Electronics > Electronics

Protection circuits for GPIO pins of a Raspberry Pi bot

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RpiHacker:

Greetings,

I am making plans to build a GPIO breakout board for the Pi B+ and intend to use two (2) SN7407 ICs to make available 12 buffer circuits (3.3V to 5V level shifter) for connection with motor controllers, servo controllers, sensors, and other bells / whistles I will be cobbling in the future as the Pi bot gets built-out.

In addition to the level shifter for GPIO outputs, I intend to include a number of voltage dividers on the breakout board to protect the Pi GPIO inputs.

In looking at the value of R1 in the attached circuit I interpret the value of R1 as shown 4k7 to be 4.7 K ohms, on the other hand, I suppose it it could mean R1 can vary between 4k and 7k as the applied voltage varies between 5V and 30V.

I would appreciate it if someone would clarify the nomenclature 4k7 and any comments or advice you care to share on building the GPIO breakout board.

The above circuits, along with other Pi GPIO protection circuits, can be found at:
 < http://elinux.org/RPi_GPIO_Interface_Circuits >

Thank you for any thoughts and comments you may share.

RpiHacker:
Issue resolved: the proper way to interpret 4k7 on the schematic is 4.7k ohms.

It appears you can teach old dogs new circuit symbol conventions!

Cheers!

Fr0stAngel:
Hello,
You may also use optical isolation to completely isolate the power of the RasPi from the other control devices. This can be useful when using high current draw devices that can cause voltage drop on the batteries (like motors). Good luck with your break out board :)

RpiHacker:

I have decided to use two battery packs one 6V for the Pi and support ckts., and one 12v for the motors.

For motor control I plan on constructing a board using IRF54ON Mosfets, as the stall current of the motors is 5A.
 
Adding photo isolation to the project is spot on as the ultimate Pi protector.  Long live the Pi!

Thanks for the advice.

RpiHacker:
Change in plans -- after reworking the motor selection process, and gaining a greater appreciation of my lack of knowledge, the J-Bot ver. 1.0 will use Pololu 6V motors product # 2275 and Pololu motor driver product # 1451with the following specs:

motors: 6V 130RPM @ 450mA Free Run with 130 oz-in 6A stall

motor driver: 5.5-24V, continuous current 12A (peak 30A), 2.5-5V logic levels (will work with Pi) PWM up to 20kHz

My circuit building activities will be limited to a GPIO breakout board for the Pi that is fitted with both opto-coupling and level shifters for output and voltage dividers for input.

At this point I finally feel comfortable enough with my plan to proceed with construction/assembly.

Cheers

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