The Ruggeduino-SE 

Special Edition

The Ruggeduino-SE Special Edition is a microcontroller development board compatible with the Arduino UNO. Please visit the Product Page for purchasing information.

Quick Look

The Ruggeduino-SE is directly compatible with the Arduino UNO SMD. It uses the same microcontroller (ATmega328P), same Arduino GUI, same clock frequency, same connectors and connector positions, and so on. The Ruggeduino-SE adds several features to make it more rugged, making sure that it will last a long time.

This table shows a quick comparison between the Arduino UNO SMD, Ruggeduino-SE, and Ruggeduino-ET.


Improved Feature Set.  Same Great Price.

NOW Shipping the updated Ruggeduino-SE and ET models.  Performance upgrades include a new Power Supply IC with virtually nearly zero ripple and improved current capabilities.  5.0V output current has doubled to 300mA from 150mA at 3.5V input.  A rock-solid 5.0V analog reference is now included for improved A-D, and a USB reset jumper has been added to the USB circuit.

The picture below highlights the physical modifications to the Ruggeduino-SE and Ruggeduino-ET. 


Power In

The Ruggeduino-SE is powered from one of three sources:

  • USB port: 5V is provided directly from the USB port. A 500mA PTC (resettable fuse) protects the computer from overcurrent.

  • DC power jack: a 2.1mm center-positive DC power adapter can supply 3.5V - 30V. This power input is also protected by a 1.85A PTC.

  • Vin: this connector pin can either source power to the Ruggeduino-SE (3.5V-30V) or draw power from the DC power jack.

When more than one power source is present, the automatic power switching circuit selects external power (DC power jack or Vin, whichever is higher) when available, otherwise USB power is used. This behavior is the same as the Arduino UNO.

Power Out

The Ruggeduino-SE can supply power to shields or external circuits in three ways:

  • +5V output: the +5V connector pin is the same voltage seen by the microcontroller. The current from this output is naturally limited by thermal protection on the on-board 5V regulator when externally powered, or limited to 500mA by a resettable fuse when USB-powered.

This +5V output can typically source 700mA when the DC power jack voltage (Vext) is 12V or higher, and less current as Vext decreases -- it can even source 150mA at +5V when the Vext voltage is as low as +3.5V!

On the Ruggeduino-SE, the +5V output is protected against applied high voltage. If, for example, you apply 30V to this pin by mistake, the Ruggeduino-SE will not be damaged. If you do the same thing on an Arduino UNO, you will destroy it!

  • +3.3V output: the +3.3V connector pin sources 850 mA (Vext is 12V or higher). Note that this is quite a bit more than the 50mA limit of the Arduino UNO. Like the +5V output, as Vext drops so does the current sourcing capability, down to 400mA when Vext is 3.5V.

As with the +5V output, the +3.3V output is protected against applied high voltage; if you apply 30V to this pin by mistake, the Ruggeduino-SE will not be damaged.

  • Vin: the Vin connector pin can draw power from the DC power jack. This power input is limited to 1.85A by a PTC (resettable) fuse. The 1.85A PTC fuse protects this input against conditions like short circuits to GND (which would damage an Arduino UNO).


The Ruggeduino-SE is programmed in exactly the same way as the Arduino UNO; the full details can be found in the Programming section of the Arduino UNO SMD product page.

In summary:

  • From the Arduino GUI, select “Arduino UNO” from the Tools->Board menu.

  • The Optiboot bootloader is preloaded on the Ruggeduino-SE so it is ready for uploading sketches.

  • The ATmega328P can be programmed through the bootloader or through its ICSP header.

  • The Ruggeduino-SE uses the same auto-reset mechanism as the Arduino UNO. Jumper J3 can be cut to disable this.


The proper USB driver must be installed for the Silicon Labs CP2104 USB transceiver. If not already installed on your computer, free drivers are available from the Silicon Labs support page.

Protection Details

The Ruggeduino-SE was designed to withstand common electrical mistakes, thus ensuring your Ruggeduino-SE will last a long time. This section describes the details of these protective circuits.

I/O Pin Protection

Every I/O pin is protected by a 5.1V zener diode and 220 ohm 30mA PTC (resettable fuse). The equivalent circuit is shown in this figure.


This protection circuit means:

  • Every I/O pin can have up to 24V applied to it and will still not be damaged (only 10V for the Ruggeduino-ET).

  • Every I/O pin can be short-circuited to ground and will still not be damaged

  • Every I/O pin can be short-circuited to another I/O pin and will still not be damaged

In addition, the 220 ohm PTC can take the place of series resistors in many applications, such as lighting LED’s, driving transistors, and so on. Here is an example of an Arduino driving a transistor through a series resistor, and how the Ruggeduino-SE simplifies the circuit due to its built-in 220 ohm series resistance.


Similarly, you can connect LED’s directly to Ruggeduino-SE digital outputs without any resistors and not worry about destroying them due to excessive current.



I/O Compatibility

The benefits associated with the protective circuits far outweigh any compatibility related issues.  We have sold thousands of protected devices and compatibility is rarely an issue.  While we can't physically test all Arduino compatible shields and accessories we can ensure you that compatibility is next to never an issue.  If a shield works with an Arduino it should work with a Rugged Circuits device.

In some instances the protective components, the 220 ohm PTC and zener,  can produce slightly different results compared unprotected circuits found on standard and clone Arduinos.  This may come at an initial surprise to the user, but being educated on the reason, it can be easily addressed.  Here is an example:

Application: Reading/Mapping analog input voltage with 68k and 12k voltage divider to measure 20 to 30VDC.

Results: With the Rugged Circuits microcontroller everything is linear and values are accurate up to 20V.  Over 20V accuracy falls off and at 29V it reads 25 volts. The divider is only measured to 3.9VDC at 29V. The exact circuit works with a Chinese Mega.

Reason and Solution:  The 220 ohm PTC and the zener are skewing the results.  With a high impedance source (68k+12k voltage divider), the leakage in the zener increases exponentially as you get closer to 5.1V.  Under ~3-4V, there should be very little leakage.  As you increase from there, the zener starts leaking microamps of current and makes the circuit non linear.  There are three methods to battle this - add an op amp buffer between the voltage divider and the analog pin, use a lower impedance voltage divider, or scale the input voltage to only use ~3V of the reference and lose a little resolution.  The first method will fix the issue completely, the second one will help, and the third should all but eliminate the issue.

If you do have an application where this built-in 220 ohm resistance is not wanted, you can easily change it. Every I/O pin has through-hole pins surrounding its PTC fuse that can be jumpered with either a wire for 0 resistance or with a standard resistor. Here is a picture showing how to bypass the 220 ohm PTC for pin D7.


Note that on the Ruggeduino-ET, the 220-ohm PTC is replaced with a standard 220-ohm resistor. This enables operation down to temperatures of -40C but only provides per-I/O-pin overvoltage protection of 10V instead of 24V.

Total Microcontroller Current Protection

It is not enough to limit the current of each I/O pin, the total current sourced by the microcontroller must also be limited to limit its power dissipation. Current-limiting device IC3 on the schematic does this: the microcontroller total current is limited to 150mA (typical) no matter how many pins are sourcing current.

The RT9728 limits the current from the +5V input to the +5V/1 output, which is used to power the ATmega328P. As soon as the ATmega328P pins start sourcing more than 150mA the +5V/1 voltage is automatically reduced by the RT9728 to limit the current to this level. You can short every one of the ATmega328P’s I/O pins to ground and set them all high in your program and you still won’t destroy the microcontroller. Don’t try that with an Arduino!

5V / 3.3V Output Protection

The 5V output and 3.3V output connector pins are current-limit protected by the shutdown feature of the on-board switching regulator. In addition, these pins are protected against applied overvoltage. Any voltage at these pins above nominal will be blocked by Schottky diodes, shown in the schematic below, at up to 30V (at minimum) of applied voltage. This will prevent the damaging voltage from reaching the other components.

Vext Fuse and Vin Blocking Diode

The external DC power jack input is fused on the Ruggeduino-SE (1.85A PTC resettable fuse). The main purpose of this fuse is to protect the Vin output when used to provide power to external devices. The on-board switching regulator provides built-in current limiting using a sense resistor to ensure safe operation.

On the Arduino Uno, the Vin output pin has no reverse blocking diode. If you apply power to the Arduino through this pin and reverse the voltage by mistake, you will destroy the Arduino. On the Ruggeduino-SE the blocking diode comes after the Vin input thus prevents this failure mode.

The schematic below shows the Vin and Vext protection circuitry on the Ruggeduino-SE and the switching regulator circuitry that provides the current-protected +5V system voltage.


RESET Pin Protection

The microcontroller RESET pin is protected by a 1k series resistor (R15) to limit current into this pin in the case of applied overvoltage.

AREF Pin Protection

The microcontroller AREF pin is protected by a 600 ohm series resistor to limit current into this pin in the case of applied over voltage.   The AREF pin is tied with a jumper to a 5V reference.  If an external reference is needed, J9 would need to be cut.  In the case of an external reference, any voltage from 0-5V can then be applied to AREF to be used as a reference.  In the case of an internal reference (AVCC or internal band gap), can be selected once J9 is cut.

USB Connector Protection

The USB data lines are protected against ESD by low-capacitance transient voltage suppressors (TVS), as is the USB power line.

ICSP Pin Protection

The ATmega328P ICSP pins have the same protection as the other I/O pins on the microcontroller (see above).

Other Features

Accurate Clock

The ATmega328P on the Ruggeduino-SE is clocked by a highly-precise 16 MHz oscillator (0.005% accuracy) rather than the resonator used on the Arduino Uno.

D13 LED Isolated

The LED connected to D13 is isolated by a transistor circuit rather than being driven directly from the ATmega328P pin. This isolation draws much less current from the ATmega328P pin allowing it to be used for other purposes.

DTR Current Limit Resistor

On the Ruggeduino-SE, 100 ohm resistor R40 limits the DTR pin current and protects the CP2104 GPIO pin.

Reference Data

  • Here is a graphic showing locations of the headers and cuttable jumpers

2-Pin VIN/Ground Connector

The center of the 2-Pin VIN/Ground connector (located next to the USB connector) is at (0.1825, 0.6975), from the origin of the PCB and the terminal block pitch is 3.5mm (0.138") - so the center of the pins are located at (0.1825, 0.6285) and (0.1825, 0.7665).  The center of the mounting hole closest to the power jack is at (0.55, 1) relative to the origin.


The Ruggeduino-SE was designed in the USA and is assembled in the USA using lead-free components and lead-free manufacturing and assembly processes.