> The attached data is for the LM339. It's called a comparator, but is it the > best choice?
It's cheap and common, but not necessarily the best choice. It has open collector outputs, so needs pull-up resistors. If you use a TLC3704 or a MCP6544, you wouldn't need pull-ups. However, if you're going to go buy chips anyway, you may as well just buy a level shifter chip like a CD4504 or 74125 and dispense with all the comparators and resistors. > > If I understand the formula correctly, if Vcc is +12v and Vin is +5v, a good > value for Vref is around 4v. I'd probably use 2 to 2.5V for Vref, that will work well with both CMOS and TTL outputs. > Using the formula, if Rref is 100 ohms than R1 should be 200 ohms. Five > resistors are needed for each of the four comparators, for a total of twenty > resistors. First, you don't need the 1M resistor, you can omit that. It gives some hysteresis, but with logic signals, it isn't needed. Second, you just need to make Vref once, you can wire it to all four comparators. You can probably dispense with the 10k series resistor as well, it's just to balance input currents. In this case, that doesn't matter. That gets you down to six resistors (or two, if you use a comparator that doesn't need pull-ups). I would use a much higher resistance for Rref, as 100 ohms will waste power. Maybe go for 10k for Rref, then you could use 2k to get a 2V Vref. Further, you can hook the high side of the reference voltage divider to +5V instead of +12V, then use a pair of 10k resistors to get a 2.5V reference voltage. > All four of the clock kits I have that use Supertex chips just run them on > +5v, with no issues. Maybe this isn't worth the trouble. For a production circuit, it would make more sense, but for your own use, it's entirely up to you whether to bother. If I were building it, I'd just use a level shifter chip and be done with it. - John -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/9B5FEF38-A721-4B51-AB0E-26CB374931D6%40mac.com. For more options, visit https://groups.google.com/d/optout.
