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Last week, Jon mentioned how several of the mechanical engineers here at Pololu were assigned a simple board to develop. Well, our new Breakout Board for microSD Card is the first board that I designed!
Electrically, this board is pretty basic. It breaks out all of the connections available on a microSD card into two rows of 0.1″-spaced pins for easy prototyping use with standard perfboards, solderless breadboards, and 0.1″ connectors. We tried to arrange the pins in a convenient order by placing all of the pins needed for SPI mode on one side of the board (along with the card detect pin). What makes this board interesting mechanically is that it is the first of our products to use a connector for a microSD card. The push-push type connector is positioned so that when a microSD card is fully inserted, it protrudes slightly beyond the edge of the board to allow easy access to the card. The integration of our electrical and mechanical procedures allows us to make 3D models such as the one below to help support our products. We currently use models like this in the dimension diagrams we publish for our boards, but we hope to eventually make the models themselves available too.
Integration with 5 V systems
There are no other components on the board aside from the microSD card connector. Since standard microSD cards use 3.3 V logic, no extra considerations need to be taken to use it with a 3.3 V microcontroller, but signal conditioning is required for use with 5 V microcontrollers. We did some tests using our 4-channel level shifter and an Arduino Uno to read and write from a microSD card using the Arduino SD library, and we had successful results; however upon closer inspection, we noticed the level shifter did not have time to shift the 3.3 V signals all the way up to 5 V, so this setup only worked because the Arduino Uno registered 3.3 V as a high signal. With a 5 V microcontroller that accepts a 3.3 V signal as high, the microSD card outputs can be connected directly to the microcontroller, and the microcontroller’s 5 V outputs can be shifted to 3.3 V using a simple voltage divider. We found the resistor values needed to be fairly low – we settled on 500 Ω and 1 kΩ resistors. Since we used the standard Arduino SD library, our tests were done at SPI speeds of 4 MHz. In systems operating at higher speeds or with more stringent logic voltage requirements, it might be necessary to use a buffer IC or other high-speed level-shifting solutions.
For more information about this breakout board, see its product page.
If you’ve checked out my company profile, you might have noticed that my focus at Pololu is on developing mechanical parts. So, I am particularly excited to reveal this basic board I designed! (Don’t worry, like Jan mentioned in this blog post, we have support structures for checking all of the work we do, so other experienced electrical engineers here assessed and contributed to my work.)
The DRV8838 motor driver carrier is the smallest motor driver we’ve made yet. With a motor supply range from 0 V to 11 V and the ability to deliver a continuous 1.7 A (1.8 A peak) to a single brushed DC motor, the DRV8838 is an exciting option for controlling any one of our plastic or micro metal gearmotors. (That includes the high power versions.)
For more information about this carrier, see its product page.
But Jon, why are mechanical engineers designing PCBs?
As our products get more sophisticated, we find ourselves wanting to integrate mechanical and electrical aspects of our design process. To give the mechanical engineers better perspective on what goes into designing electronics, we were each assigned a simple board to develop. We expect this to improve our all-around engineering abilities and also to lead to additional benefits for our customers, like better documentation and support.
When the Raspberry Pi Model B+ was released last month, one of the most exciting features for us was the availability of a second hardware PWM output that can be used for motor control (previously, only a single hardware PWM output was available on the Model A and Model B). Two is a really useful number for robot builders, since that’s what you need for a basic robot platform, so we thought it would be a great time to make a dual motor driver kit for the Raspberry Pi.
The new Pololu DRV8835 Dual Motor Driver Kit for Raspberry Pi B+ is a minimal, low-cost motor driver expansion board based on the Texas Instruments DRV8835 dual motor driver, one of our favorite motor driver chips for small robots like the Zumo because of its excellent combination of size, cost, and performance. We carry a basic breakout board for this chip and just released an Arduino shield based on the DRV8835 last week.
When assembled, the board plugs into a 2×17 block of pins on the Raspberry Pi GPIO header. Power and motors are connected to a separate block of pins, and you can use the included terminal blocks for easier wiring. The board takes an input voltage of 2 V to 11 V and provides two bidirectional motor channels capable of 1.2 A continuous or 1.5 A peak. You can also reconfigure the board for a single channel with twice the current.
Bonus feature – power your Raspberry Pi from your motor supply!
The Raspberry Pi requires a regulated 5 V supply, which is usually provided via the Micro-B USB power port. This is fine for a stationary setup, since you can just use a USB wall power adapter. On a battery powered Raspberry Pi-based robot, however, you need to somehow incorporate your own 5 V regulator capable of supplying about 500 mA. To help simplify your project, the motor driver kit includes a connection point for a three-pin regulator with the standard VIN-GND-VOUT pinout.
In the picture below you can see a complete setup using the motor driver board and our Step-Up/Step-Down Voltage Regulator S7V7F5 to drive two Pololu plastic gearmotors and power the Raspberry Pi from a set of four AA NiMH cells. This regulator is a good match for the range of input voltages of the DRV8835, though below about 4 V you might need something with higher current capability.
For more information, including schematics, pinout diagrams, and example Python code, see the product page.
We just released the P-Star 25K50 Micro, a programmable breakout board for Microchip’s PIC18F25K50 microcontroller. Although we have been using PIC microcontrollers since our very first product, the P-Star 25K50 Micro (abbreviated P*) is our first product where the PIC microcontroller can be programmed by the user. The P-Star 25K50 Micro is the same size as our AVR-based A-Star 32U4 Micro, and we designed it with the hope that it would be small and cheap enough to go into (and stay in) almost any project.
The PIC18F25K50 has 32 KB of flash program memory, 2 KB of RAM, and built-in full-speed USB functionality. Applications can be developed using standard Microchip PIC compilers and development tools (such as MPLAB X and XC8).
The P-Star 25K50 Micro can be programmed via its proprietary USB bootloader using our open source software that is available for Windows, Linux, and Mac. The bootloader uses 8 KB of flash memory, leaving 24 KB for the user. Alternatively, an ICSP programmer can be used to erase the bootloader and access the full 32 KB of program memory. (Since the bootloader cannot be recovered, we recommend this option only for those who are comfortable programming exclusively with an external programmer.)
The P-Star 25K50 Micro features a precision 16 MHz crystal, a USB Micro-B connector, and three user-controllable LEDs. A voltage regulator and power selection circuit allow the board to be powered from either USB or an external 5.5 V to 15 V source, while a resettable PTC fuse on the USB VBUS supply and reverse protection on VIN help protect it from accidental damage.
Compared to the popular ATmega32U4 microcontroller, the PIC18F25K50 has nearly the same performance and memory capacity, but it also has some compelling features that are missing on the AVR. For example, the PIC18F25K50 uses the PIC18 architecture, which has two interrupt priority levels: interrupts can be assigned to either level, and a high-priority interrupt routine can run in the middle of a low-priority one. This powerful feature is what enables our Maestro servo controllers to generate precise servo signals while still using low-priority interrupts to assist with serial communication and other tasks. Unlike the ATmega32U4, the PIC18F25K50 can operate at full speed down to 2.7 V (though the brown-out reset on the P-Star is activated at 2.85 V by default).
The PIC18F25K50 also has a 5-bit digital-to-analog converter (DAC), which is a fun feature not available on many 8-bit microcontrollers.
For more information, check out the P-Star 25K50 Micro page, and let us know what you think in the comments.
We are excited to announce the addition of two new motor drivers to our selection of Arduino shields: the Pololu DRV8835 Dual Motor Driver Shield for Arduino and the Pololu A4990 Dual Motor Driver Shield for Arduino. These miniature shields are low-cost, basic dual motor drivers for your Arduino or Arduino-compatible board. When connected to an Arduino, each provides two channels of bidirectional PWM motor control suitable for driving small brushed DC motors. The boards include various handy features like reverse protection, multiple power and motor connection options, and the ability to customize some of the pin mappings. Our open-source libraries provide a convenient way to get started controlling these motors with an Arduino.
Selecting a motor driver shield
The main practical difference between the shields is their input voltage range, so most people should probably select a shield based on their desired power supply. Here are some details:
Our DRV8835 shield, based on the DRV8835 motor driver from Texas Instruments, has an input voltage range of 2 V to 11 V and can deliver a continuous 1.2 A (1.5 A peak) on each channel. It is suitable, for example, for battery-powered robots similar to the 3pi or Zumo. As a bonus feature of our shield, you can parallel the motor outputs to get a single channel with twice the current capability.
Our A4990 shield uses the Allegro A4990 and is specifically intended for higher-voltage applications, such as projects powered by a 12 V or 24 V battery. It has an input voltage range of 6 V to 32 V and can deliver a continuous 0.65 A (0.9 A peak) per channel. The A4990 can detect and signal a variety of errors like over-temperature and short-circuit, allowing more sophisticated control and monitoring.
Basic breakout boards available
Congratulations to our summer interns!
If you carefully inspect the back of the boards, you will notice “TKern” and “izzyg” etched in copper. These inscriptions commemorate the internships of Ted Kern and Ismael “Izzy” Gomez, Las Vegas natives who were indoctrinated in Pololu design philosophies this summer while creating these new products. We wish Ted and Izzy good luck as they head/return to college at CMU and MIT!
Get a FREE copy of Circuit Cellar magazine’s August issue with your order, while supplies last. To get your free issues, enter the coupon code CIRCUIT0814 into your shopping cart. The Circuit Cellar magazine will add 6 ounces to the package weight when calculating your shipping options.
For back issues and more information, see our free Circuit Cellar magazine offers.
And don’t forget that free copies Elektor magazine’s double-sized July/August issue are still available.
Our new stepper motor bracket is designed to work with typical NEMA 17-size stepper motors like our 42×48mm and 42×38mm units (including our 42×38mm stepper motor with 28cm lead screw). It is made from 3mm-thick black anodized aluminum, which makes it both light and strong, and slot cutouts allow for plenty of mounting flexibility.
For more information, see the product page.
Securely connecting and mounting the electronics for your robot or other project is a key step in taking it from a prototype to a finished design. These perma-proto boards from Adafruit use the same basic through-hole layout as standard solderless breadboards while allowing for permanent solder connections, which makes it easy to transfer your electronics from one to the other.
We are now carrying four types of perma-proto boards:
The flexible perma-proto board is made of a thin polyamide film that allows it to be bent, flexed, and cut to fit your project. This version is 3.1″ × 1.7″ (similar in size to the half-size board) and only 0.005″ thick. It contains 30 rows of pins and three mounting holes.
Each board uses 47 mil (1.2 mm) diameter through holes to accommodate parts with thick leads and is through-plated for strength, which means that the pads are less likely to be ripped of during soldering or rework.
The Raspberry Pi single-board computer has been around for a little over two years in its original Model A and Model B versions, and in that time, it’s become a very popular platform for electronics experimentation. With many of the same capabilities as a regular desktop or laptop PC, but at a small fraction of the size and cost, the Raspberry Pi offers features like network connectivity and significant processing power for robots and other electronics projects.
We’re now selling the new Raspberry Pi Model B+, which improves on the Model B in a number of ways:
We’ve seen our customers build lots of cool projects with the Raspberry Pi, including a wirelessly-controlled Zumo robot with a video camera and a robotic ping-pong ball collector. We look forward to seeing what you’ll do with the Model B+!
Get FREE copies of Circuit Cellar magazine’s July issue and Elektor magazine’s July/August issue with your order, while supplies last. To get your free issues, enter the coupon codes CIRCUIT0714 and ELEKTOR0714 into your shopping cart. The Circuit Cellar magazine will add 6 ounces and the Elektor magazine will add 9 ounces to the package weight when calculating your shipping options.
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