servo control signal 1.8 volts - power conversion

Discussion in 'The Projects Forum' started by jeanluis, Nov 24, 2008.

  1. jeanluis

    Thread Starter New Member

    Nov 24, 2008
    3
    0
    Hello, I am a software guy, and I have a processor board that I would like to use to drive a servo or two. Right now I have the processor up and running, and the board has a 5 volt dc power rail, and ground to power the servo, it also has a 1.8vdc power rail, and my gpio that I would use for the PWM control signal is also 1.8 volts. From what I have read 1.8v is not going to be enough for the control signal, it sounds like the control signal needs to be at least 3volts when high. I have looked at LDO's and other DC to DC converters, even power muxes, but I don't know enough about these things to know if they will work on a control signal (or if I can get them to work at all without expert advice). Will these things introduce too much noise on the control line, or will they have other adverse affects? What type of circuit would you build to boost the control signal from 1.8v to at least 3v? What is the logical and correct way to do this?

    Thanks.
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    With gpio you mean a MOSFET?

    If yes, then you need about 10V to fully turn it on. With 3V you just start to turn it on.
     
  3. jeanluis

    Thread Starter New Member

    Nov 24, 2008
    3
    0
    Unfortunately I am not talking about a mosfet, the ARMv7 processor has output pins (general purpose I/Os) which the processor can drive high or low. When the signal is high, it is 1.8v, when it is low, the signal is 0v. My plan is to program the processor so that it drives the gpio to act like a PWM signal, however the 1.8v high signal isn't strong enough to make the servo move... So how can I increase the voltage when it is high?
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Yes, you can control a BJT transistor with the output of the ARM. With the BJT you will drive a MOS transistor which will drive the motor. Of course the motor will be supplied with its own power supply.
     
  5. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Jean,

    I think we have a misunderstanding here. You mentioned 'servo', which can be many things. It can mean any actuator with feedback, or it can mean the common RC-type servos.

    I am thinking that you want to supply a control signal to an RC-type servo, which is the typical 1.5mS ~60Hz control signal that varies on-time to control the angle of the servo. Is this true?

    If so, then you need to do level conversion. You can do this in a variety of ways. Try to find a buffer that is designed to run off of low-voltages, then try to find one that can accept a 1.8V signal as a high. This will effectively level-convert. Otherwise, you can use a low-gate threshold mosfet (i think they make them down to 1V now) or even a transistor. Google "Transistor level converter"

    Steve
     
  6. jeanluis

    Thread Starter New Member

    Nov 24, 2008
    3
    0
    Thank you both for your replies, and yes I am taking about an RC servo control signal. You have given me a bunch of things to look at, and now I will commence reading... :D

    Thanks for your help, I may be back.
     
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