Need help with basic circuit design.

[JPZ]

Hey guys,

I'm working on putting brushless motors in my X-UFO, and to do that I need to convert the PWM motor signal to a PPM signal. I've got that figured out, however I have a problem with inputting the PWM signal into my device.

This is my situation:

The PWM signal is generated by opening and closing the ground circuit. The positive lead is permanently turned on. Which means I can't take the positive lead and send it into my signal converter.

Basically, I need to figure out a way to send a signal to my converter whenever the ground is connected. A diagram of my problem is below.

As you can see, if I were to just take the positive lead from the pwm line, it would supply constant power from the battery to my PWM2PPM. I need some sort of circuit designed so that a signal is sent to the PWM2PPM either whenever the circuit from the battery to the electronics is complete or whenever it is not complete(I can just invert the output from the second version).

Any help would be greatly appreciated.

[jeffs555]

If the UFO's ESC doesn't have a brake, and the PWM output is driven by the open drain of a mosfet with schottky to 12v, then you could just ignore the 12v, connect the PWM output to the input of your PWM to PPM converter, and connect a pullup to the logic supply of your converter.

Better would be to connect a pullup resistor from the PWM output to 12v, then connect a resistor to the base of an NPN transistor. This should work whether there was a brake or not.

[redfoxen]

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[JPZ]

Basically like I have it setup below? Thanks a lot for the help by the way. I really have a lot to learn about electronics... I don't know much at all about pull up resistors or any of that stuff. Might I ask what is that which I marked with the red box? Never seen one of those before. Let me know if I've got this straight.

[Gary Warner]

Basically like I have it setup below? Thanks a lot for the help by the way. I really have a lot to learn about electronics... I don't know much at all about pull up resistors or any of that stuff. Might I ask what is that which I marked with the red box? Never seen one of those before. Let me know if I've got this straight.
It's an NPN transistor.

[JPZ]

I know that now. I figured it out yesterday afternoon. I had never seen the diagram symbol for a basic transistor before. :D

Can I confirm that the above is the proper way to wire this up?

[Gary Warner]

I know that now. I figured it out yesterday afternoon. I had never seen the diagram symbol for a basic transistor before. :D

Can I confirm that the above is the proper way to wire this up?

Yup. It will work.

[JPZ]

Thank you very much!