Or Flashing Joule Thief, or blinking Joule Thief, or 1.5V flasher or 1.5V flashing LED or low voltage LED flasher… I guess there could be a few more names that describe a family of circuits that use a single 1.5V cell, and flashes or blinks a LED. I’ve found a few ways to do this on the web, and I’ve designed and built a few other circuits myself to do this.
The single 1.5V cell creates limitations that make it necessary to increase or boost the voltage to that needed to light a red or orange LED, which is 2 volts, or about 3 volts for a blue or white LED. One that works with a red LED is the voltage doubler circuit. One way of V doubling is to switch a capacitor so that it is first charged in parallel with the 1.5V, then switched in series with the 1.5V cell to give a total of 3V. The size in microfarads determines the amount of energy discharged into the LED and the brightness of the flash. Cappels goes through some explanations of different versions of these. These do not use an inductor; they are not a Joule Thief. Cappels runs them at high speed so the LEDs look like they’re on steady but if the pulse rate is slowed down to less than 10 PPS then they will look like they’re flashing. There is another similar flasher circuit on Dave Johnson’s Discover Circuits website, but his uses a CMOS chip to drive the .boost circuit.
Another very similar switched capacitor circuit could be found here, but when I tried it the browser just waited without timing out. I just tried it again and it timed out within a half minute. I searched the web for this and found references to the link I gave, but nowhere else. I’ve built several of these, all without problems. They draw very little power and the cell lasts for a long time. I have attached my drawing of the schematic to this blog.
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About the circuits that I’ve designed and built I corresponded with Quantsuff and he came up with some flashers and I used a similar design that takes my Supercharged Joule Thief and adds a capacitor and resistor to cause it to flash. The single transistor in the Supercharged Joule Thief does both functions of boosting the voltage and causing the circuit to flash. One limitation with this circuit is that the flash starts to get longer as the battery voltage drops, until it just stays on solid as it gets down to 1 volt or so. The great thing about the circuit is it is very simple, with just the resistor and capacitor added.
The Joule Thief does the voltage boost simply and easily. Using the single transistor to do the voltage boost and flash the circuit puts added demands on the circuit so that interactions between the flashing and other circuit functions cause limitations. I believe this is why the SJT flasher stops flashing and goes on solid at 1 or so volts. In order to prevent this, I decided a better way to go would be to move the flasher function to a separate circuit and allow the Joule Thief circuit to perform its single function of voltage boost.
Blue Blinky On Dec 1, 2007 I finished building and put a partly used AA cell into the circuit I came to call the Blue Blinky. At the beginning of each month for 21 months I blogged that it was still running, blinking away on a desk. It became much dimmer, but it was still going even though the cell’s voltage was down to below 0.8 volts. The beginning flash rate was about 1.2 flashes per second.
Referring to the schematic and picture in the link above, the circuit consists of two parts. The left side of the schematic is the two transistor flasher which can be found on the web including here. This circuit does not have to turn on and off the total Joule Thief current, it only handles the base bias current to the Joule Thief transistor. This is just a milliamp or so, and the idle current between flashes is very low – which helps the battery last a long time. The right side of the circuit is a conventional JT except the base bias resistor goes to the flasher circuit instead of the positive V. The C5 bypasses the pulses of the feedback winding to negative and the R5 resistor quickly discharges C5 to end the pulse.
When the flasher circuit turns on, the voltage to the JT’s base bias resistor rises to nearly the full 1.5V, and the JT takes off pulsing the LED with the boosted voltage. During this pulse the battery current should be somewhere between 50 and 100 milliamps if the JT uses a 2N4401, PN2222A, BC337 or similar transistor. This pulse gives a very bright flash of light, but is very short, only two to three milliseconds. Then the flasher turns off, the circuit rests for almost a second, before flashing again. I would guesstimate the off to on ratio is somewhere around 300 to 1, so the battery will last that many times longer than a Joule Thief that’s on steady. If the Joule Thief circuit lights brightly for a few days on a fresh cell, then it should last a few years when flashing.
Astable Multivibrator flashes Joule Thief This circuit is similar to the above circuit in that the flasher and Joule Thief are separate, but the flasher consists of two transistors in a simple astable multivibrator. The astable mvbr circuit has two base bias resistors to positive, two collector load resistors to positive, and two timing capacitors each connecting the base of one transistor to the collector of the other transistor. If the capacitors and base bias resistors are the same value, the ratio of on to off will be equal, the output will be a square wave. But we want the circuit to put out a short pulse with a long time between pulses. This requires that the capacitors and resistors be unequal so that the pulses are short. But the circuit allows us to set the ratio of on to off times from equal to very unequal in any ratio. Ten percent on, 90 percent off might be a valid choice.
I got around the 2V or more LED voltage problem by running the astable multivibrator at 1.5V, which works okay. Then the output of this drives the 1k resistor of a Joule Thief, which boosts the 1.5V up to run the LED. The flashes are very bright, but the battery life won’t be as long as the Blue Blinky flasher. The typical battery life might be ten times longer than a constantly on Joule Thief. I designed a circuit board and sent it out to ExpressPCB and got back three Miniboards each with 3 flashers. I cut them up and assembled them and they work great.
Back to experimenting…