Let current flow when signal is low?

Other Hardware General Electronics
10

Or,

  • Arduino PWM drives one input of a two-input NAND gate

  • Main PWM drives an input of a second NAND gate

  • Arduino also drives a retriggerable one-shot (555?)

  • One-shot Q output drives 2nd input of Arduino NAND gate and through an inverter (4th 74HC00?):

  • drives the 2nd input of the main PWM gate

  • both of these NAND gates together drive a third two-input NAND which in turn drives

  • a solid state relay which runs the fan

While Arduino provides PWM signal one-shot keeps Q output high. If Arduino goes away one-shot times out and switches to the main signal.