but 1 per segment + 1 per digit != 12ame wrote:The classic way to drive a bunch of 7-segment displays is one line per segment, plus one line per digit. So in your case that's 12 ( don't forget the decimal point).
If that's too many then you can start using shift registers, demultiplexers, and other tricks.
Driving your display from the Pi is no different to using a Z80, 6502, PIC, Arduino, whatever. Find a solution that appeals to you and fits within the restrictions you have.
How many segments? 8 (including d.p.)evilkitty wrote: but 1 per segment + 1 per digit != 12
7 segment * 4 + 4 = 32 or 36 with the decimal
where did 12 come from?
Use a spi device with max7219 controller, dirt cheap on ebayevilkitty wrote:im planning to use 2 separate two digit displays, i will not be needing decimal points
though i guess i could use a button to switch the readout and use 1 display
i am not familiar with 7segment led stuff or using any of the tricks to increase GPIO pins
The max7219 can control up to 8, 7 seg displays (plus DP), but can also work with 4. The chips are available in DIL packages, so are quite 'hobby' friendly (no smd soldering required), and are easy to wire up on proto- or breadboards.evilkitty wrote:that looks a little too big, i only have 65mm to work with and i need to stick a mini toggle switch in the middle of that
though if i solder stuff on to a 2x8cm board i can put that out of the way
With the MAX chip you set registers corresponding to the dispaly number values, not the individual segments (it has and inbuilt 'font' so if you set 0, it displays "0" ). It handles all the multiplexing, you only need to send updates over spi when the values change.evilkitty wrote:thanks, would using a chip like that take the looping load off of the pi or would i still be changing output every few hundred nanoseconds to display multiple digits?
i figured out how the 7segment actually work
since i am going to be suing common anode panels, i guess i should use a PNP transistor as a switch for each digit
if i were going to drive the panel from the pi
i would put a GPIO to each of the 7 segments (not using decimal point)
then put a GPIO to each PNP transistor
have + 3.3 to the PNP collectors, while the emitter goes to the panel's voltage input
what would be a decent choice PNP transistor (one good for fast switching at low voltages)
(one that can be soldered to 2.54mm breadboard)
A quick Google will obtain for you the driver chip data sheet. The data sheet will answer all of your questions and more.evilkitty wrote:a quick look on ebay shows it being sold with common cathode displays, does it work with common anode?
if not is there a version for common anode?
does it have resistors and transistors built in so it can be directly connected to the LEDs?