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	Free Circuit Cellar magazine June 2014
	Free Elektor magazine June 2014

Continuing with our recent theme of tiny new actuators, we are now carrying FS90R micro continuous rotation servos from FEETECH. Continuous rotation servos are standard hobby RC servos that have been modified for open-loop speed control instead of their usual closed-loop position control, and they make convenient drive systems for robots because they are effectively a motor, gearbox, and motor controller/electronic speed control (ESC) in a single compact package. They are also very easy to use as they can be connected directly to an RC receiver or controlled by a single microcontroller I/O line programmed to output RC servo pulses.

With a weight of just 9 g, the FS90R is the smallest servo we have come across that is manufactured specifically for continuous rotation. It has great speed and torque for its size (up to 130 RPM and 1.5 kg-cm at 6 V), and at only $5 per servo, it is a very simple and affordable way to add some motion to your next project. For comparison (or if you are looking for an alternative servo that offers position control), it is very similar in size, weight, speed, and torque to the Power HD Micro Servo HD-1900A.

For more information on the FS90R micro continuous rotation servo, see the product page. For other options, you can check out our full selection of continuous rotation servos or our entire RC servo category.

Continuous rotation servo size comparison. From left to right: SpringRC SM-S4303R, Power HD AR-3606HB, FEETECH FS5106R, Parallax Feedback 360°, Parallax (Futaba S148), and FEETECH FS90R.

Related post: Understanding continuous-rotation servos and multi-turn servos

Related products

FEETECH FS90R Micro Continuous Rotation Servo

A few months ago, we introduced our new D24V5Fx buck (step-down) voltage regulator family with inaugural members offering fixed output voltages of 3.3 V, 5 V, 9 V, and 12 V, and now we have expanded that family by adding versions with fixed output voltages of 1.8 V, 2.5 V, 6 V, and 15 V.

We are particularly excited about this regulator family because of its wide operating voltage range, high efficiencies, and low dropout voltages, all in a compact 0.5″ × 0.4″ × 0.1″ (13 mm × 10 mm × 3 mm) form factor that is smaller than standard through hole linear regulators with DIP packages. For example, the picture below shows a D24V5Fx next to a 7805 voltage regulator in a TO-220 package:

These regulators operate at up to 36 V, making them especially useful in applications where there can be large variation in the input voltage, such as solar-powered systems or devices where power supply flexibility is a benefit. Since they are switching regulators, the efficiency is much higher than linear regulators when there is a big difference between the input and output voltage, and since they are synchronous, the efficiency is high even at light loads and low output voltages. As an example of the versatility of these regulators, the same D24V5F2 module can in one application be used to get 2.5 V from a 24 V battery and in another be an efficient way to add a 2.5 V node to a system that already has regulated 5 V. As the performance graph below shows, typical efficiency in the latter scenario is 90%, which could almost double battery life in portable systems when compared to linear regulators.

We consider the new D24V5Fx regulators to be next-generation alternatives to our D24V3Fx and D24V6Fx buck regulators, which have been some of our most popular products. In addition to having generally higher efficiencies (which in practice allow these 500 mA units to achieve maximum output currents comparable to our 600 mA D24V6Fx units), these new regulators have much lower dropout voltages (“dropout voltage” is the amount by which the input voltage must exceed the output voltage in order to ensure that the target output can be achieved). For example, the two graphs below show the dropout voltage of the new 5 V D24V5F5 compared to the older 5 V D24V6F5 and D24V3F5:

What this means for your project is broader operating ranges and longer battery life. For instance, a low-power 5 V system running on a 9 V battery can discharge it all the way to 5 V whereas the higher-dropout D24V6F5 regulator can only go to 6.5 V, and four-cell alkaline and five-cell NiMH packs (both with 6.0 V nominal voltages) become viable options.

For other regulator options, you can take a look at our full selection of step-up voltage regulators, step-down voltage regulators, and step-up/step-down voltage regulators.

Related products

	Pololu 1.8V, 500mA Step-Down Voltage Regulator D24V5F1
	Pololu 2.5V, 500mA Step-Down Voltage Regulator D24V5F2
	Pololu 3.3V, 500mA Step-Down Voltage Regulator D24V5F3
	Pololu 5V, 500mA Step-Down Voltage Regulator D24V5F5
	Pololu 6V, 500mA Step-Down Voltage Regulator D24V5F6
	Pololu 9V, 500mA Step-Down Voltage Regulator D24V5F9
	Pololu 12V, 500mA Step-Down Voltage Regulator D24V5F12
	Pololu 15V, 500mA Step-Down Voltage Regulator D24V5F15

We’ve just released a carrier board for ST’s newest pressure sensor, the LPS25H, and we’re also lowering the price of our LPS331AP carrier.

Like the LPS331AP, the LPS25H provides pressure readings over a range of 260 mbar to 1260 mbar (26 kPa to 126 kPa), and this data can be used to calculate the sensor’s altitude. Our LPS25H carrier mounts the sensor on a 0.4″ × 0.8″ board (0.1″ shorter than our earlier LPS331AP carrier) that breaks out all of its pins, and as usual, we’ve included level shifters and a regulator to make it easy to use in a 5 V system. Continued…

We are expanding our selection of products from Adafruit, and the latest addition is the Adafruit Pi Case, a stylish protective enclosure for the Raspberry Pi. And to celebrate, we are dropping the price of our Raspberry Pis!

Related products

	Adafruit Pi Case - Enclosure for Raspberry Pi Model A or B
	Raspberry Pi Model B, Revision 2.0

Need a “little” help with your next electronics project? Get it up and running with our sub-micro plastic planetary gearmotors! Measuring a minuscule 6 mm in diameter and weighing just over a gram, these gearmotors are even smaller (and much lighter) than our popular micro metal gearmotors.

26:1 sub-micro plastic planetary gearmotor next to a Micro Metal Gearmotor and a LEGO Minifigure for size reference.

Our sub-micro plastic gearmotors are available in two gear ratios: 26:1 and 136:1. Click on those ratios to learn more about each motor!

Motor accessories

While there are no mounting holes, their cylindrical bodies makes them perfect for snapping into 1/4″ (6 mm) fuse clips, and their small scale makes it easy to affix them with tape or glue. We are also now carrying tiny 14 × 4.5 mm wheels, which are compatible with the sub-micro plastic gearmotor output shafts.

26:1 sub-micro plastic planetary gearmotor being held by a 1/4″ (6 mm) fuse clip.

Sub-micro plastic planetary gearmotor with a 14×4.5mm wheel.

But, Jon, what can I do with such a tiny, adorable motor?

I’m glad you asked! The way I see it, you really only have two options:

1. Spin something really tiny and adorable.
   
2. Make something tiny and adorable like this line follower made by Pololu engineer Kevin (blog post coming soon!):

I’m just kidding; there are definitely plenty of interesting things that can be made with these motors. We can’t wait to see what you use these motors for!

Related products

	26:1 Sub-Micro Plastic Planetary Gearmotor 6Dx16L mm
	136:1 Sub-Micro Plastic Planetary Gearmotor 6Dx19L mm
	14×4.5mm Wheel Pair for Sub-Micro Plastic Planetary Gearmotors

We are now carrying metal servo horns that work with Power HD’s ultra-high-torque HD-1235MG giant servos, which can deliver a whopping 560 oz-in (40 kg-cm) at 7.4 V. If you want to get the most power out of your HD-1235MG, I recommend substituting one of these anodized aluminum horns for the included plastic horns.

Related products

	Metal Servo Horn for Power HD Giant Servo HD-1235MG
	Power HD Ultra-High-Torque, High-Voltage Digital Giant Servo HD-1235MG

We are now carrying four exciting new sensors from Interlink Electronics:

• 0.25″-diameter circle, short tail force-sensing resistor (FSR)
• 0.6″-diameter circle, short tail FSR
• 1.4″ × 0.4″ force-sensing linear potentiometer (FSLP) strip
• 4.0″ × 0.4″ customizable-length FSLP strip

The two force-sensing resistors (or FSRs, for short) are short-tail versions of the small, circular FSRs we already carry, which allows them to be integrated into applications with tighter space constraints. These sensors act just like variable resistors that depend on the applied pressure, so you can put them into a simple voltage divider circuit and measure the force on them with a single analog-to-digital (ADC) microcontroller input.

0.6″-diameter short-tail force sensing resistor (FSR) next to a 0.6″-diameter FSR with a standard tail.

The two force-sensing linear potentiometers (or FSLPs) take the force-measuring functionality of FSRs and add in the ability to detect the location of the force, all while being an entirely passive component that is incredibly easy to use.

The two force-sensing linear potentiometers (FSLPs).

These FSLPs are exciting because they enable fun new touch interfaces, not only for you to interact with your project but for your project to interact with the world. We decided to make a quick demo for the Las Vegas Mini Maker Faire 2014 to show just how easy it was to do something cool with this sensor. The video at the top of this blog post shows the demo in action.

In the demo, a 4.0″×0.4″ FSLP is used with an Arduino Uno R3 to meassure the position and pressure of the user’s finger. (For applications where space is tight, smaller modules like our Arduino-compatible A-Star Micro can be directly substituted for the Uno.) Using the strip requires four connections to a microcontroller and one additional resistor. Two of the required connections must be analog inputs. Four connections for one sensor might seem like a lot to deal with, but step-by-step procedures in section 5 of the sensor’s integration guide (513k pdf) make it easy to get the sensor working, and the Arduino code used in this demo is available on github to help get you started. A diagram of the connections made between the sensor, Arduino, and LED strip in this demo are shown below.

The connections shown in the diagram also work with the shorter 1.4″×0.4″ FSLP (referred to as “standard FSLP” in the integration guide), though the pin numbers that correspond to each of the sensors outputs (SL, D1, and D2) are different for the two sizes of FSLP. The pin numbers for each FSLP can be seen in Figure 9 of the FSLP Integration Guide. In the guide the 4.0″×0.4″ FSLP is referred to as a “10 cm FSLP”.

Once the Arduino reads the position and pressure data from the sensor, it sends signals to a WS2812B addressable LED strip that control the number of LEDs that turn on and their color. The further along the strip your finger moves the greater the number of LEDs that light up, and the more pressure you apply the more the color of all the LEDs changes from blue to red.

To make the demo easy to transport and able to be left on all day, a 9V wall adapter was used to power the Arduino and 5V step-down regulator. The power connections from the regulator’s 5V output to the power input of the LED strip were also simplified by using a DC barrel jack to terminal block adapter and a DC barrel plug to terminal block adapter. The structure of the demo was laser cut from 1/8″ clear acrylic, and aluminum standoffs were used as spacers.

If you guys do something cool with our force-sensing linear potentiometers or resistors, we’d love to hear about it!

Related products

	Force-Sensing Resistor: 0.25″-Diameter Circle, Short Tail
	Force-Sensing Resistor: 0.6″-Diameter Circle, Short Tail
	Force-Sensing Linear Potentiometer: 1.4″×0.4″ Strip
	Force-Sensing Linear Potentiometer: 4.0″×0.4″ Strip, Customizable Length
	Addressable RGB 60-LED Strip, 5V, 1m (WS2812B)
	Arduino Uno R3
	Pololu Step-Down Voltage Regulator D15V35F5S3
	DC Barrel Jack to 2-Pin Terminal Block Adapter
	DC Barrel Plug to 2-Pin Terminal Block Adapter

Level shifting is a common issue when interfacing multiple microcontrollers or other digital logic devices. For example, you cannot directly connect an Arduino running at 5 V to the Wixel, which runs at 3.3 V. Our Wixel Shield for Arduino contains several level-shifting circuits to help you do this.

In some cases, such as connecting a digital sensor output to your microcontroller, a simple voltage divider or transistor inverter might be good enough. However, in many cases a better solution is necessary. I²C, for example, is a common protocol that makes use of a bidirectional communication line. Luckily, a relatively simple circuit consisting of a MOSFET and two pull-up resistors can be used for general-purpose bidirectional level shifting:

Schematic of a single bidirectional logical level shifter.

We have used this level shifter circuit on many of our breakout boards operating at a lower voltage, such as the MinIMU-9. It works like this:

• When Lx, the lower-voltage input, is driven low, the MOSFET turns on and the zero passes through to Hx.
• When Hx, the higher-voltage input, is driven low, Lx is also driven low through the MOSFET’s body diode, at which point the MOSFET turns on.
• In all other cases, both Lx and Hx are pulled high to their respective logic supply voltages.

The circuit works for any pair of voltages (within the limitations of the MOSFET) and can be used with most common bidirectional and unidirectional digital interfaces, including I²C, SPI, and asynchronous TTL serial. You can read more about it in NXP’s application note on I²C bus level-shifting techniques (54k pdf).

Today we released a logic level shifter board featuring four of these bidirectional channels:

Our board can convert signals as low as 1.5 V to as high as 18 V and vice versa, so you can use it for almost any logic-level signals that you might encounter in your project. It is also, as far as we know, the smallest bidirectional logic level conversion board out there:

Note the use of a more internationally-appropriate size reference than our traditional U.S. quarter. After we put together this image, nobody believed that the board was actually that small, but we verified it several different ways to make sure.

Anyway, with this board’s small size, low cost, and versatility, we think it is something that everyone should have in their toolbox. For more information or to order, see the product page.

Related products

Logic Level Shifter, 4-Channel, Bidirectional

Get FREE copies of Circuit Cellar magazine’s May issue and Elektor magazine’s May issue with your order, while supplies last. To get your free issues, enter the coupon codes CIRCUIT0514 and ELEKTOR0514 into your shopping cart. The magazines will add 6 ounces and 7 ounces, respectively, to the package weight when calculating your shipping options.

For back issues and more information, see our free Circuit Cellar magazine offers and free Elektor magazine offers.

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	Free Elektor magazine May 2014
	Free Circuit Cellar magazine May 2014