In North Carolina, there are 4 seasons, and none of them too bad. It does make getting into the Holiday Spirit a little harder when there is no snow. But fortunately, technology can step in and fill the gap with a little creativity. Enter the ubiquitous solar garden light, typically had for less than a buck each just about everywhere.
Solar garden lights are adequate but underpowered with the most commonly used configuration of a single NiMH or NiCd battery. The light usually runs out power long before the earth’s terminator comes back.
Scavenging the workshop for 18V tools with failing battery packs provided the substrate for a battery boost. Using the subC cell batteries from the packs as a replacement cell for the garden light, the lights went from about 4 hours of useful light to “all night long” in the words of Lionel Richie.
With duration optimized, the next step was to introduce the element of holiday color. The garden light circuitry eeks out enough voltage to drive the stock LEDs. But would it be enough to also drive a simple astable multivibrator circuit attached to the tail of the photoresistor/charger/illumination circuit?
The answer is an unequivocal yes and that leads to the next phase, injecting holiday cheer. This part is simple after constructing the oscillator circuit. Pick the color LEDs and apply them. The current limiting LED resistors were discarded as unnecessary however, this will need to be determined by the specific application.
The circuit is a 2 NPN transistor-2 capacitor-2 resistor affair. A 555 timer prototype was constructed but the 555 has low current flow at voltages below its specifications and at 2.9V (the output of the garden light’s booster) it proved inadequate in empiric testing.
After construction of the additional circuit and placement, the final product is shown below:
There are dozens of ways to improve on this idea. Moving to a microcontroller is the most obvious. If retaining the analog circuitry, a simple pot to adjust the frequency of the oscillator is the next step. If a 555 tickles your fancy, the links below could be used to pump the voltage to a 555 friendly level.