Currently, I have a board that powers 4 DC fans. A PWM signal is received from an external microcontroller to control the fan speeds. Unfortunately, the microcontroller sometimes fails to send the PWM signal, and I can not change it at this stage. When the PWM signal is not received the fans ramp up to maximum speed due to an internal pullup resistor in the fans. I want the fans to default at a speed anywhere between 20% and 50%.

I would like to make changes to this board by adding a default PWM signal which drives the fan speeds when no external PWM signal is received.

##I have the following constraints and specifications:

1. All PWM signals have a frequency of 25kHz

2. I have 12V available to the board.

3. The board is tiny (the chosen components must be small and few).

4. The budget is low (the cheapest viable solution is prefered).

##Below I have made a diagram of how I want to solve this problem. I have a few questions:

1. Is there a simpler way of solving this problem?

2. Is the 555 timer a good option for a PWM generator?

3. What logic level switch would be appropriate for a PWM signal?

4. How will I be able to convert a 25kHz frequency to 5V? (IC suggestions?)

5. Any advice, comments, critiques, component suggestions will be much appreciated.

##Thank you in advance for any help or comments.