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.

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.

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.

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.