Introduction: PCB UV Exposure Unit

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A UV exposure box is an extremely useful piece to make PCBs.

I have searched and found that they are too big and too expensive for my uses. In addition, all the devices require an external power sorce.

Thereupon I decided to build one that fits better on my needs.

My requirements:

  • Compact dimensions
  • USB charging
  • Long battery runtime
  • No buttons or knobs
  • Very simple use

So I built a box, with which you can exposure up to 184 PCBs on a single charge!

Step 1: The Components

Mechanical Parts:

-Glass pane:
From an old picture frame. 145 X 95 x 2,2mm

-Top cover:
Wood (152 x 99 x 3mm) with glued foam (152 x 99 x 7mm).

-Screws:
4x M3x 5mm
2x M3x 6mm
6x M3x 10mm

-Washer
4x for M3 (3,5 internal)

-White Paper
1 (A5 or larger).

Electrical parts

-Battery pack:
Li Ion battery (from old laptop batteries)
2x flat 2,2Ah (98 x 40 x 6mm)
1x round 2,5Ah (18 x 65mm)

-Li Ion USB charger (TP4056):
In every household you can find USB chargers -> no problems to refuel. Link

-UV LEDs:
The LEDs are 5-10 times more efficient than fluorescent lamps. This helps to improve the operating time and keeping the form factor small. In addition, only a MOSFET with a PWM signal is necessary for driving.
35x 390nm, 350mcd,120°, 3,3V, 20mA

-LEDs
2x green 5mm
1x red 5mm

-ATTiny:
I opted for an ATtiny85, because it is small, has enough outputs and functions.

-Mosfet:
The 4468 Mosfet is in a small SOIC-8 package, thereby saving space.

-Capacitive sensor:
The sensor comes under the front to have no visible buttons and knobs. Link

-Resistor:
To protect the ATTiny output from damage.
1K SMD1210

-Perfboard
140 x 80mm for the light pannel
Optional one for the control circuit

-IC socket
Optionally, to replace the microcontroller without soldering.

-Pin / Socket strip
2x 2pin
1x 3pin
1x 4pin

-Cable

Step 2: The Box

I've designed all parts in Google SketchUp.

As a guide, I had all components and especially the glass sheet.

The main body part is cut in half, because my small printer has a plate with only 150 x 150mm.

Then I export the STL files, slice and print them on my printer.

All Files are in this zip folder:

  • stl
  • skb
  • EAGLE
  • Arduino sketch

Attachments

Step 3: Circuit / PCB

The circuit and the PCB is designed in EAGLE.

It would be nice, to solder all parts an the designt PCB, but to etch the PCB you will need the circuit :(
So either you assemble the circuit on a breadboard and then build the final control circuit, or solder all on a perfboard.
I use pin / socket strips to easy connect and disconnect all components, but you can also solder them direct to the PCB.

LED panel
The LED Panel is a 140 x 80mm Perfboard uniformly equipped with LEDs.
Between the lamps and the Perfboard is a white sheet of paper to reflect the light to the PCB.
All LEDs are paralell.

Step 4: The Code

The program originated Arduino typically in C ++ in the Arduino IDE.

If the microcontroller is not in use after a certain time he will go in the sleep mode to save power.
To wake up the µc you only have to touch the sensor at the front. Through this you can also choose between the exposure times and also can cancel it.

I flashed the ATTiny with an Arduino UNO and a DIY programming board like this.

PWM Signal for LED:
The batteries have a voltage of 4.1 volts in the charged state. Since the LEDs only tolerate 3.3V, so I use a PWM signal to drive them.
I'm not using all the Power of the LEDs, because then the quality is good even with poor templates.
-> AnalogWrite (Mosfet, 205);

If you want to shorten the exposure time, they can also use the complete power of the LEDs (larger PWM value).

Sleep Mode:
To use the sleep mode, I include the libraries "avr/interrupt.h" and "avr/sleep.h"
In this mode the ATTiny only uses 0,5µA!
The touch sensor uses much more (6,5µA).
-> If the device is asleep it consumes only 7μA.

6,9Ah of battery capacity results in gigantic 112.5 years in standby mode.

When the UV LEDs are on, the system needes 720mAh.
On time = 6,9Ah / 0,3A = 23h
With my Bungard PCBs are 450 seconds (7,5min) optimal.
-> 184 PCBs with one battery charge!

Step 5: Assembly

After everything has been printed and soldered, can be assembled jew started with.
At first I glued together the two main parts.
While these dried I got down to the front. For this I glued the LEDs and the touch sensor with superglue in. Then I soldered the wires to the LEDs and the sensor.

I soldered the cables to the Battery and conneted them in paralell to the USB Charger.

Then i connetet everything together and tested the hole thing.
And YES, it works ;)

After that i screwed the LED panel and the front in place.

Now the is moment, to place the glass plate in the top.
And yes, this fits.

Now I screwed on the lid with the attatched hinges.

Finisht, looks good. Now plug it in to fully charge and make PCBs.