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MOSFET Circuits Ultimate Flasher

It was implied that the first transistor circuit shown was the simplest possible, but substitute a FET for the transistor, and you won't even need a limiting resistor!
Most FETs do not switch on until 2 or more volts, and even with the BS108, you may have to do some selecting before you get one with a low enough Vgs(on). But the Rds(on) for this device is as high as 8-ohms, which means we will not get full performance out of our circuit. However, it will drive up to 3 LEDs in series easily.

Another approach: a hybrid circuit by Watt-sun:

The MOSFET-driven circuit was discussed here on Watson's Blog and this is my take on a single MOSFET LED circuit. The DPDT switch charges a small 0.1uF (100nF) capacitor in the 'off' position. This charge is used to increase the supply voltage to 'jump-start' the 2N7000 MOSFET, which needs about 2-volts to start working. After that, the 1N4148 diode taps a small amount of the output to keep the circuit going.

Here's an interesting adaptation of the above circuit. Instead of relying on a switch to charge the capacitor, we are using the body's charge to start the circuit. Adjust R1 between 100K and 1M-ohm for LED current. The BS270 is a higher performing version of the 2N7000 (or BS170), able to handle up to 2-amps and has a lower on-state resistance of about 1-ohm. This design will drive a 25mA LED at full brightness with a 78mA draw on a NiCd battery.
It is also very adaptable - replace the BS270 with a "Logic-level" N-type power FET (Like the FDN3055L) and you can drive LEDs up to 150mA without changing any other parts.

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See my other Projects on Instructables.com

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	It was implied that the first transistor circuit shown was the simplest possible, but substitute a FET for the transistor, and you won't even need a limiting resistor! Most FETs do not switch on until 2 or more volts, and even with the BS108, you may have to do some selecting before you get one with a low enough Vgs(on). But the Rds(on) for this device is as high as 8-ohms, which means we will not get full performance out of our circuit. However, it will drive up to 3 LEDs in series easily.
Another approach: a hybrid circuit by Watt-sun:
	The MOSFET-driven circuit was discussed here on Watson's Blog and this is my take on a single MOSFET LED circuit. The DPDT switch charges a small 0.1uF (100nF) capacitor in the 'off' position. This charge is used to increase the supply voltage to 'jump-start' the 2N7000 MOSFET, which needs about 2-volts to start working. After that, the 1N4148 diode taps a small amount of the output to keep the circuit going.
Here's an interesting adaptation of the above circuit. Instead of relying on a switch to charge the capacitor, we are using the body's charge to start the circuit. Adjust R1 between 100K and 1M-ohm for LED current. The BS270 is a higher performing version of the 2N7000 (or BS170), able to handle up to 2-amps and has a lower on-state resistance of about 1-ohm. This design will drive a 25mA LED at full brightness with a 78mA draw on a NiCd battery. It is also very adaptable - replace the BS270 with a "Logic-level" N-type power FET (Like the FDN3055L) and you can drive LEDs up to 150mA without changing any other parts.
RETURN to MAIN Page.