Introduction: The Solar Garden Light: Adding the Flash

In the previous attempt to increase the output from a basic Solar Garden Light (SGL), publishedhere, we quickly reached a limit: the brighter we make the light, the quicker we deplete the battery, or more properly, the charge we can obtain from the sun during the day.

The solution was apparent: make a flashing SGL. Not only is it much more readily noticeable, and hence, more effective against would-be intruders; it also conserves battery power, so that the battery will last a long time.

The difficulty was designing a circuit that could be made from easily obtainable and inexpensive parts. The result is a simple flashing circuit using only ONE transistor, powered by a single rechargeable battery.And, a simple enhancement of 2 parts will more than triple the light output.

And it is effective - after 4 days of storm & rain, with only the rechargeable from the old light, it's still going strong.

Step 1: The Circuit

The material you will need for the light:

R1,R2,R3,R4: 1.5K-ohm resistor
C1: 470uF (micro Farad)
C2: 1nF (.001uF) or 680pF for 2 x orange or red LEDs
D1,D2: 1N4148 or 1N914 (any low-power signal diode)
Q1,Q2: BC337 (do not substitute!)
T1, made from 3 pieces of thin, insulated wire each 8ft (2m5) long
Solar cell, 1.2volt battery, LED, casing: from old Garden Light

The hi-lited components (Q2,R2) are optional, but will greatly increase the brightness.

The basic circuit is a Joule Thief which is designed to drain the charge in C1 to light the LED. This causes the LED to flash. After that C1 has to recharge through R3, which takes about 3/4 seconds, at which point the cycle repeats. The optional parts increase the power to the LED, and make the flash longer as well.

This circuit is also unique in that not only will it switch off the flashing when the battery begins to charge; it will also turn the light back on to avoid overcharging the battery.

Step 2: Making the Coil

The heart and soul of the circuit is the coil - T1 in the schematic. Despite its complicated lines, it's just a hank of 3 x 8'-long wires that have been jumble-wound together using a penlight cell as a form.

A closer look at the construction shows the 3-wires. The Green is a #32 wirewrap, used for the secondary; the Yellow is #26 for Q1 and the enamel (magnet) wire is a #24, used for Q2, which carries more of the current. Note also the plastic-ties used to hold the wires tightly.

Make sure you identify the start and ends of each wire: and label them P1a, P1b (yellow); P2a, P2b (copper), S1a and S1b (green). They will help you see where they should go later.

For the technically-minded, this makes a 1:1:1 air-core transformer measuring about 20uH per leg.

Step 3: Putting the Pieces Together

The circuit can be assembled in 4 steps:

1) Wire the components together from the schematic as shown

2) Attach part B to A

3) Attach C to the Garden light, leaving as much of the negative lead as possible.

4) Finish attaching the coil and the light.

Step 4: Performance Stats

Each flash of the LED lasts about 1/20-second, sending out brief pulses of up to 150mA through the LED, but too quickly to cause over-heating in the light. This also allows us to conserve battery consumption to about 10mA on average.

That is why even a low-capacitor battery will keep the light working up to 2 days, even without charging under direct sunlight.


It is possible to use this circuit with different color LEDs: whites, blues and UV are basically interchangeable.

For red, orange and yellow, two in series can replace the single white. One small adjustment is to lower C2 to 680pF to allow for the higher LED voltage. A word of caution: in some communities, red and/or blue flashing lights are used to signify a crime or emergency.

Check out my Website for more information on this and other LED circuits