Introduction: Battery Powered Portable VU Meter
Step 1: Materials Required
For this project, you will need the following materials:
1. 1 LM3916 IC
2. 1 LM386 IC
3. 10 LEDs
4. 1 UV Reactive PCB Board
5. 1 18 pin IC socket
6. 1 8 pin IC Socket
7. Various SMT Resistors
8. 1 Dremel tool
9. 1 UV Exposure box
10. Developing chemical
11. Etchant (I use Ferric Chloride)
12. 1 Fine soldering pencil
13. Fine silver-bearing solder
14. 4 3v coin cell batteries
15. 2 Sockets for 2 coin cell batteries each
16. 1 switch
17. 1 electret microphone
18. 3 (1 uf SMT capacitors)
19. Denatured or Isopropyl alcohol
In a pinch some of these components may be purchased from Radioshack but your best bet is to buy them from DigiKey.com or from Frys Electronics, or other equivalent local electronics parts retailer.
Step 2: Preparing the PCB Artwork
I created the PCB artwork in a program called ExpressPCB, which is available for free download and is surprisingly functional. The resultant artwork is pictured on this page. Next, I printed out the PCB artwork on a transparency. When printing the top copper layer of he PCB within ExpressPCB, the yellow component outlines are not printed, only the red traces are printed. I then cut out the printed portion of the artwork. This will define the size and shape of the PCB. The third picture is a screen shot of ExpressPCB showing labels for all of the components.
Step 3: Cutting and Preparing the PCB for Exposure
To make PCBs, I use the UV exposure method, which is only slightly more difficult than and significantly more precise than the toner transfer method. To start out, I cut the PCB to be the same size as the outline of the PCB positive. I first drew a rectangle the same dimensions of the PCB on the protective layer of UV Reactive copper covered fiberglass board, then cut it out using a Dremel tool equipped with a diamond wheel. Make sure that once you have removed the board from its protective package it will not be exposed to any UV. When I am working with UV reactive PCBs, I keep the garage illuminated with a single incandescent bulb. Fluorescent and halogen lights both output enough UV light that they will expose the board through the protective layer of plastic. Additionally, ensure that you are wearing proper protective gear when cutting fiberglass.
Step 4: UV Exposure
Now that you have the UV sensitive PCB cut to size and the PCB positive cut to size, you are ready to expose the board. Only remove the protective layer from the PCB right before you place the positive on it, or else dust particles will attach to the board, which will mar the final PCB. I made a UV exposure box by buying a common blacklight and attaching it to the inside of the top of a large plastic box. Thi has worked flawlessly for me so far, and is far cheaper then purchasing a premade UV exposure system. To expose the PCB, first remove the protective layer, place the positive transparency on top of the board, and place it in the UV exposure box. An exposure time of 10-11 minutes is recommended.
Step 5: Preparing the Developing and Etching Solutions
Now you need to use a little chemistry. Once the PCB has been exposed, turn off the UV light and prepare the three chemicals you'll need. Mix the developing agent with the amount of water prescribed on the bottle, and place in a plastic container large enough to lie the PCB flat in. Next, fill a similarly sized with water, and fill another identical container with Ferric Chloride or similar copper etchant. Make certain that the container you place the etchant in is made plastic, copper etchants and especially Ferric Chloride are rather fond of eating through any metal they come in contact with. In the main picture shown below, the blue fluid is the developing agent (it started clear) the orange fluid is the rinse stage, and the very dark brown fluid is the Ferric Chloride.
Step 6: Developing and Etching the PCB
Once the board has been exposed, drop it into the developer solution. Make sure to wear chemical resistant waterproof gloves to protect your hands. I recommend long-wristed thick rubber gloves that which be purchased from the average grocery store. These are superior to average latex gloves in that they protect the wrist, they are more resistant to tears and abrasion, and they can be reused. Once the board has been developed to the point that only the desired traces are visible as remaining etch resist (the green coating on the board) and the surrounding area is exposed copper, you will want to rinse the board. If all the etch resist comes off, the board was likely exposed before you wanted it to be or it was left in the developer solution too long. If none of the etch resist comes off, the board likely wasn't exposed properly. Once the board has been rinsed, you should be able to see the desired traces in the green etch resist, as shown in the primary picture of this page. The board is now ready to be etched. The Ferric Chloride works faster when heated and agitated, but work just fine with neither. Drop the board in the Ferric Chloride, checking on it at half hour or hourly intervals, until all the exposed copper has been etched away, as in the second picture. Once the board has been etched, remove it from the Ferric Chloride and rinse it thoroughly in the rinse stage. Finally, remove the etch resist on the desired traces using either denatured or isopropyl alcohol. The PCB is now ready to be drilled.
Step 7: Drilling
Now you need to drill holes in the PCB for the through-hole components. My design for this VU meter uses as many SMT components as possible to streamline theboard and to minimize drilling, which I find to be one of the most tedious parts of making any PCB. Mkae sure to use a drill press, or the drill bit will almost certainly break. I used a 3/32" drill bit to make the holes. The drill bit is a dremel tool drill bit purchased at Home Depot. The first image shows my drilling setup and shows the board when it is partially drilled, while the second picture shows the board with all the holes drilled except for those for the battery holders, which require a larger hole as the leads are thicker.
Step 8: Soldering the Components to the Board
It is assumed that you possess intermediate soldering skills, as I will not cover the extreme basics of through-hole soldering here, there are many Instructables which cover this very skill, I will only go in depth with regards to soldering SMT, or surface mount, components. To solder SMT components, first heat up one of the two SMT pads and melt some solder to uniformly cover it, as shown in the first picture. Next, hold the soldering pencil on the pad with the solder on it, maintaining it in a liquid state, while holding the component in place with a pair of fine pliers. Then remove the soldering pencil, allowing the solder to cool. Finally, heat the other pad and melt some solder there, ensuring a good mechanical bond and a good electrical connection. The optimal shape of solder you are going for is shown in the second picture. The third picture shows the size of the SMT components I used, as compared to a 5 mm LED. The fourth picture shows all the SMT components attached, where the fifth picture shows the type of solder I used. I recommend using fine silver-bearing rosin-core solder, such as this solder I purchased from Radioshack. Finally, solder on all the through-hole components.
Step 9: Preparing for Testing and Completion
Insert the four batteries (2 per holder) and the VU Meter should be fully operational. Turn it on using the switch and it should now respond to people talking as well as other ambient noises. Assuming it works as planed, the VU meter is now complete.
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