User Manual

m3pi general-purpose 2/3 voltage

dividers and MOSFETs with

schematic diagrams.

The m3pi expansion board has four general-purpose 2/3

voltage dividers that can be accessed by the set of four upper

“HV” and “LV” pins (pictured to the right) located between the

Wixel and XBee socket pins. These voltage dividers can be

used to safely connect 5 V outputs to 3.3 V inputs (the Wixel

and XBee do not have 5V-tolerant pins): connect the 5 V

signal to one of the four HV pins and then connect the

corresponding LV pin to the Wixel or XBee 3.3 V pin of your

choosing. The voltage dividers are not connected to anything

by default.

The expansion board also has two general-purpose MOSFET

circuits that can be accessed by the set of two lower “HV” and

“LV” pins (pictured to the right) located between the Wixel and

XBee socket pins. These circuits can be used as inverters,

level-shifters (e.g. to convert a 3.3 V Wixel output to a 5 V

signal), or for driving larger loads (up to 200 mA) than you can

with a Wixel or Arduino I/O pin alone (e.g. high-current LEDs

or relays). The MOSFET circuits are not connected to anything by default. The circuit incorporates a

BSS138 MOSFET (N-channel, 50 V, 200 mA, 1.5 V maximum gate threshold voltage).

4.e. LEDs

The eight red LEDs on the expansion PCB connect directly to mbed pins P13 through P20. The LEDs

connect to the pins sequentially, with the outer-most LED connected to pin P13. The LEDs connect

to ground through an appropriate current-limiting resistor, so they can be turned on by driving the

corresponding mbed output high. If you aren’t using an mbed, you can control the LEDs by connecting

I/O lines to the appropriate mbed socket pins.

Schematic diagram of the m3pi LEDs.

m3pi expansion PCB LEDs.

4. The Expansion Board in Detail Page 32 of 39