Servo pins on the proto board?
cavelamb
Posts: 720
The Proto board has four connectors labeled "servo", but for the life of me, I can't find any reference to what pins are connected to them!
The schematic skips over that little detail,and I've not been able to figer it out from te code examples yet.
Is it a secret? Or have solar flares fried by brain? (should have worn my tin foil hat!)
Richard
The schematic skips over that little detail,and I've not been able to figer it out from te code examples yet.
Is it a secret? Or have solar flares fried by brain? (should have worn my tin foil hat!)
Richard
Comments
The set of holes closest to the edge of the board are ground, the next row are either 5V or Vin depending on how the jumper is set. The next set of holes are the signal holes. The signal holes are electrically connected to the fourth row of holes. You solder a wire to this fourth hole with the other end of the wire soldered to one of the pin holes around the center of the board.
You decide which of the Propeller's I/O pins you want to use with the servos. They are not already connected like some of the Stamp boards.
On the other side of the large capacitor from the servo connections is a three hole row where you'd install a three pin header. The center hole is connected to the power row of the servo connection. By jumping this position to either the "5+" position or the "Vin" position you can choose how much voltage you want to send to your servos. If you don't use a jumper the servo connectors wont receive any power.
Make sure to remember to move the Proto Board's switch all the way on to the "servo" position. The first "on" position doesn't turn on the power to the servos. This lets you leave your servo plugged in while programming without your robot walking away in the process.
There is an example of using the object Servo32v7 in the demo folder in the library of the Propeller Tool v1.3.
Post #15 of my QuickStart servo tester thread also has a very simple demo program using Servo32v7.
Is this really needed? The servo isn't using the line as an output so the Prop should never see 5V on it. I personally don't use a resistor.
Edit: I remember now one of the reasons I don't use resistors. I had a servo that wouldn't work with a 3.3V signal if there was a resistor in series but without the resistor it worked.
Thank you, Duane.
That explains why I couldn't figure it out with a meter too...
And the schematic was telling the truth after all!
In fact, I've dissected a servo and found that it already had a 4.7k resistor in series with the input pin.
But, as Mike points out, it might protect you from user error, such as applying 5V to the pin yourself..
On the other hand, adding resistance may make the servo not work...
With the one I looked at the current through that 4.7k resistor went to the base of a transistor.
So, the transistor was used as an input buffer. It's possible that adding resistance there could
make it so the transistor doesn't turn on enough with 3.3V applied...
There are definitely different perspectives on this topic. If you're comfortable running without a resistor inline that is certainly a personal choice and may be fine for the particular servo you are using. However I wouldn't recommend that to people to start with. I don't think all the servos are created equal and all may not have the same input circuitry. There could very well be some off brands that lack protection and may end up hurting the Prop. For the pennies it costs for a resistor it is cheap insurance. It's my opinion and not everyone agrees with it. That's fine too.
If you know for sure that the particular servo has the proper protection built in then I suppose it's ok.
That could definitely be one cause of that. I still probably start out with the resistor and if it isn't working then back down the value or see if it make sense to remove it.
Most of the interfacing issues I've run into ended up being that the 3.3V wasn't just quite high enough. The last one was with a MAX7219. Some people said they were able to connect the propeller pins directly and they worked ok It didn't work for me. After looking at the datasheet it showed the min logic high was 3.5V. I guess some chips would accept 3.3V as a high and some don't. I used a 74HC14 with two inverters back to back which accepted the 3.3V as a high and then had something closer to 5V on the output. That took care of the issue.
I'd be interested to know what the input circuit of the Parallax Store servo's looks like...