Fan Tachometer

Hello,

I have created 2 seperate systems (I know i could do 1, but wanted to do 2) where i have a PIC16f micrcontroller providing feedback control of a 12VDC fan. The rpm is fedback into one of the Ports on the pic and basically with a P controller inside the pic i maintain the speed.

Then i am using an arduino uno to display the RPM of the fan. This was easiest as i have to demonstrate by disconnecting the rpm wire of the fan from the PIC the fan will reduce its speed as its not a feedback loop anymore.

Here is the issue, i am not able to connect both the PIC and the arduino to the same tachometer output. I have the fan tach pulled up to 12VDC with a 10k ohm resistor (open collector). If the wire is connected to just the PIC the system works, if its connected to just the Arduino i get RPM. If the tach is connected to both the fan, the PIC, and the Arduino, the RPM is 0 on the LCD display.

I am assuming i am getting a floating input or something. Does anyone have a circuit or schematic where i can have both the PIC and the Arduino connected to the same signal wire.

Thanks for your time.

I have the fan tach pulled up to 12VDC with a 10k ohm resistor (open collector).

Why? It is a bad idea to expose either PIC or ATMega inputs to voltages > 5 V.

Have you connected all the grounds together?

The tachometer output of the fan is an open collector circuit. It needs to be pulled up Since i am using a 12VDC fan i am pulling it up to its source voltage.

All of the grounds are tied together.

Pull it up to 5V.

Last time I pulled it up to 5v with just the pic there was a ton of noise. You couldn't see the pulses anymore. Pulling it to 12v solved that issue.

I don't think pulling it to 5v will help.

Last time I pulled it up to 5v with just the pic there was a ton of noise.

You probably made more than one mistake.

Good luck with your project!

So put in a voltage divider for the Arduino. Otherwise the protection diodes inside the chip will limit it to 5V maximum.

Pulling it up to 5V didnt make a difference which i didnt think it would. Also trying a voltage divider did nothing as well. I am not at the maximum voltage or current levels at either port. There is some type of high impedance when both inputs are plugged in thats causing the tachometer circuit in the fan to stop outputting a signal or something.

Has anyone connected to controllers into 1 tachometer output before?