PCBs in the Wild!

I have been working on a couple of prototyping boards for general “hey I need a PCB for this oh I have one here” type of projects. Since I’m working more with Surface Mount Technology (SMT) these days and have more of those part in my “stock”, I designed some prototyping boards with this in mind:



I have sent a few of these to some people that I chat with on the #eevblog IRC channel, and one of them was kind enough to send me an “action shot” of a little LED-based project he used my PCB for!


And another one from c4757p:



Power Designs Inc 6050C Display Mod

Here’s a modification that I have been meaning to do for a while. It involved replacing the PCB in one of my power supplies with a modified version designed by me that upgraded the size of the 7-segment LED display.


I ordered the board from OSHPark. My experience with them has been positive; the turn around time was about 2 weeks and the gold finish is very nice. I’m fine with the colour of the solder mask, however note that the mask is a matte finish rather than the typical “pearl” or “glossy” finish that I am accustomed to seeing on PCBs. Not a big deal, but something to think about. Also, the traces are a bit difficult to see through the solder mask.

IMG_5737 IMG_5739 IMG_5741

I of course did not fully check my notes when throwing together the schematic for this project which resulted in the boards I received having the ‘a’ and ‘g’ segments reversed. A few cut traces and a jumper wires later and all was working as expected.

IMG_5745 IMG_5744  IMG_5742IMG_5743

I originally tried to think of a better way of re-attaching the new display board to the existing display measurement/logic board, but in the end the simplest solution won out and  I just re-soldered the new display board back. The header pitch is 3.81mm and was a total pain to find (yay eBay!).


The end result is a bright, clear and LARGER display. Here I have contrasted it with the model right after the 6050C, the 6050D which has a larger digital display (and also does not display the measurement mode as the 6050C does, E or I).IMG_5748If I decide to pick up any more of these supplies, I think I’d make the same display modification to them as well. I have been looking at the 6050A models (which can usually be had for cheaper): these might also be good for a “digital makeover” involving removing the analog meter and designing a new digital display PCB.


Tektronix DMM 916 Back-light Mod

In keeping with the theme of back-light mods, I have another one here for you all. Recently I was able to get my hands on a well-loved (read: had the piss kicked out of it) Tektronix DMM 916. The specs are nice:

  • 4.75 digits
  • 40,000 count
  • Basic DC accuracy of 0.06%+1 count

The only problem which I didn’t know until I got the meter in my hands was that the back light was horrible:


Wait, where is that back-light?IMG_5671

Still can’t see it? Turn off the lights!IMG_5670

I’m not sure if this is “factory standard” or just a sign of the age of the unit, but either way it needed some change. The first thing I did was to open up the meter and check out the display:


There is a small slot on one side of the display assembly where the lamp bulb pokes into the light pipe. At first I thought I might use a standard through-hole LED, but realized that I wouldn’t be able to mount it without either cutting the trace (for the limiting resistor) or cutting the display. I didn’t want to mod the board, in case I or someone else wanted to restore it back to a incandescent bulb. So I choose to use a SMD chip LED and resistor, and build it “tee-pee” style on the top of the display PCB, so that the LED and resistor would stick up vertically into the display light-pipe recess:


The LED is blue, Digikey part number 475-2816-1-ND with a 270ohm current limit resistor. The bulb sank about 20mA while the LED uses ~18mA, so a bit more efficient. I’m still not sure about the blue, but I figured it matches the theme of the case, so why not:


And as is evident, it is much brighter even with the lab lights on. Curiously, it is not much more legible in the dark in terms of the digits on the screen as I would have thought.

HP8642B Signal Generator Mod

A quick mod post here. I saw this post by Kerry Wong, and having the same hardware myself (and finding the backlight ridiculously dim) I thought this mod was a great idea and wanted to try it myself. First, here is a shot of the original backlight:


I pulled the front panel apart, and decided to use white SMD LEDs for my replacement mod:


After soldering up everything and reassembling (and of course cursing a lot due to the number of defective white LEDs that I didn’t realize that I had), the result is beautiful!



And here is the same Easter egg that Kerry found (hold down the MSSG key while powering on the unit):



Lab Frequency Standard

I’ve been rather silent for a couple of months now, but I have been busy! This time it is a laboratory frequency standard! This project is modeled on the work done by Gerry Sweeney over on his blog. I have been assembling the various pieces needed for this build for several months now, and just recently decided to take the plunge and do the build.

It started off with a Rubidium frequency standard from Frequency Electronics, the FE-5680A. These can be purchased used off of EBay for ~120$ CAD. Buyers beware: I had success with my first unit, but others have not been so lucky. There are numerous configurations of this unit, with various features present or missing, including the 10MHz sine output. This project requires at a minimum the 10MHz out. Some units will have this via a dedicated BNC or SMA connector, others (like my unit) will have it present on a pin from a DB-9 connector.

The second most important component is the case which will house the completed project. I was originally going to choose a basic project box, but Gerry was pointed towards a distribution amplifier for video. Since the signal from the FE-5680 would need to be replicated on multiple outputs to be useful, some sort of amplifier was going to be needed. It just so happens that 10MHz lies in the range of off the shelf video amplifiers, and the model I got by Extron, the ADA 6 Component was perfect for the job AND was only 40$ on EBay.

Other than just putting the Rb standard in a box with a bunch of outputs, I wanted to have an indicator LED that lit up when the Rb was locked on 10MHz. My particular unit will pull a pin on the DB-9 connector LOW when the Rb is locked. A quick transistor circuit with a regulator feeding off a 17V supply and I had a small prototype board completed:


Once I had the “interface circuit” figured out, I started laying out what the mechanical construction of the project would look like. I decided to use the case itself as a heat-sink and seeing as how the Rb standard likes to be warm (haha, non-heat-sinked, the case measures ~65C), I decided to mount it to the top half of the box:

IMG_5577 IMG_5578

The Rb standard requires 2 different power inputs: 5V and15-18V. The distribution amplifier also required 2 different power rails: 6.2V and 17V. I decided that the easiest way to power it all would be to pick up a 18.4V laptop power brick, rip the guts out, and add a LM317-based regulator to get the 6.2V to my existing interface circuit. I discovered that the Extron was actually using the 17V rail to power a buck converter-inverter circuit, so the 18.4V input didn’t change it too much.

IMG_5580 IMG_5581 IMG_5585 IMG_5587 IMG_5588 IMG_5589 IMG_5590 IMG_5591

I wired up a small wiring harness to go from the Rb standard’s DB-9 connector to the various parts of the interface board. I then modified the amplifier board to change the input/output impedance from 75ohm to 50ohm:


Not having any 50ohm resistors handy (and they’re damn expensive as well!), I used 2 100ohm resistors in parallel stacked on top of each other:

IMG_5584 IMG_5583

I soldered in the coax to the center channel, and then jumped the signal from the center channel to the 2 other ones with some wire. This means I will have a total of 18 10MHz outputs on this standard 🙂

The testing and assembly:IMG_5593 IMG_5597 IMG_5598 IMG_5600 IMG_5601

I ran into one problem when testing, which was a deformed lower part of the sine wave on the outputs (all of them). You can see it on the oscilloscope to the upper right:


I started probing around (“THOU SHALL CHECK VOLTAGES!”), and discovered that the negative rail to the op-amps was running at ~0.3V, which was not right. Running back through the circuit on the amplifier board I realized that I had flipped the 6.2V and 17V rails on the input connector after splicing it into my interface board. After switching them back, everything was working as expected!


Turns out my frequency counter on the upper right is probably off by 0.5 Hz… not too shabby. There are a couple of things that need to be done: adding a heat-sink to the top of the case for extra dissipation and adding some passive ventilation holes on the top/back of the case and on the bottom.

Another Nixie!

I found the first Nixie-Tube clock that I built from a kit so awesome, however the only problem was that it got requisitioned for the living room! So of course I had to order another one for my office.

Again, Pete from PVElectronics did a great job on getting the clock kit to me, and assembly went smoothly. Towards the final stages of the build, there is a step that tests all the tubes, the micro-controller and the high voltage generator with a test pattern that counts up from 0 to 9 and then cycles over. It was at this step that I ran into a weird issue where all of the tubes would display all digits when they should have been displaying 4 or 8. I finally isolated the problem to a single tube (although why it would affect all tubes was unclear at this point):

After Pete and I scratched our heads for a few days, we finally came to the conclusion that it must be some sort of internal short in the actual tube (weird!). He promptly mailed me a new tube + circuit mounting board and I was back in business and finished the clock:

And the color cycling:

And here are a few build pictures I took along the way:

IMG_0265 IMG_0268 IMG_0278 IMG_0279 IMG_0281 IMG_0282





And some shots of the questionable tube:


IMG_0290 IMG_0291 IMG_0292 IMG_0293 IMG_0294 IMG_0296 IMG_0297

And the test/debugging setup that I put together.  Here (and I’ve said this before) I’m using two of the test points on the board. I have soldered in two single-pin sockets so I can easily attach a breadboard/other test components to the live board:

IMG_0306 IMG_0307 IMG_0308 IMG_0309 IMG_0310

And the case being assembled:

IMG_0276 IMG_0277

USB to Serial Adapter Project

Not satisfied with the “typical” USB to serial cables that one can readily buy, I decided to design my own.  The basic reason was just another excuse to practice designing a circuit and laying out a printed circuit board (PCB), but to also create a very small unit that can be easily transported. I additionally wanted to have some sort of visual feedback of data transfer over the adapter, so two LED’s (receive and transmit) will address that.

The circuit design came from the application notes on the FTDI chip that I am using, the FT230X:


I did a few tests with a bread-boarded version of this circuit. You will notice that there are 2 bread boards; the bottom one has the USB to serial circuit, the top one has an Atmega8 micro-controller with a some program that echoes back characters it receives on its serial interface (used for testing):



A close-up of the USB to serial circuit:


And the two circuits/bread-boards separated:IMG_5307

The other requirement was size: that whole circuit will need to fit within this little box:


I have actually laid out the board, and so will be finalizing it in the coming days. I will write up another post with the board layout, 3D shots and a bill of materials. In terms of price, this will not be saving me any money over buying a pre-built adapter.

MOSFET Driver Board, Rev C

It is said that one should learn from one’s mistakes. In the process of learning how to design and get a Printed Circuit Board (PCB) manufactured, I have indeed made a few mistakes. I would like to share them with you here.

First, a quick update on the board that I was designing. My requirement was to be able to drive a spinning light assembly (pictured below) from a micro controller (MCU). Rather than use the traditional relay-based system (which would use a transistor to achieve MCU control) I decided to design it using a MOSFET (a big ‘ol voltage-controlled switch). I also figured that this would present a good opportunity to learn how to design and lay out a very simple PCB.

Always, ALWAYS, Double-Check Your Work

I don’t know if I can emphasize this more: a core theme in all of my mistakes during this project have been due to this. My first board, Rev ‘A’, suffered from a major circuit design error that occurred when I moved my circuit design from the bread board to the schematic CAD tool (I got the circuit backwards: I needed one that worked with a “NPN” transistor, but designed one that worked only with a “PNP” transistor).

Here is my awesome not working circuit (Rev A):


Here is the corrected circuit with the motor BEFORE the NPN MOSFET:


My second revision, Rev ‘B’, was not manufactured; it had the corrected circuit design, but I wanted to tighten up the physical PCB layout so I created Rev ‘C’:


I had this revision manufactured. The mistake that I should have caught on this time was the adjustments I had made to the hole diameters for the terminals (for the inputs and outputs); I had forgotten to adjust the copper (called “annular ring”) around the holes to make them larger as well. This wasn’t a show stopper, as I was able to solder the components on the board still, but it was borderline.

As an aside, I noticed that the front solder-mask (the blue color) was not uniform between boards; some were blue and others were slightly blue-greenish, about half and half. I also noticed that the silk-screening was less than awesome on all the boards. Note that this board was Hot Air Solder Leveled (HASL) and NOT gold plated like the first revision board.



Thou Shall Check The Data Sheet. Again.

The second biggest mistake that I made was not re-checking the foot prints (the “physical” sizing of a component) during and after I completed the layout of the PCB. DO THIS. It avoids embarrassing mistakes such as holes that are too small to fit the leads of a through-hole component. You should also re-check the pad/leads in the data sheets to make sure they fit with your component symbols and circuit design. Don’t assume anything!

Always Have a Reason

When you place a component, connect a trace, add a connection, always have a reason for doing so! This will prompt you to ask yourself why you are doing this and perhaps lead you to discover a mistake in the making.

Test Points

This board was a bit too small to warrant test points (PCB pads that are designed specifically to be accessible to probes for testing properties of your circuit) but remember to think about adding them in. This can make debugging a malfunctioning circuit much easier later on. Also remember that it may be a good trade-off when designing a prototype device to take a little more space but offer debugging facilities such as test points and wire jump points.

Remember To Add Your Name!

If you truly are happy with your design and PCB layout, then you won’t mind putting your name, date, revision information and other useful identification information on the PCB. Do it! This will make differentiating board revisions easier as well as identifying you as the designer. Take some credit!