This is related to my time machine display project for my wife's school. Which you can read more about here:
https://www.eevblog.com/forum/projects/time-machine-display-for-my-wife's-school-ideas-needed/I'm trying to come up with a cheap way to blink a variety of LEDs as cheaply and with as a few components as possible.
I'll be using some kind of MCU as the brains behind it. These LEDs will most likely be all 5mm in different colors.
Ideally I'd like to be able to do some other things with the MCU while it's blinking these LEDs and I will be driving some 7 segment displays separately unrelated to this part of the circuit.
The total number of LEDs I guess depends on budget and usage of pins. The more I can drive, the better. If using a 2N3904 and 2N3906 on the same pin will let me alternate between two LEDs, I might go that route to double my LEDs.
I would say in order of importance:
1. Cost
2. Low part count
3. Ease of implementation
4. Conservation of MCU pins
I'll be ordering all my parts from Tayda unless I absolutely have to get something they don't have.
These are my TAYDA part contenders:
74HC164 S > P non-latching - $0.21 each
CD4511BE BCD to 7 Seg - $0.23 each
M74HC595B1R S > P latching - $0.25 each
2N3904 NPN - $0.02 each
2N3906 PNP - $0.02 each
8550SL PNP 20V 0.7A - $0.02 each
1/4W Resistor (Most Values) - $0.01 each
Ceramic Caps - $0.01 each
Film Caps - $0.03 each
Electrolytics - $0.03 to $0.04 each
I'm guessing an update interval of maybe 250ms and the input for these doesn't need to be technically controlled, I mean I could just easily have it spew random garbage off a few MCU pins and have the chips interpret it in some way and change the lights.
I could live without latching unless there was an easy way to allow myself to hold the pin states until changed on the shift register or BCD decoder and we're assuming I already have the LEDs.
I have 2 pins on the MCU which will be constantly flipping from + to - and back (to drive the analog clock to spin quickly backwards) and I can invert and hold their state as quickly as every 18ms (that's the fastest I can go and have the clock respond) in case I might be able to steal that signal for a clock on the shift register if we go that route.
We'll assume a max budget for this part of the circuit using the parts above of $1.50.
Conservation of MCU pins would be nice, so stacked shift registers might be a good idea, but can the BCD to 7 seg handle LED current directly without a transistor?
What do you think? I'll probably be ordering the parts tomorrow evening since I have a discount code that expires then.