A volunteer colleague in our local Energie cooperation asked me for some help for a project he wanted to do. He found an interesting tool he wanted to build, more for the fun of it than that he had an urgent need for the tool, but that's the way it goes with many of my own projects as well.
His own, very antique, logic level probe/continuity tester died on him, so his original idea was to use the housing of that probe. When I looked at it, I found that it had enough room, but would have been an ugly overkill for the design he found.
That design and description can be found on a website here. The picture below from the original design is from that site.
The original designer, Pieter-Tjerk de Boer, even included the KiCad details, unfortunately, they were from a much older KiCad version I am using, and by the time I could work with it, and made some changes, I decided to start with a new KiCad schematic and redid most of the layout. Not that the original one was poor, but KiCad gave me too many problems with the verifications and library conversions between the two KiCad versions.
The design of the tester is really neat, well done Pieter-Tjerk! This tester will show (audibly) when there is a good trace connection and is also sensitive enough to pick-up very small Ohm value changes, an indication of a poor trace.
Here is my version of the design, with a few small changes based on the original one.
I changed the way the tester is charged, by eliminating the series LED in the USB 5V path, which is not needed in my opinion. You know when you are charging it. As an indicator to show a fully charged Super Cap it did not work really well for me. The LED in the original design will only turn off when the Super Cap is really fully loaded and there is no voltage differential across the LED anymore. I tried it, and it takes way too long to my liking. The way I understand this, is that the topping-up of the Super Cap goes against the current draw from the circuit. Eventually the LED will diminish and go off, but I had another idea on how to use the probe, without a permanent USB connection. In any case, with the series LED gone, I could also reduce the series resistor to a much smaller value, making the charge time less then a minute for a full charge of very close to 5V, even with more than double the capacity for the Super Cap, which is now 0.22F
With the charging cable gone, what is now more important for this circuit is to have a warning when the supply voltage, from the Super Cap, is getting too low. According to the datasheets for the Opamp, comparator and Schmitt-trigger that was 3.3V, so I added a two transistor circuit that would turn on an LED when the voltage is too low for a proper operation. You are relying solidly on the beeper for a good connection, so the circuit must work reliably. The two resistor values I used for the tripping voltage setting are pretty low for an "ultra-low power" design, but that can be changed by increasing the values for R11 and R12 and make them 150K and 520K, or even higher, as long as there is enough of a Base current for Q1 to function. You can also increase the value of R10 to reduce the current some more.
I do not envision that you will be using the probe for an extended time while searching for a bad connection. In any case, just topping it up within a minute will get you going again.
Another change in the layout was required because I did not have a 33MOhm resistor in my 0805 or 0603 collection. I reverted to using a 13M and a 20M value combination with an 0805 package instead of the 0603 I use for most of the other ones. You can use any combination as long as it is close to 33M. Also R8 is an 0805 package due to the higher charging current when the Super Cap is empty.
Finally, I changed the USB connector for the charging. There is no need for the USB lead to be connected, you can now stick the probe into a USB slot for a minute to fully charge the Super Cap. After that it's much easier to use when you are probing around and don't have the USB cable connected.
By the way, with this design, the Super Cap is always charged and only the connected circuits will drain the charge. The discharging process will go faster as soon as the voltage is below the trigger point when the circuit with the LED will drain it faster. Keep this in mind when you store the probe while it still has a charge. I contemplated using a micro sliding switch to remove the power from the circuit, but then the Super Cap would keep it's charge "forever". I could also have used a draining resistor and a switch to drain the charge of Super Cap itself but didn't like that idea either. It is what it is...
Here is a picture of the 3D viewer in KiCad to have a better look.
As with the original design, you can use a USB connector with THT pins, as well as the SMD version.
I found a test pin that was appropriate by searching for PCB test pins. The one I ended up buying came with a qty of 20 and was called P100-B1. I do not think that this is a standard, so be careful what you order. The link for this part is in the BOM.
When we build our version of the tester, we could verify that these probe pins, that have a spring in them, need to be soldered together to reduce the resistance. With a solid connection, there is a firm buzzing sound. However, with a not so perfect connection, the buzzer emits a distorted sound as a warning.
I also reduced the length of the PCB test pin a bit, but just cutting of about 12.5mm from the back-end.
One of the other boards was used as the second probe. I created a pair of holes in the PCB to act as a strain relief for the connecting wire. I used a small piece of blank wire (from a THT resistor), that I soldered together on the backside, without melting the plastic insulation, of course (;-))
The design information, Gerber files and BOM are on my GitHub site here:
I may add more later... Stay tuned...
If you like what you see, please support me by buying me some Java: https://www.buymeacoffee.com/M9ouLVXBdw
For those that already did, thank you!
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