drives...

You'll have to find out the two Pins for the vertical coil and the laserdiode. If you replace the built-in photodiode against a larger model you'll have no problems with soldering.

The laser-Diode needs a constant current, about 80mA. This can be easily realized with a LM317 and some resistors.

Step 2: The Speakers for the X/y-movement

You'll have to look for suitable speakers with little movement. I took some small ones with a resistance of 24 Ohm. A metall bar is glued to the membrane. At the end of the bar a small piece of copper-foil (0.5 mm thick) is soldered with the bars.

To drive the Speakers I use a MCP4922-DAC. It offers two voltages between 0 and 5V. With some resistors and a Transistor I'm able to vary the current through the Speakers up to 250mA. With a potentiometer the magnification of the microscope can be changed. The larger the value of the potentiometer, the higher the magnification.

Step 3: How to Focus

Focusing right is a difficult work because we are speaking about µm.

Usually cd-drives use photo-diodes with more than 4 areas (ABCD in the Picture). With those signals you're able to find out the Focus quite easily. It's in the focus, when the Signal (B + C) - (A + D) gets zero.

Using a single photodiode we just get one signal. But don't worry, there's another way to find the right Position of the coil. Changing the current through the focuing-coil you'll see, that the Output-voltage of the photodiode first increases, going through a Maximum and then decreases. Exactly in that maximum the laserbeam is focused right. To make this easier, you should use a 10-turn-potentiometer (in my case a 100Ohm-type parallel to a fixed 100Ohm-resistor to get 0-50 Ohm).

If no object is on the moving-table, I get voltages in the range of 0.2V. If something is lieing on the table, the voltage slightly increases. The arduino is able to read voltages between 0 and 5V. Therefore we first have to eliminate the Offset (0.2V) and second we have to amplify the signal up to 100 times. For this purpose I use a LM358. Before scanning I turn the gain-potentiometer to get voltages as high as possible.

Step 4: The Visualization of the Data

You'll have two ways of visualization, with the Software processing or with a TFT-display. I recommend a 480x320 Display (no touch), which you can get on ebay for less than 10 USD.

If you prefer processing you'll have to send the data in the following way:

Serial.print(xPos);

Serial.print(",");

Serial.print(yPos);

Serial.print(",");

Serial.println(value);

Using the TFT-Display you'll need the UTFT-library. Just search for "rinky-dink electronics" (http://www.rinkydinkelectronics.com/library.php). There you download the UTFT-library for arduino.

Step 5: The Complete Setup & Code

Finally everything is mounted on a wooden plate and all the outputs and potentiometers are mounted too. The only additional parts you'll need are a multimeter for controlling the photodiode-voltage and a power supply (9V, 0.6A). If you want you can take a 7.4V lipo and a DC DC step up module to work everywhere you want without the need of a Computer or power supply.

Step 6: The Calibration

To be able to estimate the magnification I bought a cheap calibration slide on ebay. The finest scale is 0.01mm, so 10µm. With this slide put on the moving-table you can easily find out the magnification.

Step 7: The Results

Luckily I found a piece of a waver with very tiny structures. But the main Goal of my Project was to visualize the pit-structure of a compact disc. For this project I had to cut an old windows-CD into pieces (so window's isn't quite worthless ;-) ).

Voila', there are the 0's and 1's. To get this resolution I had to increase the resistance of the potentiometer nearly to the maximum.

I hope you've enjoyed my work. Maybe you'd like to take a look at my YouTube-channel (https://www.youtube.com/user/stopperl16/videos)

more physics projects: https://stoppi-homemade-physics.de/

Comments

[mehrbanreza]

Hello
I'm have a problem

In the processing
I see this text:
ArrayIndexOutOfBoundsException:3

What should I do?

[stoppi71]

Hi! Have you used those two programs, one (Arduino_Lasermikroskop_Processing) for the arduino and the other (Lasermikroskop_processing.pde) for the processing itself? Processing gets 3 values: x-position (within 0-128), y-position(within 0-128) and value from the photodiode (within 0-255)...

[Vinimiquelin]

Hi, congratilations for the project! Maybe it´s too late to get an answer, but I will try: I am trying to reproduce the xy movements with the speakers and i was thinking in how did you program the Arduino to control them? I used the SPI to send values between 0 and 5V to the outputs of the MCP4922, but I don't know how the loop need to be for a good control. Did you variate the value, sent to the DAC, between 0 and 4095 and made the outputs oscilate between 0 and 5V in a loop, or did you use other logic?

[stoppi71]

Hi! For the speaker-movement I use two for()-loops and the loop for the x-movement is within the loop for the y-speaker. The MCP4922 gets a digital value from 0 to nearly 4096 (f.e. 240 steps * 17). With the 10 kOhm potentiometer at the base influences the magnification. The higher the resistance of the potentiometer, the lower the current through the base and the collector of the transistor and the lower the movements of the speaker --> higher magnification

[NeilRG]

This is a fascinating project. I'm wondering if the voice coils from disk drives could work as the x y positioning elements. I have not studied them in enough detail to know if they have the needed resolution. What do you think.

[zposner]

Incredible!

[psicopato]

Very nice project! Kudos to you!

[hansthegoodknight]

Hi. I am a beginner. Can you please explain to me how are speakers used for xy movement? Thanks

[stoppi71]

Because of the current through the Speaker, the Membrane is moving outwards (or inwards if the current direction is turned around)...

[losmilosmi]

nice idea, cool project!

[StefanB54]

Fascinating project, thumbs up :-)

[MarceloN]

Tienes mi voto !

Gran Trabajo !

[stoppi71]

Muchas gracias ;-)

[eljonco]

Well done. At the expense of doubling your scan time, you might be able to increase image quality by unidirectional scanning. After a left-to-right scanline, increase y position and move too far left for the x=0 position. Then fly in from the same side every time and scan your x positions. This likely rids you of the interleaved looks of your images.

[stoppi71]

Hi! Thank's for that advice. I've already noticed the "steps" in my pictures too. I'll try out just scanning from one side....

[tswaehn]

ziemlich cooles Projekt. Daumen hoch für den Contest :)

[stoppi71]

Danke, danke ;-)

[Venkes]

It seems that we were developing an LSM on about the same time. You also managed to nail it! Well done!

There are indeed differences in approach between our LSM's. Both work well but both has pro's and con's naturaly. Where mine ( "https://www.instructables.com/id/Laser-Scanning-Mic... for the interested!) has the freedom to use different lasers, yours is probably slightly easyer to build.

Just a thought, I think you can get sharper images if you should use a DVD pick-up (red laser), or better, a blu ray pick-up (uv laser) instead of a cd pick-up( ir laser). Where a blu ray pick-up uses uv light, the shortest wavelength, and a cd pick-up ir light (in your design), the longest wavelength. The shorter the wavelength, the smaller the lightspot can be and will produce, theoretically, the sharpest image. These differences in sharpness can you see in my results were images are taken with red, green and uv lasers.

I really want to congratulate you with this result because I know what a long way you have traveled, just as I did.

I am allways looking for ways to improve my builds (probably you do to!), so I'm gonna take a closer look at your photodiode solution as that is maybe a better way to detect the reflected light!

Mine does half an hour to complete one (256x256) image. What I really want to know is: how long take an image with your LSM to complete?

Greets,

Venkes.

[stoppi71]

Hi Venkes!

Thank's for your kind words. I started with this Project around christmas with buying used cd-drives... Concerning the laser-colour (the lower the wavelength the sharper) I have my doubts, because when I'd use f.e. a bluray-diode I should use bluray-disc's too, because they are coordinated with each other (reflective colour and so on). And if I do so, I wouldn't have an advantage, because the distance between the pits on bluray-disc's would be smaller too. Can you say something about using red laser not for DVD's but CD's (which are tuned to infrared-laser) or blue laser not for bluray-disc's but CD's? If you get better results than I'll try at least DVD-heads...

Cheers Christoph

[Venkes]

Hello stoppi71,

In my opinion only the (re)writable ones are doped with a corresponding colour: bluish for DVD, reddish for blu-ray. If you lay a few readymade different types with the label side down on a table its hard to tell wich is wich, they are all silvery. I have used four different (red, green, blue and uv) lasers to magnify a cd, and the uv-type (shortest wavelength) has the sharpest picture (see my samples below). Which is on the opposite side of the optical disc spetrum.

So look at my results and judge for yourself if it is worth to try, if it's not to much of an effort, to put in a DVD pick-up instead of a CD pick-up!

The differences I get with an LDR are: (LDR without a pinhole) 4,2 V when in focus 1,1 V when nothing in the light path. (LDR With a pinhole): 3,6 V in focus and 0,3 V with nothing in the light path. I don't eliminate any offset

Greets,

Ron.

CD samples: Both a cd, 1st with uv, 2nd with red. I think also that the optical quality of my red laser's collimating lens isn't really good (what partially should explain the difference in image quality). In a laser pick-up however it must be excellent.