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Power Estimates
07/30/2023 at 23:41 • 0 commentsI assembled two of the test boards, one with ST25DV and the other with NT3H2211, then took some measurements with different loads.
Turns out the ST25DV does not regulate the output voltage, and it drops off with load fairly steeply. The NT3H2211 is better regulated, and maintains the voltage better when loaded.
The CH32V003 needs at least 2.7V to run and will consume between 2.3 and 1.9 mA depending on how many peripherals are enabled.
The measurements are listed below and a plot of the output voltage vs load curve fit to the measurements. This shows that the NT3H2211 provides a much more consistent voltage and stays above the minimum 2.7V up to a higher load current. This suggests it will be easier to power the CH32V003 and a few peripherals with the NT3H2211, but there is not much margin.
I hear they are coming out with a low power CH32L103. I wonder how the price of that will compare to the CH32V003. It is available in the same TSSOP20 pin package, but it has more memory and advanced features like USB, which is not a requirement in this case.
Load ST25DV NT3H2211 No Load 4.01 3.09 1000 Ohm 2.72 2.82 500 Ohm 2.37 2.66 -
Course Correction
07/30/2023 at 03:41 • 0 commentsIt has been a while since I posted on this project. I had some inspiration at CrowdvSupply Teardown 2023 and have been putting in some more time on this. I’ve got a few issues with the NXP micro. It only generates 1.8V from NFC which is only enough to drive a red LED without a voltage doubler. It is also missing support from their SDK on GitHub.
With the WCH CH32V003 available for about a dime in volume, I could use it with a I2C NFC EEPROM for about the same price, and drive an RGB LED without a voltage doubler. The CH32V003 is not specifically a low power device, so I decided to see if it would be feasible to drive this from the NFC harvested power. It only runs down to 2.7V, so I need to know if the NFC chips can deliver that when loaded. There are a few options available with energy harvesting output. I’ve used the ST25DV in the past, and NXP has a couple similar devices like the NT3H2211, or NTP53321. The data sheets are pretty sketchy on how much power is available, so I decide to measure it for myself.
I designed a board to test a couple of the device. The ST25DV and NT3H2211 are both designed with 50pF load capacitance, but the NTP53321 is designed for a different load cap, so it would require a different tuned antenna. I already have an antenna I’ve used for 50pF load, so I designed a board to test the two 50pF chips. Unfortunately they have different pin outs, so I had to put down two footprints in parallel. I added the CH32V003 and a current sense amp so I could see how much current it is using. I used my badge as a template and shrank it to save a little on the OSHPark price. I kept the bannana plug/alligator clip pads for measuring voltage and current. I’ll share more about what I found in another post.
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More details on B5
04/20/2021 at 01:19 • 0 commentsMy progress been delayed by samples getting stuck in customs. I would have preferred to have tested out the design before sharing more, but I can at least provide some of the theory of the design that I ordered from OSH Park. It looks really good in after dark.
Here is the schematic for the design:
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A Dilemma, A Hedge & A Hack
03/25/2021 at 03:04 • 0 commentsTo improve performance while operating from harvested power, the LPC8N04/NHS3152 suggest adding decoupling capacitors to a couple of the high drive GPIO pins. There are four high drive GPIO pins that also happen to be the timer/PWM pins. This allows us to use two for decoupling and two for the piezo buzzer. The dilemma is that the SWD signals are also on two of these high drive GPIO, so I have to choose: do I want to give up decoupling or buzzer when I am programming/debugging?
I can provide power externally when programming/debugging, so I could easily give up the decoupling, but SWD may not work with the capacitors installed, so I may need to add and remove them often. On the other hand, I only need to remove a single resistor to eliminate the load of the piezo buzzer, and if I use a large enough series resistor, SWD may even work with the resistor present. But, if I share the buzzer with SWD, I can't use them at the same time. To hedge my bets, I will put decoupling caps and a buzzer on both pairs so I can extend my indecision until I assemble the boards.
The buzzer is one of the largest parts on the board, so adding a second one adds bloat, but there is really no point installing them at the same time so they can share some space. I placed them so they overlap on the board. You might think the hack is ignoring all the placement errors in KiCad, but the real hack is that the silkscreen for the buzzers goes right over the pads for the other buzzer. I know that OSH Park clears silkscreen off pads so I left it there. I can clean it up in the next pass when I figure out which stuffing option to keep.
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Zero height connector
03/21/2021 at 23:06 • 0 commentsA business card or badge should be as close to flat as is possible. With the new features I am adding I want to provide convenient connections without adding to the thickness of the assembled board. A 4mm plated through hole with an oversized pad is ideal for this application. It is probably the most utilitarian connector you can put on a PCB. It adds zero height and zero cost, and it works with banana plugs, alligator clip, and screws. Banana plugs are the perfect connection for a multi-meter.
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'B' is for Banana Plug
03/19/2021 at 04:03 • 0 comments -
Displaying data without an app
03/14/2021 at 03:52 • 0 commentsIn my latest spin, I traded the LPC8N04 for the NHS3152 so I can make use of the added analog features. The added ADC will allow for reading voltages, so this business card will be able to act as a volt meter.
The NFC Forum defines NFC Data Exchange Format so NFC devices know how to communicate with each other. Using one of the standard formats allows phones to recognize the data and act accordingly. The business card uses a standard v-card type to share contact information, but there is no standard type for a volt meter. I could create an app to implement a custom NFC protocol, but there are some other standard formats that can be utilized without requiring a custom app.
One very flexible format is the URL type. This format exists so that you can tap a tag that will direct you to a website. To enable reading the voltage without an app, I will create a voltage display web page and pass the data to the page in the query string of the URL.
This way the business card/badge will also serve as a volt meter that doesn't need batteries and never needs to be recharged.
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I think I found another "B"
03/08/2021 at 03:38 • 0 commentsI have been working on an update to light up the blue and green LED's and to add some additional features. Just placed the order with OSH Park. I'm excited to try their after dark service. Here is a sneak peak:
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To Do List
02/28/2021 at 18:51 • 0 commentsThe current design is a functional Beeping and Blinking Business card Badge, but there are a few improvements I would like to make. Here is some of what I hope to accomplish in a future update:
- Re-enter design in KiCad and try to move past the shame of the disgusting schematic I created in upverter.*
- Add voltage doubler to light green and blue LEDs from harvested power
- Add cap to an I/O pin to store harvested energy
- Migrate to NHS3152 to get analog for new features
---------- more ----------* The tool had nothing to do with the quality of the schematic. I have no one to blame for that but myself. I am moving to KiCad because it is a much more capable tool and I have been burned by upverter updates in the past. I promise the new schematic will be a vast improvement.
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Lessons Learned - NFC
02/27/2021 at 03:09 • 0 commentsTo be useful as a business card, I can't expect a stranger to load an app to get the vcard. This means I need to use the standard vcard NDEF and live with the power that provides. That only lets me play a couple quick tones. I was hoping to find a way to trick the phone into reading longer, but I haven't found it yet.