Thu Aug 13 19:49:10 PDT 2015

A Simple, Battery-Free, LED Flasher that Works Forever

My Infini-Flasher (in its stylish cassette case) (click to enlarge)
(My Infini-Flasher (in its stylish cassette case))

Are you interested in about making an electronic device which does something (slightly) useful, doesn't require an external power supply, and might last longer than you? I found this proposition enticing, and so, inspired by Kevin Horton's Infini-Flasher, I had a go at making my own version of Kevin's cunning device.

I firstly put together the components using a simple breadboard and found that I needed to make some changes to the component values listed in Kevin's design. I suspect that this is simply a result of using different transistor types. (This type of oscillator is fussy).

The changes simplified Kevin's circuit slightly, I omitted a pair of diodes, which are only needed if the supply voltage is high (I think), and changed the ordering of the NPN and PNP transistors. I also found that it was important to use a suitable value for the limiting resistor, between the super capacitor and the flasher circuit. I started out with 100k here, as in Kevin's circuit, but found that the circuit would not start to flash as the leakage through the flasher circuit was too high to allow the flasher circuit to reach a high enough value to actually start flashing. Clearly the circuit is a little sensitive to component tolerances. I recommend that you put together your circuit using a breadboard first, then solder everything together once you are sure that the component values are satisfactory.

This oscillator circuit works well for this application, as we want a very short duration of current usage, so that the relatively small amount of charge available from the capacitors lasts as long as possible. A simulation of the circuit shows that the on condition for the output transistor is very brief. This provides a brief spike of power to the inductor and in turn flashes the LED. As human eyes are very sensitive to short pulses of light (through having evolved to avoid the glinting teeth of sabre tooth tigers) this provides the most electrically economical means to light the LED. Even if the pulses were longer, the human eye would not appreciate the large expenditure of power that much.

Here (below) is the circuit diagram. Be warned that this battery free flasher is fascinating! The super capacitor (1F on the diagram) charges up from the solar solar cells in about 30 minutes under a lamp, or quite happily during the hours of daylight on a desk. When the voltage supplied to the flasher circuit reaches about 1.5 volts, the LED starts flashing. The current consumption is around 10 microamps, so the charge in the super capacitor lasts a long time, certainly more than a typical night time. So far my version has been flashing away happily on my desk for about a month. It should last as long as the electrolytic capacitors that it contains - that should be at least 20 years - or perhaps longer...

The Infini-Flasher Schematic (click to enlarge)
(The Infini-Flasher Schematic)

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Posted by ZFS | Permanent link | File under: electronics
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