NXT: How to use external power supply for non-LEGO-sensors?

[noob405]

Hi everybody,

I allready posted this to the German Mindstorms forum, but did not get any final hints.
Maybe You guys can give me some ideas. This is what I do:

I have several ultrasonic sensors (Devantech SRF10) connected to my NXT brick.
The I2C circuit for this is straight forward and even I- as a complete noob when
it comes to electronic an soldering things - could build it.



With 4 sensors connected to one NXT-Sensor port this works quite well.
I can read all sensor values and use them to navigate my bot.

If I connect a fifth sensor this does not work anymore! I guess that the internal power
supply just can´t handle more than four SRF10s on one port. So I thought about
using an external power supply.

This is my current attempt.

Instead of wiring 4,3 V and GND from the NXT I connected a 5V LIPO to my little I2C
board. After blowing just one sensor because of wrong polarity , I could power
up another sensor properly. Now I still struggle with my programming because I don´t get any
readings back from the sensor.

I guess that my idea of replacing NXT Power and NXT GnD with a LIPO power and Lipo GND
was a bit naive and tha the correct wiring for an external power supply on a I2C chain
is different.

Can someone tell me where I´m wrong? I´d like to avoid to blow another sensor or risk
my NXT. Has anyone done this before or has an idea how it should work?

Thank You in advance

noob405

[mattallen37]

Make sure the NXT, sensors, and battery all have a common Gnd (very important!).

Make sure the lipo output voltage is correct. Is the lipo a single 3.7v cell? two cells in series? is it regulated at 5v? I'm not sure about those sensors, but the NXT I2C bus would be fine with 3.3 - 5v (probably 2.5v - 5.25v). Make sure the lipo output is within that range, and make sure the sensor is okay with the voltage you feed it (too high and you could burn it out, too low and it might not be reliable).

[noob405]

Hi,

thanks to You Matt for the quick reply!!

Lipo output is regulated at 5V and that´s fine for the sensors.
Good to know, that the NXT can handle that too.

This "common ground thing" still gives me some headaches.
Do I have to connect the GNDs of the NXT with the GND of the Lipo,
like that?



Any help apprechiated.

Thanks
noob

[mattallen37]

Yes. Connect battery GND, sensor GND, and NXT GND (pins 2 and 3), just like in that last picture.

[noob405]

Will do this week and report
what happend.

Thanks again Matt
Noob

[noob405]

Hi guys,

took a while to test the circuit.
It doesn't work!

Is it possible that I need different
pullup resistors?

Cheers
Noob