How do the adjustable knobs in my car work?

In my car there are, for example:

–A knob I can turn to move up and down the radio spectrum. This knob can go infinitely in one direction; if I turn it clockwise until I reach the highest frequency and keep going, it resets to the lowest frequency and keeps going up. In an old car where you’d have to stop at one end of the spectrum and reverse direction to go back down, I’m guessing the knob would have controlled a variable capacitor? (Been a long time since I built a kit radio…) What is the tuner knob in my current radio doing?

–A knob to control air temperature. This goes from “Very cold” at the extreme left, to neutral/ambient in the middle, to “Very hot” at the extreme right. Once you hit the extremes, you have to reverse direction. My guess for this component (old school) would be a rheostat, but is that what I’ve got in 2018?

I’m more familiar with the controls on audio equipment, but I’m guessing that the tuner knob actually just sends a pulse to the digital tuner circuitry as you rotate it. For example, it may just send a “pulse” (momentary contact, breaking an optical sensor, or something else) for every 60 degrees of rotation. They could achieve the same thing by having you push a button repeatedly. That’s why the knob turns freely in either direction as much as you care to turn it. (It’s trivial to design a knob to differentiate direction.)

The temperature control may be a potentiometer (variable resistor) or it may have some other type of design. (We tend to shy away from calling them rheostats now unless they are actual, wire-wound variable resistors.) Interesting fact: pots are available in many different configurations, but two of the most common are “linear taper” and “audio taper.” Linear tapers are used in many applications and, like the name suggests, they adjust the resistance in a linear manner (i.e., turn the pot 20% and you get 20% of the resistance). Audio taper pots “bunch up” the resistance on one extreme in a logarithmic manner. Replacing a linear taper pot with an audio taper pot can give you very fine adjustment at one end of the range and very coarse adjustment at the other. This often produces side-splitting results…I’m laughing tears just thinking about it.

Ever seen the old iPod with the scroll wheel?
A computer knows if you are executing a movement (turn knob, or run your finger in circles around a disc face on an iPod).
It can translate this into a direction and speed.
I have no ide how, but… for the iPod (and the ipad touch screen) it uses capacitance areas slightly smaller than a fingertip, and infers relative position from how much relative capacitance it detects on adjacent pads. (Finger is 30% on one pad, 70% on other - therefore 30% of the distance from the center of the first pad.
For a twiddling knob, I could see the equivalent being a series of resistance pads around a contact on the shaft. As you turn the shaft, the contact percentage on each resistance pad, it sees the resistance increasing on one and declining on the adjacent, until it moves on to contact a third pad, and so on in a circle. Position means nothing - the device detects this change and the direction of change.

In the good old days, an LC tuning circuit and some transistors was the height of technology; today’s devices store and display and synthesize radio station frequencies digitally or sense/adjust cabin temperature digitally, decode embedded text from radio stations, etc. Should you be surprised they can detect 1-2-3-4… sequence of contacts and translate that to “increase the volume”? As for the temperature - with the radio, it makes sense to restart tuning at the bottom of the band. It’s not a clever interface where when you turn the heat up too high, you suddenly return to bone-chilling cold. Simpler to have the computer control act like it was a mouse that hit the edge of the screen - It’ve moved the setting to ts upper limit, going to 11 won’t help… but if you turn it down, it will stat going down.

So basically - computer control listening to your twiddling.

The “infinite spinner” knobs are a form of rotary encoder.
Cheap ones use contacts, and they are available for under $1 each. Fancy ones, like the silky-smooth knobs on high-end audio gear, use optical sensors.
There are two distinct types - absolute and incremental. Most consumer products use incremental (quadrature encoded), since they are much cheaper.

Rotary encoder is the correct answer.

As for the temperature control knob, it could well be a resistor that sends signals to open the mixing door. Or it could just be a cable that directly opens that door. If it is a modern one that indicates temperature for the climate control, it would be a resistor I guess.

You never know what a manufacturer thinks is better. The speed control on a towmotor we had used a rotating plastic disc linked to the foot pedal. The plastic started clear and gradually got more and more opaque around the circle. A light shined through the disc to a sensor to control the speed.

Thanks for the replies. So the rotary encoder sends signals to the radio tuner–does the digital tuning circuitry operate in a way that is similar to an analog tuner, or completely different?

Well, that all depends…
In the old, old, old days, the tuning knob was connected to a variable capacitor. That capacitance changed the resonant frequency of the tuning circuit directly.
Later radios used a variable capacitance diode to do the tuning, and the diode’s voltage was changed by the tuning knob controlling a variable resistor. More modern radios use “software defined radios,” where everything is done digitally. Those could just take the current “count” of the rotary encoder as input to the SDR.

It’s interesting that the radio tuner knob has the same kind of connection to the radio (software connected “by wire” to determine position and movement) that your throttle has to your engine in all modern cars.

Most likely a quadrature encoder. this is what used to be in computer mice when they had roller balls: you move the mouse, the ball rolled, and in turn it spun a shaft attached to a quadrature encoder wheel that interrupted a light beam (early versions actually had electrical contacts). Quadrature encoders are useful when you only need to know the relative movement (direction and magnitude) of the shaft, and never need to know the absolute orientation.

That’s the old tech way of doing it. The modern way would be to have the temperature contol be an input to a microcontroller which takes the value of the setting and the value of the temperature sensors and then determines how much power to send to the heaters and fans, and in sophisticated systems might also open and close solenoids or use stepper motors to open or close various gates and vents. The control itself could be a potentiometer, or just another type of encoder. It depends on the system.

In modern vehicles, wiring for various switches and controls is usually digital to save on wiring and reduce complexity. Older cars filled with modern conveniences like power windows, air conditioning, power door locks and other electronic doo-dads were a maze of analog wiring, which over time led to failures and maintenance headaches. Ask someone who owns an old 70’s era Cadillac.

Today, there’s more likely to be a common information bus and all the various sensors and switches and knobs provide digital signals which can be decoded by a central computer. This is much cheaper and much more reliable in the long run when things get complex. In fact, it’s probably necessary with modern cars as there are hundreds of sensors - abs, traction control, climate, parking sensors, tire pressure monitoring, cameras, power windows and doors, engine sensors, pollution controls, airbags… If you had separate wiring runs to every sensor and switch in a modern car it would be unmanageable.

As it is, wiring systems are still incredibly complex. The new Bentley SUV has seven different computer networks running inside it, communicating with 90 different control modules. The wiring harness for the vehicle weighs 110 pounds. But if it had to be done with all analog switches and relays like the old systems, it would probably not even be possible,

Thanks for all the replies. I got lost in “software defined radio,” but everything else more or less makes sense.

A standard FM radio will take an input from an antenna and run it through a tuned circuit to isolate the frequency you want, then mix the signal with an internally generated one to create an intermediate frequency which is then amplified and run through a demodulator then through an audio amplifier to the speakers.

Other forms of radio signals require different hardware, so general purpose radios that can do it all require a lot of circuitry.
In software defined radio, the entire broad spectrum signal received by the antenna is processed by the computer, using software filters and demodulators to isolate and decode what you want to hear. This minimizes hardware and makes the radio software updateable. You can buy an SDR dongle for your computer that will let you listen to AM/FM, shortwave, or a number of other frequency ranges and modulation types. You can buy them for around $20, because all they are is an antenna plug and a bit of circuitry to interface with the PC. All the heavy lifting is done in software.

Most mice still have a dial like this, just for the scroll wheel, not for the movements of the mouse as a whole.

Thanks, much more comprehensible than Wiki.

Yes, a radio unless it’s really simple uses a “beat frequency” - mixes it with the received signal frequency to get a standard intermediate frequency. (In old FM radios, this IF was IIRC 10MHz).
So in the good old days, an FM radio had 2 ganged variable capacitors, one to tune the incoming reception and one to tune the oscillator for a frequency 10MHz different. Then any further work on the signal before it was decoded to audio was done at the standard IF frequency, so no need for variable tuning.

I think today the radio uses a computer chip to synthesize the exact frequency digitally to produce the signal to mix and get that IF, so no need for variable capacitors, for tuning, etc. Much simpler circuitry, and less risk of “drift” - frequency that old device is tuned to might change as for example, temperature affects the capacitance or inductance of the components doing the tuning, while digital circuits are not affected.

So real simple - the computer controller for the radio sees the knob turning, and uses that info to change the number displayed as the tuned frequency - up or down. Position of knob is irrelevant. From the number displayed (and stored) the computer synthesized the frequency required to tune to that station.

Similarly, the computer control determines the temperature to set the cabin air to; it has a temperature sensor on the air intake, I presume, and depending on the difference adds heat (or air conditioning). Presumably a significant difference increases the fan speed, as long as the outgoing air is hot enough that you are not making the situation worse by blowing more air. it’s all programming that determines the decisions the controller makes.

That’s for a temperature feedback controlled system. But there are still plenty of heating control knobs (like the OP described) that just go from "cold to “hot”. Those systems are just setting the temperature of the output air, they do not regulate to a desired cabin temperature.

You can usually tell by the feel of the knob if it is pulling a cable or turning a pot. On my Scion the hot to cold knob is a cable driven mixing door. The fresh air /recirculate knob is a motor driven flap.