<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div>Brian,</div><div><br></div><div>I propose for consideration a third option, power the MCP23017 from the 5V rail and use an i2c-safe level converter ( <a href="https://www.adafruit.com/products/757">https://www.adafruit.com/products/757</a> ) to connect your Pi to the i2c bus. This would scale well, if needed, as you could keep hooking up more MCP23017 chips to the 5V i2c bus.</div><div><br></div><div>Don't forget to connect the grounds of your 5V and 3V3 buses together. </div><div><br><div><span class="Apple-style-span" style="-webkit-tap-highlight-color: rgba(26, 26, 26, 0.294118); -webkit-composition-fill-color: rgba(175, 192, 227, 0.231373);">Jeff :)</span></div><div><span class="Apple-style-span" style="-webkit-tap-highlight-color: rgba(26, 26, 26, 0.296875); -webkit-composition-fill-color: rgba(175, 192, 227, 0.230469); -webkit-composition-frame-color: rgba(77, 128, 180, 0.230469); "><br></span></div></div><div><br>On Aug 11, 2014, at 10:11, Grawburg <<a href="mailto:grawburg@myglnc.com">grawburg@myglnc.com</a>> wrote:<br><br></div><blockquote type="cite"><div><span>Many of you are aware of my traffic light project with the Raspberry Pi. I currently have 12 LEDs that are connected to the Pi through an MCP23017 I/O port expander. The Pi provides 3.3 VDC to the LEDs with 470 ohm current-limiting resistors. This combination works just fine (I'll have it with me tonight.) But I want to also make a set-up that uses an external 5 VDC to power the LEDs; I will still use the MCP23017 to provide the additional output pins. There are two options I am aware of and would like some comments tonight.</span><br><span></span><br><span>1. Use 12 2N3904 transistors (with appropriate resistors)</span><br><span>2. Use three 4-channel LTV-847 opto-couplers (also with appropriate resistors)</span><br><span></span><br><span></span><br><span></span><br><span></span><br><span>Thanks,</span><br><span>Brian Grawburg</span><br><span></span><br><span></span><br><span></span><br><span></span><br><span>_______________________________________________</span><br><span>Triangle, NC Embedded Computing mailing list</span><br><span><a href="mailto:TriEmbed@triembed.org">TriEmbed@triembed.org</a></span><br><span><a href="http://mail.triembed.org/mailman/listinfo/triembed_triembed.org">http://mail.triembed.org/mailman/listinfo/triembed_triembed.org</a></span><br><span>TriEmbed web site: <a href="http://TriEmbed.org">http://TriEmbed.org</a></span><br></div></blockquote></body></html>