OK, here is another idea that performs a similar process, however it can only work from a higher voltage level down to a lower one (like a TTL 5V down to a BBB 3.3v level, NOT the other way around...you'll have to use the optoisolator approach for that). How a voltage divider works is by the ratio of the two resistor values you select:
<https://lh3.googleusercontent.com/-KNkptwmAZDQ/WJLs8FsPciI/AAAAAAAAAE0/mRI1uCDhObsfZ53bQyKRSddnuKHnMe4uACLcB/s1600/voltage_divider1.png> OK, how I always conceive of the schematic of a voltage divider is the schematic on the far left. Your Vin would be the 5V digital signal (NOT VCC, the signal itself) you were trying to divide down to 3.3v (remember it can't do the up conversion, it'll only go "downhill") which appears on the Vout. I even have the optimal resistor values for you that make sure the currents right, strong enough to drive the logic but weak enough that it doesn't consume any power really. For 5V to 3.3V signal conversion: R1 = 1700 Ohms (or a 1.7K resistor) R2 = 3300 Ohms (or a 3.3K resistor) Breadboard it out first. Take two resistors of the previously stated values, manually apply a continuous +5VDC to the Vin, tap it between the two resistors (don't forget the ground connection its how voltage division actually occurs) and verify with DMM that the output voltage is indeed 3.3v. Now you have a nice little way of doing that voltage conversion. Add it to you bag of tricks. For the newbies, whatever you do DON'T dream of passing ALL your signals over big arrays of optoisolators and voltage dividers, just for elegance sake. What you do is you run multiple voltage "domains" on your board and then when you absolutely, positively have to do the conversion, do so. For example, don't surround a TTL MCU with several optoisolators, run it off of a 5V bus (common ground though) and thread through the single signal (like a UART_TX from the BBB through the optoisolator). On the board level remember you don't necessarily have to thread the UART_RX back a lot of time (like for status). A good example is my PIC based motor speed control...it only receives a TX. You don't have to check at the board level, it always works...saving you a voltage divider back to the BBB. On Tuesday, November 8, 2016 at 7:39:33 AM UTC-7, mzimmers wrote: > > Hi, all - I thought I posted something about this last week, but a search > doesn't turn it up, so here goes again. > > I'm working through Molloy's book, and trying to build the opto-coupler > circuit in chapter 6. I'm not a hardware guy, so I'm feeling my way along > here. The diagram doesn't show specifically how to wire up the four > connectors. I looked at the data sheet for the device, which was helpful, > but still doesn't get me home. > > I could trial and error, but I've already fried one component, and they're > not easy to come by in my area. Can anyone help clarify this configuration? > > This is the specific device: opto-coupler > <http://www.farnell.com/datasheets/2050347.pdf?_ga=1.151569401.1989118912.1477330330> > > Thanks... > -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to beagleboard+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/beagleboard/df04ef3e-e2e8-4146-a221-2041913192b6%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
