Can a servo be controlled (requiring other circuitry) by two binary input pins?

Question

I am working on a project where I would like to control the position of a servo without a normal input, e.g potentiometer. The servo would be constantly powered on the red wire as required, and then I would have 2 input pins. Ideally, if one is positive the servo goes to 90 degrees, if the other is positive to -90, and if both are off to 0. I am aware of the 555 timer IC to generate the PWM, but don't know how the frequency?speed? of the PWM can be controlled without a resistor. Any suggestions on if/how this is possible would be greatly appreciated, and sorry for any wrong terminology etc, I'm not super experienced in electronics.

Answer 1

Yes, you can do this, but it's not 100% straightforward since you want (presumably momentary N.O. switches) to both increase and decrease the resistance. Here is one way, there may be better ways:

^{simulate this circuit – Schematic created using CircuitLab}

I have wired Y0 to Y3 so pressing both switches at once returns the servo to the null position. It could be wired to one of the other Y pins depending on what you want to be dominant.

You can substitute fixed resistors for the pot+resistor combinations when you know the value you want.

The 555 circuit is one of the standard hobby servo controller circuits. It produces a 1-2ms pulse with about a 50Hz repetition rate.

The 74HC4051 connects one of the 8 mux 'Y' pins to the Z pin depending on the state of the three 'S' pins.

You could also simply use an Arduino or another kind of MCU.

Answer 2

speed up-to 50 pulses per second (not critical)

pulse-width 1 to 2 milliseconds.

Build a 50Hz oscillator with a 1.5 second pulse width connect use your positive or negative voltage to skew the on-time of the oscillator, eg by appying iot to one of the timing nodes of the oscillator through a resistor.

maybe something like this:

^{simulate this circuit – Schematic created using CircuitLab}

R5 sets one extreme R1 the other R2 sets the mid point. R3 sets the pulse rate