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I am designing a control circuit for a Sabertooth 2x60 motor controller. When this controller is in analog mode, it runs in full reverse (0v control signal) until my software starts and applies 2.5v to the signal pin. I have currently integrated a voltage divider and a relay circuit to apply 2.5v from a source independent of the computer (5v vin divided by two 47kOhm resistors) which works but I am hoping to get rid of the mechanical component (relay) if possible.

It has been a long time since I have worked with transistors and mosfets and was hoping if someone could assist me in the design and if there is an IC I can use? To recap requirements; Apply 2.5v from a voltage divider circuit until the software applies a 5v signal (Digital I/O) to the relay which then switches vout to use the analog output of my control software instead? There will be two analog inputs to my control circuit, one 2.5v from the voltage divider and one 0-5v from my software. Using a Hi/Lo pin, I want to select which input the circuit should output.

Thanks in advance for any assistance you can provide me :)

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  • \$\begingroup\$ It's not entirely clear -- do you have two inputs, one of which is a digital "use the DAC" input, and the other of which is the DAC itself? If so, please edit your question to clarify. \$\endgroup\$
    – TimWescott
    Commented Apr 1, 2019 at 23:19
  • \$\begingroup\$ @TimWescott I have made the recommended edits which hopefully answer your questions. I have also made comments on your answer addressing this. Thank you for your reply. \$\endgroup\$
    – Grant Dare
    Commented Apr 1, 2019 at 23:57
  • \$\begingroup\$ Is the 5V created from the controller Vbat input 6V-33.6V ? Output current: 60A per channel \$\endgroup\$ Commented Apr 2, 2019 at 1:30
  • \$\begingroup\$ Hi @SunnyskyguyEE75 , This was the intention however with the relay circuit I found there is a few seconds between the battery terminals connecting to the motor terminals and the 5v output from the sabertooth coming up. This causes the machine to go into a full reverse for a few seconds taking no shins as prisoners.The 5v will now be created from a 24v to 5v regulator. \$\endgroup\$
    – Grant Dare
    Commented Apr 2, 2019 at 2:00
  • \$\begingroup\$ If the 5V comes from the same battery used to drive controller then this starts up with 2.5V ( stop). If you derive it with delays then you get full reverse for that delay. \$\endgroup\$ Commented Apr 2, 2019 at 2:05

2 Answers 2

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Assuming that you have two inputs: a digital "use the DAC" input, and the DAC itself, then you can use an analog switch to do this job. Depending on your needs, a 74HC4053 may be ideal.

Like this. Note that this is untested -- but it really ought to work:

enter image description here

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  • \$\begingroup\$ Hi @TimWescott, Just made an edit to clarify the questions you have asked. I understand now why you were confused, the computer is connected to a 1208FS-PLUS from MCC which I refer to as a DAC. This has a "analog output" pin. The circuit I am designing will have two inputs and a single output which is decided by a HI/LO pin which my software sets to HI when it is appropriately running and ready to control the motors. \$\endgroup\$
    – Grant Dare
    Commented Apr 1, 2019 at 23:42
  • \$\begingroup\$ I checked the IC you recommended and have never worked with an analog multiplexer myself. If I am correct, I would supply 5v on pin 16 and 7, GND on pin 8. A1 on pin 12 and A2 on pin 13. The HI/LO on pin 11. Tie pin 6 to GND. The output would then be pin 14? \$\endgroup\$
    – Grant Dare
    Commented Apr 1, 2019 at 23:43
  • \$\begingroup\$ I made a picture. It may be exactly like you said (except that VEE goes to ground), but I'd have to make a drawing from what you said anyway, so I just made mine. \$\endgroup\$
    – TimWescott
    Commented Apr 2, 2019 at 0:05
  • \$\begingroup\$ Hi @TimWescott , I went and picked up an analogue multiplexer from my local electronics store. I have built the circuit and can confirm this is appropriate for my application. Thank you very much for your assistance. \$\endgroup\$
    – Grant Dare
    Commented Apr 2, 2019 at 7:29
  • \$\begingroup\$ I have also edited the title to better reflect the solution (since we didn’t use NPN transistors). This way the title is not misleading to the answer \$\endgroup\$
    – Grant Dare
    Commented Apr 2, 2019 at 7:32
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Your motor controller accepts Input voltage: 6V-33.6V, Output current: 60A per channel and 120A surge ( which depends on battery & motor ESR, I=(V/DCR)

  • Controller Voltage drives a full bridge
    • 0V = Drives to Vbat full reverse
    • 2.5V = stop
    • 5V = Drives to Vbat full forward.

enter image description here

details

  • Use 0.5% equal R's for the Bo input. ( 1k to 10K)
  • You may have outputs defined as floating inputs , so a 10K R to gnd on DAC En prevents a floating input.
  • The Motor Control is intended for a Pot Control but input impedance is not known. So 10k is suggested.
  • This switch resistance is 7 Ohm (Typ)
  • RF decoupling caps 0.01uF on Vcc and across OUT to reduce stray motor noise loop induced noise.
  • Shielded twisted pair is suggested to reduce control noise with the shield connected to gnd of the 5V uC.
  • layout should be kept very small and avoid large area loops. Cost < $1
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