Short Throw Smartphone Projector




Introduction: Short Throw Smartphone Projector

About: I am an automation engineer but I will give anything a go. I don't know if you call if pessimism or just being an engineer, but I look for problems everywhere, then I look for some weird, left field way to s...

I know there have been directions on the construction of short throw smartphone projectors for as long as there have been smartphones.

I have wanted to make one for some time but there was always 4 things that bothered me:

  • you have to open the projector to interface with the phone inside
  • you have to open the box (reducing the brightness of the projection) to set the focus
  • the sound is poor due to the small phone speaker inside a box
  • watching an entire movie or a heap of youtube videos with the screen at full brightness, really depletes your battery

This instructable will explain how to make a projector that addresses these issues and more.

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Step 1: The Lens

Many of these projects start out with a shoebox, I had one of the folded heavy cardboard types but the dimensions are similar.

I also wanted to use a fresnel lens rather than a standard convex lens. The one I got came from my local bookshop as a small text magnifier and cost 99c.

A fresnel lens is effectively a flattened out convex lens, similar to the ones they fit to lighthouses.

I used a straight edge to find the centre of the short side of the box, I then marked around the lens.

I drew a smaller opening to give the lens something to mount on and cut through all of the layers.

I opened the flap and trimmed away more of the material of the middle layer.

Using double sided tape, I mounted the lens. IMPORTANT: the textured side of the lens must face inwards to focus correctly.

I then closed the flap up and contained the lens inside the wall.

The flap from the side of the lid will also require removal of a section to clear the lens.

Step 2: Focus

Okay, this is where I tackle one of my major gripes with existing designs.

Normally when you make one of these things, you will make a stand for your phone, however, initially you will not know exactly where you need to place the phone to get an in-focus image from the lens. Finding this spot means opening the lid to reposition, therefore letting a lot of the light out and dimming the image you are trying to focus.

My solution was to install a nut and lead screw I had acquired for a CNC project I had in mind.

Cutting a hole in the back of the box for the lead screw to protrude and fixing the supports down with double sided tape worked well. I also installed the motor coupler that came with the lead screw as it makes a nice focus knob.

Note: I also had to add a thin sheet of cardboard to stabilise the bottom of the slider block.

Now twisting the focus knob moves the whole assembly back and forth, this allows you to set a blurry picture on the projector screen and adjust the focus to sharpen the image without opening the box.

Step 3: Phone Mount & MORE POWER!!!

I needed to make a multifunction mount for the phone it needed to:

  1. Allow the easy support of the phone
  2. Fix the phone to the slider block
  3. Support the power bank
  4. Leave room for speakers (later step)

I started out with a small cardboard box, I trimmed the lid to sit just under the phone, this was to prevent the case of the phone running against the lead screw and marking my phone. The side flaps were cut until they were just as wide as the phone. A couple of medium binder clips hold the phone in place and just nearly touch the sides of the box when installed, providing left-right stabilisation (this was planned, I swear (",))

Using double sided tape I mounted a 6600mAH power bank with 2 USB outlets (1 for the phone and the other for the powered speakers) and ran the cable to the phone.

Everything was mounted onto the slider block with some hot glue.

Step 4: Software

So now we nearly have a working projector, I wanted to install some apps to make life easier.

First, any convex or fresnel lens will invert your image, the graphic above shows this effect.

Note: this is not my graphic, it came from here and there is other great lens info on this site.

You can use the onboard screen lock option on Android, however, I had set which way my phone would sit and it was not correct, there is probably a way to change it with stock apps but I went to the play store and found Set Orientation, this allows you to manually lock your screen orientation in any mode. So simply select landscape (inverse) and job done.

I also said earlier that I didn't want to have to open the box to interact with the phone to select videos and so on.

There is a great app called Tablet Remote, this allows you use one Android device as a remote control for another, perfect in my case as I have both a personal and work Android phone. Simply install on both devices, pair the Bluetooth and run the app. Your remote control gets an easy navigation remote screen with pointer, keyboard, volume and brightness control.

Step 5: A Quick ScreenTest

With all of the visual equipment ready, I decided to do a quick screen test.

I set the screen against a cream coloured wall (no white in the building) and went for broke with about a 36" picture, I didn't think I could focus it at this size but to my surprise, with 3 twists of the focus knob, I got a pretty good picture.

The photo does not do justice as it was taken in complete darkness on a mobile device, it's not exactly HD but I would be happy to watch a movie on it.

Step 6: Make It All Pretty

Just to take away the look of an old cardboard box and to add a little strength, I wrapped the whole unit carefully in silver duct tape.

Step 7: Let There Be Sound

My final issue was to improve the sound.

The speakers on a smartphone are weak at best, when you close them inside a box it gets even worse.

I was going to use passive speakers (no external power) but then I saw some USB powered ones on special in my local supermarket for only €8 so I snapped them up.

I marked out the location of the speakers at the back of the lid, this is regret as it shortened my focal length and maxed my final sharpened image to about 30", if I were to do it again I would put them to the front.

I found the centre of the speaker using a simple cross and then cut out a circle the size of the speaker itself.

The speakers were mounted using plenty of double sided tape and hooked to the power bank via USB and the 3.5mm plugged into the phone.

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    3 years ago

    If you spray-paint the inside of the box with a matte black (IMPORTANT! DO NOT USE GLOSS BLACK) your projected image will be a lot cleaner and crisper. The black absorbs all of the ambient light put out by the cell phone so you don't end up with a washed out image from light reflected off of the walls of the box and thru the lens. I've made several of these using a portable television and an 8 x 10 Fresnel lens.