View Full Version : propstick/usb V-bat input??

01-04-2009, 05:27 AM
Hey guys quick question I have been doing some interfacing on my propstick and it hit me these are 3.3V devices right? I have been using VDD out pin 32 of the· prop stick for my 3.3 volts but I am using 9 volts on Vin pin 12 so my question is , is there a voltage regulator built into the propstick device I also have a few prop 40 pin chips that I almost hooked up the same way so on a 40 pin prop chip is Vin have to be regulated down to 3.3 V thanks

and one more on my propsticks I have been using pullup resistors 1-10K to +5 volts on the input pins is that ok or am I over driving this thing?· example 5V------1K-resistor-----LED-----prop-pin of course one side of the led is going to ground just cant draw it

01-04-2009, 05:39 AM
Yes, there is a regulator, an NO if you hook anything about 3.6V to the prop, well, you're going to need another $12.99 plus shipping and handling. :)

If you use the 5V with an LED, you should be OK. The LED is probably going to drop around 1.6V (give or take), so this brings the actual voltage to 3.4V, well within limits. The limiting resistor is a must, however, to limit the current. With a 1K resistor, I think you should be just fine. If the LED doesn't drop enough voltage, then it may still appear "on", although dim. This is because when the output goes HIGH, you have 5V as the supply, a drop across the resistor, and a drop across the LED, then down to 3.3V from the prop. Let's say your LED only drops 1.3V, then that means there is 0.4V across the resistor(5-1.3-3.3 = 0.4). If you are using a 1k resistor, then that is 400uA of current, enough to just barely turn the LED on.

Like I said, it would damage the chip, but it's not a good way of doing it.

01-04-2009, 06:36 AM
So Phil in the course of building stuff for the prop lets just say for now , interfacing LEDs, and pushbutton switches and maybe even some NPN or PNP to drive some toy motors will the 3.3V from the prop VDD pin 32 have enough capacity to power these simple interfaces for the purpose of learning Spin? and running the class room instruction examples? or would I be better off suppling rebulated 3.3 for all the examples?

Mike Green
01-04-2009, 06:46 AM
Read the documentation! Like most Parallax products, there's a link from the webstore page to documentation.

Parallax's documentation clearly shows the regulator that's used (LM2937) and their webstore has the datasheet for the regulator. The amount of current available is at most 400mA and depends on the heat produced. This depends on the input voltage. Best to run it from a 5V supply, 6V maximum.

01-04-2009, 07:11 AM
Yes, it will have the capacity. It's best to use a transistor to drive these devices, though. Like Mike Clause said, read the documentation. I've asked alot of questions on the forums, spent many days waiting for a response, then realized "oh... here it is right here!" in the propeller manual. Questions are great for the forum, and welcomed, but it really is a benefit to you to go ahead and read through it.