fernando_g
Senior Member
This thread got me thinking about this project:
http://www.picaxeforum.co.uk/showthread.php?t=14374
Back in the 70s, the LM3909 was introduced. This wonderful circuit would flash an LED from a single cell battery for many, many months. The key for its skimpy current drain was to pulse the LED with a short, bright pulse, and then wait a couple of seconds. Thus the average current would be quite low.
But times have changed, the LM3909 is no longer available, low cost PICAXEs are here, and there are supercaps. Could they be used as a LM3909 replacement.
Yes indeed!. By using the 08M's low voltage capability, and pulsing the LED briefly and immediately going to sleep with DISABLEBOD, the average current can be made real small.
I employed an 08M, which would feed a small high brightness red LED via 330 ohm resistor and used the following ultra-simple, almost trivial, code:
I took a 1000 uF capacitor (actual 950 uF, measured with a Peak Atlas) charged it to exactly 5.00 volts and allowed to power the circuit while I timed it with a stop watch.
252 seconds later, when the cap voltage reached 1.88 volts, the LED finally stopped flashing.
Some quick calculations yield an average current of ~11.8 micro amps.
Extrapolating these values, a 1 F cap should be able to power this circuit for 254,000 seconds or almost 3 days!
Of course, components have tolerances and your mileage may vary. I suspect that the supercaps (especially the cheaper ones) may have substantial self leakage which would shorten the actual time.
And one could use a small coin lithium battery and power the circuit for months. The Duracell CR2032 is rated for 180 mAH, and thus could last almost two years.
http://www.picaxeforum.co.uk/showthread.php?t=14374
Back in the 70s, the LM3909 was introduced. This wonderful circuit would flash an LED from a single cell battery for many, many months. The key for its skimpy current drain was to pulse the LED with a short, bright pulse, and then wait a couple of seconds. Thus the average current would be quite low.
But times have changed, the LM3909 is no longer available, low cost PICAXEs are here, and there are supercaps. Could they be used as a LM3909 replacement.
Yes indeed!. By using the 08M's low voltage capability, and pulsing the LED briefly and immediately going to sleep with DISABLEBOD, the average current can be made real small.
I employed an 08M, which would feed a small high brightness red LED via 330 ohm resistor and used the following ultra-simple, almost trivial, code:
Code:
#picaxe 08m
main:
pulsout 1, 600
'selected 6 msecs since that was the pulse width from the LM3909
disablebod
sleep 1
enablebod
goto main
252 seconds later, when the cap voltage reached 1.88 volts, the LED finally stopped flashing.
Some quick calculations yield an average current of ~11.8 micro amps.
Extrapolating these values, a 1 F cap should be able to power this circuit for 254,000 seconds or almost 3 days!
Of course, components have tolerances and your mileage may vary. I suspect that the supercaps (especially the cheaper ones) may have substantial self leakage which would shorten the actual time.
And one could use a small coin lithium battery and power the circuit for months. The Duracell CR2032 is rated for 180 mAH, and thus could last almost two years.
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