#066 Digital Logic Switch - N-Channel Enhancement FET

Using a small-signal n-channel MOSFET as a switch, with turn-off delay.

Here’s a quick video of the circuit in action:

Notes

N-Channel Enhancement FETs are easy to use as a digital switch:

• ON when gate-source voltage is much greater than threshold voltage (into saturation region)
• OFF when gate-source voltage is below threshold voltage (0.8-3V for 2N7000)

This circuit demonstrates the basic switch, with the addition of a turn-off delay.

The turn-off delay is implemented with a simple RC circuit: the R1=220kΩ C1=10μF pair having a time constant of 2.2 seconds.

Construction

Credits and References

• LEAP#106 SwitchPFET - same circuit but using p-channel MOSFET
• LEAP#286 SwitchPowerNFET - low-side power switching variation to this circuit
• 2N7000 datasheet
• MOSFET as a Switch