allanhurst:
Looks jreat I- didn't think you'd get it all in.You shouldn't need the 555. Are you using current limiting resistors with you LED dsiplay ? - otherwise you'll take far too much current ...
Do you mean 7805 instead of 555? No, I am not using current limiting resistors. I did some reading on current limiting resistors (Sparkfun Tutorial), and then found the datasheet for the controller on the backpack used to drive the LEDs (HT16K33 Datasheet). But I don't see anything on the sheet related to forward voltage or forward current. I assume this is because the datasheet is for the chip and doesn't take into account the LEDs it's driving.
So I went and found, what I believe, is the datasheet for the LED display itself (4 character, 7-segment display datasheet), and it shows forward current as 30mA but doesn't show forward voltage.
The schematic doesn't seem to show a limiting resistor on the backpack itself.
Then I found This Tutorial Page which give some basic info on the LEDs. So if I use 2v as the forward voltage, 5v as the source voltage, and 30mA as the forward current, I come to 100ohms. The tutorial says to double the value for determining the limiting resistor ...
so I should place a 200ohm resistor in series with the 5v feeding the LED backpack?
allanhurst:
A medium capasitor (?100u) across the zener may help...
I do have some 100uF 50v capacitors here. I'll try placing one on the leads of the zener diode.
Thanks Allan!