# Designing a 4053 IC robotic control circuit for interfacing with Sabertooth motor controller?

I am designing a control circuit for a robot and was recommended to use a 4053 IC to achieve analogue multiplexing.

PROBLEM

The Sabertooth 2x60 motor controller I am using is running in analog input mode. This means its control scheme is 0v-2.5v is reverse, 2.5v is stop, and 2.5v-5v is forward. I am using ROS and using a 1208FS-PLUS as the DAC to interface to the motor controllers. Until my ROS software starts, it is at 0v and this is full reverse.

GOAL

The design goal is that I require a 2.5v source until my software is started and able to control the robot. My approach was to use the 4053 IC to multiplex my analog signals. While the software is running it supplies a high and switches the multiplexer to use the signals from the DAC. If it is low it uses a 2.5v source from a voltage divider circuit.

APPROACH

Below is a schematic (and built on proto board). I am using 2x10k resistors to build a voltage divider circuit (as per the relationship). The output is using an RC filter to achieve a smoother operational output (as per the Sabertooth user manual recommendations).

PROBLEMS AND HELP

• I have tested this on the bench and it works however the voltage divider produces ~2.8v on average which causes the motors under no load to drift. Could anyone recommend a way to achieve as close to 2.5v as possible. Solution can be active as the 4053 is an active component.
• The input is not always 5v despite the specs of my DAC specifying it will be. I was thinking of using an op amp to cap my voltage at 5v? Could anyone please assist me to evaluate the applicability of this?
• The Sabertooth manual doesn't specify the control signal currents (I don't imagine they would be too large). Should I use a buffer (voltage follower) to somewhat amplify the current from the 4053?

Thanks in advance for any help :) -Grant

• If you're asking about the output current of the 4053 you probably don't want to use it. You can look for its internal resistance and calculate the expected voltage drop, however. – TimWescott Jul 2 '19 at 23:36
• @TimWescott the sabertooth motor controller doesn’t specify any electrical characteristics of the input and so I was curious as to if the IC could support it. Perhaps my edits will make the question and issues more clear – Grant Dare Jul 3 '19 at 6:58