UV Light Box

Home made DIY printed circuit boards (PCBs) are a must for the electronics hobbyist. Well at least for those that don't have ready access to a commerical PCB manufacturer that has reasonable prices for "one-offs" and or limited production runs. What's more, being able to DIY a PCB means "getting on" with a project without waiting for a PCB to arrive in the mail (fitting a hobby into "spare time" is often nerve-racking). Also, imagine waiting those precious few days for a PCB to arrive and there was a mistake in your schematic - insert obligatory expletive!

There are a wide variety of techniques for DIY printed circuit boards ranging from literally hand drawn with indelible ink pens, to the toner transfer method through to CNC machine routing of the copper PCB surface. I have had some success with the toner transfer method, but for surface mount components and other electronic components with fine pad and trace width requirements, recommend the photo-resist method.

A photoresist is a light-sensitive material that covers a copper clad board (the PCB to be) and after exposure to appropriate wavelengths of light (generally ultraviolet UV) becomes soluble (a positive resist - the "reverse" is true for a negative resist) with a suitable developer chemical solution. Then after washing away the exposed, developed photoresist, the copper surface not now protected by the remaining photoresist can be etched away with a suitable chemical, leaving behind the desired pads and traces of the PCB (1).

Even though sunlight (which obviously contains UV wavelengths and not just visible light) can be used to expose a photoresist, clearly this does not provide much control of the process. Therefore, a UV exposure box is necessary. The commerical UV exposure box products are rather expensive, but conveniently, a DIY UV exposure box is a relatively easy project, and can be generally made predominately from salvaged parts (except usually for the actual UV lamps).

There are a number of UV exposure box projects detailed online, the common thread being that each is relatively unique due to the particular components (i.e., what was available in the "junk box") utilised. So while the circuit and construction details discussed and documented in the following sections may not be exactly relevant, as you may have different availability to particular items, the overall approach and design process should be helpful when "rolling your own".

The basic requirement for the circuit is a timing function that powers-on a set of UV lamps for the user input desired period. Ancillary functions include accepting user input denoting the desired time period, providing user feedback as to the remaining time the UV lamps will be illuminated and safety aspects such as ensuring that the UV-lamps are powered down if the illumination compartment lid is removed. The maximum time period that the UV lamps will require illumination is less than 15 minutes (assuming 2 or 3 10W or similar UV lamps are used).

Such functionality is readily applicable to a combination of discrete components and specific function IC's (e.g., the ubiquitous 555 timer with 7-segment LED displays and driver chips such as CD4511 or similar). However, a microcontroller approach was taken, more as a learning opportunity at the outset, but which proved to provide a low chip count, low cost and extensible solution.

Power Supply

A wall-wart power supply was initially favoured rather than constructing a dedicated DC power supply dropping/converting from a AC wall socket. Surplus chargers from laptops are readily available which provide not only a safer option but also a much more economical option (generally zero cost for a surplus charger). However, since the UV light box was expected to draw relatively high current and require AC power for the UV lamps in any case, a AC to DC power supply built around a transformer was included as part of the power supply for the circuit.

A 230VAC to 12VAC (10A) transformer steps down the mains AC and feeds a full bridge rectifier (BR1) to produce 12V DC. The 12V DC is then converted and regulated to 5V for supply of the logic and microcontroller circuitry by a LM7805 linear voltage regulator (U1). Filter capacitors C2 and C3 are as per the datasheet recommendation for the LM7805.

Basic Microcontroller Connection

The minimal circuitry required to operate the typical PIC (PIC16F84A in this case) include 5V regulated supply, oscillator (generally external crystal oscillator), in-circuit serial programming connections and a method to connect to the various I/O ports.

The power supply is discussed previously (LM7805 regulator). The PIC datasheet discusses required type of crystal or ceramic resonator and necessary capacitance, however, a 33pF capacitance is readily available, falls within the middle of the recommended ranges, and in practice does not seem particularly critical.

Resistor R2 provides the necessary connection to 5V for the MCLR pin, and switch SW3 provides a reset function for the microcontroller. Connector J5 is for the in-circuit serial programmer (ICSP) which allows hex code (i.e., the firmware) to be loaded into the PIC flash memory.

I/O Connections

The I/O provided via the microcontroller hardware/firmware include 2 x 7-segment LED displays for showing timing information (both setting desired time of UV lamp illumination and once running, time remaining), a piezo-buzzer to provide audio feedback that UV lamp exposure has completed, two switches (SW1 and SW2) for setting timing information and start/stop UV lamp operation and finally, controlling a relay (RL2) via transistor Q2 to energise the UV lamps when required.

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  UV Light Box Schematic

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  UV Light Box PCB - Top Copper

UV Light Box PCB - Top Copper

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  UV Light Box PCB - Silk Screen

UV Light Box PCB - Silk Screen

This project is still under active development.

This project is still under active development.

Note: Image loading can be slow depending on server load.

ref001: https://en.wikipedia.org/wiki/Photoresist.

ref002: This project is still under active development.