Introduction: 3W LED Strobe - 2 AA Batteries and Joule Thief
This LED strobe light allows 2.4V to be used compared to 4.5V for most 555 timer circuits. It uses the Joule Thief to turn on a 4V MOSFET, reducing the number of cells required. It's also suitable for low powered LEDs and PWM dimming.
Step 1: Materials
- 3W red/yellow LED
- Resistor (optional)
- 2 AA rechargeable batteries
555 timer circuit
- Low power 555 timer (eg. ICM7555, TLC555)
- N-Channel Power MOSFET (e.g. IRFZ44N)
- Diode (for duty cycle less than 50%)
- 0.01 uF ceramic capacitor
- 10 uF electrolytic capacitor
- Resistors R1 and R2: Must be higher than 1k (values depend on desired frequency and duty cycle)
- 2x Enameled copper wire
- Toroid bead (you can find them in spent CFL lamps)
- NPN transistor (e.g. 2N3904)
- 1k ohm resistor
- 2 x Diode
- 10 uF electrolytic capacitor (at least 25V rating)
Step 2: Joule Thief for Driving MOSFET
This strobe circuit consists of a 555 timer powered by the Joule Thief. The LED runs directly from the batteries, but you can use a resistor. You must use a resistor if you are using 20 mA LEDs. Using fewer cells reduces the power lost by the current limiting resistor. If 12 volts is used to power this, it'll lose 80% of the energy (9.6V) as heat through the resistor. The battery voltage should be similar to the LED's forward voltage. For UV/blue/green/white LEDs, use 3.6V. For red/yellow LEDs, use 2.4V. If you're using IR LEDs, its possible to power them with a single cell if the forward voltage is 1.7V or less. To add more LEDs, solder them together in parallel.
The Joule Thief circuit is famous for powering a blue LED with 1.5V, but it can also be used to turn on 4V MOSFETs, which are easier to find. Compared to NPN/PNP transistors, MOSFETs don't require much current to turn on as they do not amplify current. They also have a lower on-state resistance, which means you can drive red LEDs at full brightness with 2.4V.
Since 2.4V is too high for the Joule Thief to operate, a diode must be used to drop the excess voltage. For higher battery voltages, use more diodes. I've also included a diagram that shows you how to wind the toroid bead. Three turns should work. The Joule Thief and strobe circuit draw about 45 mA on standby.
The Strobe Circuit must use Low Current Components
A low power 555 timer must be used here because, at higher current, the voltage supplied by the Joule Thief decreases. That's also why we needed to use MOSFETs.
Make sure there is always a load for the Joule Thief. Without a load, the capacitor's voltage may overcharge and damage the 555 timer and MOSFET when you turn on the strobe circuit. If the capacitor does get charged too high, disconnect the batteries and short the capacitor to discharge it. It is always a good idea to test the voltage with a multimeter.
Step 3: Frequency and Duty Cycle of Strobe
This calculator can be used to determine the frequency and duty cycle of the strobe light for N-channel MOSFETs.
If you want to know the duty cycle for P-channel MOSFETs, it is 100% minus that of the N-channel MOSFETs'.
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