fernando_g
Senior Member
I would assume that most of the forum browsers know about the joule thief circuit. This is an ultra-simple circuit which boosts the voltage from a single cell alkaline battery to a voltage high enough to light up a white LED (about 3.4 to 3.8 volts).
Turns out that this voltage is enough to power up a 08M PICAXE too.
Thus this little project. It is a single cell emergency beacon, that turns on for a second, then sleeps for over two seconds. Great to locate that switch in a darkened basement. And its low average current drain greatly increases the battery's lifespan.
The code is trivial:
The 0.01 uF capacitor in parallel with the PNP transistor, bootstraps the unit for the joule thief to start oscillation. The voltage builds up, and the PICAXE starts up. It immediately will pull out2 low, fully turning on the PNP transistor which provides base bias to the main NPN oscillator.
The PICAXE will then turn on the white LED on out for 1 second. After it turns off, it will maintain the joule thief enabled for some 5 more milliseconds to allow some charge to build up in the 100 uF capacitor. The PICAXE will then turn off the PNP, and then go to sleep. It will then turn on the PNP and the LED again, repeating the cycle.
The 0.1 and 22 uF in parallel with the 1.5 volts really allow to get the very last drop of energy from the battery.
With the 8.5 uH transformer one gets about 9 milliamps to the LED.
The 5.1 volt zener is mostly dormant, but is included to prevent the voltage from building high enough to damage the PICAXE.
A pair of yellow LEDs in series may be substituted for the single white LED.
Turns out that this voltage is enough to power up a 08M PICAXE too.
Thus this little project. It is a single cell emergency beacon, that turns on for a second, then sleeps for over two seconds. Great to locate that switch in a darkened basement. And its low average current drain greatly increases the battery's lifespan.
The code is trivial:
Code:
#picaxe 08m
'out1=led, out2=fet
low 2 'turn on fet
disablebod
main:
high 1 'turn on led
pause 1000 'for 1 sec
low 1 'turn off led
pause 15 'allow cap to charge
high 2 'turn off fet
sleep 1 'low power mode
low 2 'turn on fet
goto main
The PICAXE will then turn on the white LED on out for 1 second. After it turns off, it will maintain the joule thief enabled for some 5 more milliseconds to allow some charge to build up in the 100 uF capacitor. The PICAXE will then turn off the PNP, and then go to sleep. It will then turn on the PNP and the LED again, repeating the cycle.
The 0.1 and 22 uF in parallel with the 1.5 volts really allow to get the very last drop of energy from the battery.
With the 8.5 uH transformer one gets about 9 milliamps to the LED.
The 5.1 volt zener is mostly dormant, but is included to prevent the voltage from building high enough to damage the PICAXE.
A pair of yellow LEDs in series may be substituted for the single white LED.
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