A better flasher

Ok, so in my previous blog entry I talked about the fun I had fine tuning my single LED flasher for my little solar / battery relay controller. Well, as it turns out what I produced wasn't the best idea I've had all week.

I went back to my CAD software (Eagle) to update the schematic only to realize the original 555 based implementation had the two blue LEDs in parallel. Yep, darn, I didn't even give the second LED a thought - I could have stuck with a traditional flip-flop after all. Furthermore, my original schematic had the ULN2003 wired incorrectly so as a bonus I've also realized I don't need the original zener diode either. My circuit already has a 5v rail that will be provided by the Arduino.

So, being the perfectionist I am, I went back to the breadboard and put together a basic flip flop (almost identical to the previous circuit) and played around with different resistor / capacitor combinations to see how close I could get to a 1Hz flash rate. Based on the parts I had available I ultimately decided to stick with the standard 10K resistor and 100uF capacitor (provided a rate of 1.07Hz).

While I had all the gear out I thought I'd also optimize the current flowing through each LED. I measured around 5.2v from my power source (my test setup was using a 12v source and a 5.1v zener) and found the LED had 3.3v across its pins (I expected it to be around 3.2v). This leaves 1.9v so for a maximum current of 20mA I would require 95ohms between +VCC and the transistor's collector (with 100ohms being the closest available). I decided to halve the maximum current and use a 200ohm resistor instead. I put it all together and measured the current between the transistor's emitter and the LED's anode - it was spot on 10mA.

Below is the oscilloscope's output, the updated circuit and a small video showing the flashing LED (I used two inputs of a tri-colour LED to emulate two individual LED's.