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What kind of DC is this?

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stefan

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I am a total beginner at electronics and I am cutting my teeth on attempting to hack in to an RC transmitter and replace two pots with a low pass filter on a PWM signal.

I got the PWM signal smoothed out with an RC filter, however after hooking up the transmitter to a scope I'm seeing a square DC line.

When both channels of the scope are calibrated the same my PWM>DC signal looks perfect at 1V, but the top of the the transmitter's wave reads about 7.5V when the volt meter reads 1V.

I'm assuming something on the TX board is causing this to pulse somehow but I'm not sure how to replicate it.

Any direction is appreciated!
 

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Where is the schematic of the circuit you chopped up and the filter you added?
We don't know where on the circuit you connected the 'scope and we don't know the settings of the 'scope.
 
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The original circuit board I want to interface with is intact and un-altered. My PWM circuit is not connected to the transmitter at this time.

In that picture I connected the scope to the high side of the potentiometer. If I connect it to the middle signal pole I can adjust that pulse from max (as shown) down to 0.

The scope is set on .5ms Time/Div and 2 Volts/Div.

The circuit I've built is coming from 3.3V Netduino running at 10khz. I've attached a diagram.

Previously I disconnected the middle pole of the potentiometer from the Tx's circuitboard and attached the Out in this diagram to the board at that point and then connected ground from the board back to the Netduino's ground. I can meter the correct V readings on both sides of the potentiometer's points on the board doing this and adjusting the PWM's duty cycle up and down, but the RC vehicle is not reacting as intended.

Thank you,
Stefan
 

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You show an RC lowpass filter. If it has a load then its voltage changes.
You keep talking about a potentiometer that we know nothing about.
You keep talking about a transmitter that we know nothing about.

Post the schematic of the transmitter and point to the potentiometer you are replacing with this lowpass filter.
 
Sorry I do not have a diagram of the transmitter. It is a toy grade rc transmitter that uses two pots, one for motion and one for steering.
 
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Ok what I have determined is the wave above is the full output of the 9v battery and the width of the pulse is defining the voltage. Turning the pots lower and raise the amount of voltage by adjusting the height of the pulse. The two pots have different max voltage and different width pulses but the same height, analog PWM. The pot tweaks the PWM range.

Can I replicate the pulse duration with a 555 timer or is there another better method?
 
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I've attached a front and back of the transmitter PCB showing the disconnected pot signal and where I soldered the DC input in and clipped the ground out.

Thank you for your time.
 

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  • 2011-01-19_00-26-22_697.jpg
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  • 2011-01-18_18-53-14_875.jpg
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