Joule Thief With Ultra Simple Control of Light Output

The Joule Thief circuit is an excellent entrée for the novice electronic experimenter and has been reproduced countless times, indeed a Google search yields 245000 hits! By far the most frequently encountered circuit is that shown in Step 1 below which is incredibly simple consisting of four basic components but there is a price to be paid for this simplicity. When powered with a fresh battery of 1.5 Volts light output is high with commensurate power consumption, but with lower battery voltage the light and power consumption drop away until at around half a Volt light output ceases.”