interfacing the Launch pad with a 5V device

[JMLB 24]

I am trying to interface the launch pad with a device that works on 5V

 

It is my understanding that the launch pad works on 3V. My volt meter is dead so i couldn't confirm this. this was just by reading the data sheets.

 

so my idea for interfacing the launch pad with this device was for the output of the LP to the Device I would use a NPN transistor.

 

I would have the base connected to the out put pin of the LP, the collector connected to a 100? and the Input of the device and the resistor would be connected to 5V VCC and the Emitter to the ground. I am aware that this will invert my logic but that is easy to deal with in code.

 

Ill try to Illustrate with ASCII.

 

                   LP out
                      |
          100?      __|__
5V VCC  ---\/\/\--|---/   \-------|
                 |             __|__
                 |              ---
                 | 
           5V Data Input

 

for the reverse, Input to the LP from the 5V device I would use a voltage divider. I would take the output of the 5V device connect it to a 100? resistor then connect it to the input of the LP and a 200? resistor and connect that to the ground

 

ill try to illustrate with ascii again

5V Data  -----------
                  |
           100?   Z
                  |---------  Launch pad input
           200?   Z
                  |
                -----
                 ---
                  -         

 

I might have to get bigger resistors depending on the max input current, but I think the ratio would be good. I am much more of a software guy than a hardware guy. So I am just trying to get feed back on how you think this would work or if you have any suggestions or improvements. I am not looking to order any parts though.

[Joby Taffey 8]

Here's a good summary of techniques.

 

http://www.sparkfun.com/commerce/tutori ... ials_id=65

[gatesphere 45]

If you want to go the direct, overkill route, you can tap into TP1 and TP3 which are located next to the USB port. Those provide 5V and GND respectively, and should allow you to power the device, at the least. However, if you are using the device as a sensor, make sure you use voltage limiting hardware, such as a zener diode, to protect the MCU from high voltage (higher than 3V3) spikes.

 

I've outlined the process of tapping into those lines on my blog, where I had to power a PIR sensor module from TP1 and TP3. Check it out here: http://blog.suspended-chord.info/?c=11

[JMLB 24]

Lol that is exactly what lead me to your blog the first time. Tapping into TP1 and TP3.

 

Does any one have any Idea on how to drive the 5V device? It does not seem to respond to the 3V

 

of course this is assuming it works on 3.3V I don't have a volt meter for the moment so I can't test this fact.

[GeekDoc 226]

Perhaps if you could tell us what the 5v device is, we could be of more assistance?

 

Some work fine on 3.3v, some don't. Some will recognize a 3.3v signal as high, some won't.

 

-Doc