SamCoVT wrote:
That simulation is helpful as it shows the resistor values (470 Ohms series resistor on each line). In that simulation, I see the TOTAL current from the 5V supply in your simple simulation is about 25mA, however the 74HC166 will only be driving one of the three paths (eg. you will need one '166 for PIX1, another for PIX2, and a third for PALLETE (unless you want to use something simpler like a latch to control the pallete).
Actually that particular simulation is using a single '166 for all three lines. On my current board, I have the monochrome display setup like falstad is set up there. BUT, as you are looking ahead, yes, the PIX1 and PIX2 will be driven individually, along with PALETTE. Correct.
Quote:
In your simulation, you can see there is only 9.2mA going down a particular branch, and that's what the '166 will be sourcing for current (probably a tiny bit less, if you were to measure). That is comfortably within the limits of the part.
For your dioded version, having values on your schematic (even if you aren't sure if they are right) will allow a more thorough analysis. It looks like PIX1 and PIX2 will have to source a bit more current if PALLETE is low, but you should be looking at the branch currents rather than the total from the supply because that's what the IC driving that line will have to source (or sink, in the case of PALLETE with the diodes).
-SamCoVT
Exactly, I remember seeing a lot of load going through PIX1 and PIX2 when PALETTE was grounded. Like, 40mA was going through the system, and it was coming all from a pair of '166s. But when only one PIX is high and the other low, that 40mA has got to come from a single '166 which is just too much!
I can redo the falstad simulation for the diodes, but I really like BillO's suggestion to use transistors. I was looking online and some folks are saying the 2N2222 will go like 500 MHz or something crazy. Still, I didn't see that in the datasheet (though I don't think I looked terribly well either).
Thank you Sam!
Chad