Servo control board

[Timmahw]

Hello! Long time listener, first time caller.

Nutshell question: Do servo and/or servo controllers pose a danger to RPi when GPIO is used only for PWM?

Backstory: Just getting started with RPi, robotics and .. electronics, really. I have a good grasp on the key circuit principles, I think. Anywho, I am moving on from controlling my little KNEX car with L293D, and into servo controls. My ultimate goal is to drive a KK2 Flight Controller via RPi, which accepts PWM via typical 3-pin servo connector. Link. My Google-fu and forum searching produced several servo control boards, but I am insisting on making my own for the sake of education and practice:



Goals:

• Produce several 3-pin male headers for use with servo connectors.
• The pins should be Gnd, V+, and Signal.
• The signal pins should be attached to GPIO pins.
• The V+ pins should be switchable; 5v from RPi or AUX supply.
• Each GPIO channel should have a status LED.
• LEDs should fade with PWM, for program testing.
• LEDs should operate even without servo connection, for program testing.

Notes:

• Not in diagram: hookup wire is run from the GPIO pin (yellow line) to the left resistor terminal.
• Not in diagram: 0.1 jumper is used to switch from 5v to AUX. Thought it would be safer than a switch.
• Probably would never need/want to run 9 servos; I can still use the GPIO pins for other stuff and still take advantage of the LED status! (I think.)
• Since I plan to use this with a *copter, I left the right half blank for any future sensors etc.

Current (ha!) concern:

• 3.3v from GPIO is enough to feed PWM signal, but is there a danger to the RPi without resistors?

I've been "that guy" at the local electronics store recently. I'm operating without any real tutelage, so I would appreciate any and all forms of criticism and words of wisdom from the based RPi gods.

Thanks

[joan]

Not quite sure what question you are asking.

The Pi's gpios are 3V3. Not sure what you mean when you say 5V gpio.

[Timmahw]

Joan,

My concern is indeed with the 3.3v GPIO pins being used for PWM, not the RPi's 5v pin. I mistakenly called it a '5v GPIO pin', which I see now is way off.

Thanks

[joan]

Joan,

My concern is indeed with the 3.3v GPIO pins being used for PWM, not the RPi's 5v pin. I mistakenly called it a '5v GPIO pin', which I see now is way off.

Thanks

I'm not aware of any problems with using PWM from the Pi's gpios. Lots of posters have used it (through a resistor) to change the brightness of LEDs.

Similarly lots of posts about using PWM to control DC motor speeds (via motor driver boards) and to provide servo pulses.

I tend to connect the gpio direct to the servo control wire or motor driver board. It may be safer to have an in series resistor.

[Timmahw]

Joan,

Thanks very much for the quick response. Going to hold off on the resistors.

Tim

[thagrol]

I've had no problems so far using the pi's gpio for the servo pwm pulses in a couple of projects.

Power can be an issue though. I've been able to run a couple of sub-micro servos directly from the pi's 5v pins but standard size ones need a seperate supply to the pi to avoid brownouts and reboots. Just remember to connect the grounds (-) of both supplies but keep the + sides seperate.