Connect PINGs to Servo Ports on Activity Board?

atomicpenguin

The KickStart manual suggests using a 1k ohm resistor between the PING signal and Prop pin - presumably to drop the 5v signal from the PING down to the 3.3v that the Prop wants to see (?). I note in the schematics for the Activity Board and Prop BOE that there are 3.9K ohm resistors between the signal lines on the Servo Ports and the associated pins (e.g., Pins 12 through 17 for the Servo Ports on the Activity Board.

So . . . can I just directly connect PINGs to the Servo ports?? (no additional resistors?)

Thanks

Gary

Phil Pilgrim (PhiPi)

I've done this with my ActivityBot class. But you definitely need to solder in a 2.2K resistor in series with the white signal wire in the extension cable, and make sure the jumper is set for 5V.

-Phil

atomicpenguin

Phil - Thanks for your reply, but I am still a little confused here. Sheet 9 of the schematics for the Activity Board shows no resistors between the pin header strip and the Prop pins. And accordingly, the "Build the PING))) Sensor Circuit" of the Activity Bot tutorial shows (as you suggest) a 2.2K resistor in series with the PING signal when the PING signal is connected directly to the pin header strip on the Board. This makes sense to me.

However, Sheet 5 of the schematics shows 3.9K resistors in series between the Servo port connector signal lines and the corresponding Prop pins (12 - 17). Thus, it seems to me that the servo port lines have some built-in additional "protection." So why can't I just rely on these apparently additional 3.9K resistors to protect the Prop from the PINGs 5v signal? (I have actually tried directly connecting the PINGs to the Servo ports (no additional resistance) and they seem to work fine (no blue smoke - at least not yet!)

Can any of the Parallax boys and girls comment on this??

Thanks again for the help.

Gary

Phil Pilgrim (PhiPi)

Gary,

Sorry, I wasn't aware of the 3.9K series resistors. In that case, just a cable between the Ping))) and the header will do.

-Phil

Steph Lindsay

Hi Gary, and welcome to the forums.

Good sleuthing! You are correct. The P12–P17 servo ports on the Propeller Activity Board each do indeed have a 3.9 k-ohm resistor in series between the signal pin in the port and the Propeller I/O pin. This is more than adequate for connecting the Ping))) sensor's bi-directional 5 V signal pin with the Propeller chip's 3.3 V I/O pin. For details why this works, see the application note "Low-cost Bidirectional Mixed-voltage Interfacing."

http://www.parallaxsemiconductor.com/an010

I apologize that you had to dig to this level to confirm your observations — I will make additions to the Product Guide and various tutorials.

Steph (a Parallax girl)

atomicpenguin

Thanks again to Phil, and thanks very much to Steph for confirmation of what I was seeing/thinking.

I think making additions to the various examples and tutorials is a great idea - using the Servo Ports for PINGs is really convenient!

Thanks again

Gary

Phil Pilgrim (PhiPi)

Gary,

One thing to keep in mind when you hook it up that way is that the power switch has to be slid all the way to its final position to power the Ping))). You probably knew that, but it did manage to trip up a couple of my students who wondered why they were getting no Ping))) results with the switch in the middle position.

-Phil