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. This reduces the number of batteries required. It is also suitable for low powered LEDs and may be used for PWM dimming.
Step 1: Materials
3 watt red/yellow LED
2 AA rechargeable batteries
555 timer circuit
Low power 555 timer (eg. ICM7555, TLC555)
N-Channel Power MOSFET (eg. IRFZ44N)
Diode (for duty cycle less than 50%)
0.01 uF ceramic capacitor
10 uF electrolytic capacitor
Resistors R1 and R2: Must be greater 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 (eg. 2N3904)
1k ohm resistor
2 x Diode
10 uF electrolytic capacitor (at least 25 volt 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 mean that less power is lost by the voltage dropping resistor. If 12 volts is used to power this, 80% of the energy (9.6V) would be lost as heat by the resistor. The heat can be significant when 700 mA is being drawn.
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 are using IR LEDs, it is possible to strobe them with a single cell if the forward voltage is 1.7V or less. To add more LEDs, simply 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 easy to find. Compared to NPN/PNP transistors, MOSFETs do not 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.
In the Joule Thief circuit, the LED has been replaced with a diode and the 10 uF capacitor smooths out the voltage ripples that are produced. 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 or a reverse biased. I have also included a diagram which shows you how to wind the toroid bead. Three turns should work. The Joule Thief and strobe circuit draws 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. This also means that NPN/PNP transistors may not work so MOSFETs must be used.
Make sure there is always a load for the Joule Thief. Without a load, the capacitor's voltage may be charged too high and when you turn on the strobe circuit, the 555 timer and MOSFET may be destroyed. 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'.