I want to connect the output pins of NXP MBED controller to 16 electromechanical relays,and 32 LEDs,what is the best way to do this?The relay would require a current of at least 300mA and LED would be 1.5-2.5V rating LED.And where can I find a datasheet that gives the input and output voltage and current specifications of each of the 40 pins in NXP MBED contoller's DIP package?When I look for it in google only the datasheets for the LPC1768/65/67,etc. turns up but not for the 40 PIN DIP MBED controller

  • 1
    \$\begingroup\$ What is "NXP MBED controller"? Start with a partnumber, then check if there is a circuit diagram available and finally search for the exact partnumber of the chip on it. If you add this information to your question, we might be able to help. \$\endgroup\$ – jippie Jan 25 '13 at 7:33
  • \$\begingroup\$ Sorry ,I meant MBED microcontroller module \$\endgroup\$ – Lotsofdoubts Jan 25 '13 at 8:59

The MBED microcontroller modules are merely the respective NXP microcontrollers implemented in a PCB with a wide DIP form factor, with supporting components. bed NXP LPC11U24 mbed NXP LPC1768

In other words, they are like an Arduino Nano or Pro Mini. They are not made by NXP just like the Arduinos are not made by Atmel.

There is nothing unique about the mBed that would significantly alter the on-board microcontroller's ratings.

Thus, the input and output voltage and current ratings would be the same as for the respective microcontroller used in the board in question. The relevant datasheet will provide the information you need.

From the question, a microcontroller will typically not be able to source or sink 300 mA, that the relay drive coils require. Not only that, sourcing that much current from the regulated lines on the mBed boards will either overheat or destroy the on-board voltage regulator on those boards.

A recommended approach would be to power the relay's primary from an external power source, switching it via a logic-level MOSFET or BJT: The 2n7000 might be borderline suitable, as it is rated for 400 mA not counting requisite derating.

The MOSFET's gate would then be driven by logic outputs from the mBed, suitably protected by a small resistor (100 Ohms should do) in series with the gate. Also, a 10 kOhm pulldown resistor on the MOSFET gate would address spurious switching of the FET when the logic outputs are in high-impedance mode.

| improve this answer | |
  • \$\begingroup\$ Thanks.I have verified that each I/O pin of the mbed module sources or sinks a current of 40mA.Does this refer to per pin handling current capacity of the module. \$\endgroup\$ – Lotsofdoubts Jan 25 '13 at 8:58
  • \$\begingroup\$ @Gayathri Yes it does, but there are two factors to consider: Derating of the 40 mA value due to thermal factor, and the maximum current that the microcontroller can handle overall, which would be separately specified on the datasheet. \$\endgroup\$ – Anindo Ghosh Jan 25 '13 at 9:09
  • \$\begingroup\$ Thanks. And yet it says maximum of 40mA can be sourced or sinked \$\endgroup\$ – Lotsofdoubts Jan 25 '13 at 9:35
  • \$\begingroup\$ @Lotsofdoubts In ideal conditions, that 40 mA figure can be met. This is typical of datasheets, that there is a specific rating for a specified set of test conditions, and then a derating graph or rider note for variation from said conditions, most commonly by temperature. \$\endgroup\$ – Anindo Ghosh Jan 25 '13 at 10:36
  • \$\begingroup\$ I'm pretty sure the mbed is actually produced by NXP, as an outreach project. \$\endgroup\$ – Connor Wolf Jan 26 '13 at 3:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.