Some radios, particularly the ones used in cars, featured a row of buttons that could be used to store and select preset stations. Only one button could be pressed at a time, after which it would lock down and the previously pushed button would pop back out.
How did these radio buttons work? That is, what mechanism ensured that only one button was activated at once? Can someone explain this to me like I am a five-year-old, preferably with the aid of some nice schematic diagrams and/or photos of the inside or rear of the button mechanism?
Oh, and how did the buttons remember and restore the station frequency? These things have been around for longer than computers so I assume the memory system is entirely mechanical.
I don’t know how they worked, but I remember how to set them to a station. You used the knob to tune in a station, then you pulled on the desired button and it would slide out a ways. Push the button back in fully, and the station was set. Usually people would use the button nearest the indicator on the dial, but any button would work. I used to enjoy setting the right button to a station far on the left of the dial and vice versa.
For old mechanical devices like this it’s all about rods, bell cranks, sliders, and cams. In this case, they often used a heart-shaped cam for each button.
Here’s a similar cam from the mechanism of a chronograph watch: Heart-shaped cam
You can see that if you push on the side of the cam with some kind of rod, the shaft will always rotate to the lowest point.
Now imagine a mechanism where there are five of those cams on a horizontal rod behind the buttons in the radio, and each button has a pin that pushes on its associated cam, turning the shaft to wherever its cam was set.
Now, how are they set to begin with? A mechanism loosens all of the cams slightly, allowing the button to zero the rotational position of its cam on the shaft–at exactly where the dial currently is.
There are many ways of doing it. Here’s a drawing I came up with for one way of doing it:
As can be seen, when a button is not pressed, there is 5 V on the input channel. When a button is pressed, there is 0 V on the input channel. In the drawing, button #2 is pressed.
When the microprocessor (or microcontroller or ASIC) sees the voltage on a channel go from 5 V to 0 V (a.k.a. falling edge trigger), it means a button has been pressed. It then produces an interrupt and immediately executes a subroutine associated with that particular interrupt.
So what happens when one button is pressed and held in, and then another button is pressed? One way to handle this is with priority logic (priority encoder). I’m sure there are other ways to do it, but I’m not an embedded programmer.
Car radios used a bank of mechanical latches. When you pushed one in, all of the others released. The slide (button) you pushed in had electrical contacts on the back and a capacitor and a tunable inductor that became the tank circuit for the local oscillator. To set up the radio, each inductor was tuned, by hand, to a different station.
Sorry, you completely lost me. Remember, I am just five years old. Could you please explain, at minimum, “tunable inductor”, “tank circuit”, “local oscillator”, and “phase lock loop”?
Well, not really, but then again, I’m pretty bad at visualizing mechanical things. But I did some web searches and came up with this video. Is this what you’re talking about?
A radio selects energy waves by providing a circuit that resonates at only one frequency. Circuits that do so are called tank circuits. A tank circuit has a capacitor which is two metal plates with an insulator between them and and inductor which is a copper wire wound around an iron core. The value of the inductor is determined by how far the core protrudes into the coil of wire. So the core is on a screw that allows it to be moved in and out of the coil. Changing the core position changes the frequency the tank circuit is tuned to - hence the radio station it picks up.
Without getting into hetrodynes and mixing, that is essentially how it worked.
The mechanical setup I took apart once worked like this:
There are two bars running horizontally behind the buttons, one above and one below. At the ends they’re held in a seesaw frame such that if you push one bar back, the other moves forward. The angle of the bar seesaw adjusts a variable capacitor, in the form of parallel plates that intermesh more or less as the mounting shaft for half the plates rotates. The seesaw turns that shaft.
Each button has a flat plate mounted on its rear end, like a spatula on a handle, the plate being vertical. It’s mounted on the end on a pivot that is normally clamped firm. When you push that button, that plate pushes on the bars until they’re at the same angle the spatula edge is at.
Pulling the button somehow loosens the clamp at that pivot (sorry I forget how this detail works). Tuning the radio by the knob also positions the seesaw bars at the right angle for that station. Pulling a button now loosens the spatula, and then pushing it shifts its spatula end to the right angle and clamps it tight there.
It would have been about 50 years ago I took this apart, but I remember it pretty well, except for the spatula clamping mechanism.
My parents had an old console radio/phonograph from probably the 1940s. Unlike a car radio, each pushbutton was attached to a separate tuner set to one station. Pushing a button disconnected the main tuner and connected the small tuner. To retune the pushbuttons you had to take the back off the console and use a screwdriver to manually adjust the tuner to another station. Obviously too big a system to fit into a car’s dashboard, but you didn’t have to worry about one broken pushbutton (or all five broken pushbuttons) making the radio unusable.
For those interested in this type of thing, here is an excellent two part series (slightly less than an hour long) about how electromechanical jukeboxes work.
Part 1: Covers record selection and playing
Part 2: Covers how it remembers which records were selected
There’s a short summary of the two videos starting at 21:15 in Part 2.
That’s it. Imagine the button being on the end of that long rod and pushing down. That will always cause the heart-shape to orient itself so the little wheel is in the divot.
At that point in the video you can see the horizontal brass rod–that’s the tuner, and it’s connected to a variable capacitor on the right side (lots of thin interleaved plates).
The buttons would attach to those five thin aluminum bits sticking up at the top of the frame. The heart-cams are visible nearly edge-on on the tuning shaft, in line with each button.
In that particular radio I believe there was a screw at the end of the tuner rod that one loosens or tightens that locks all of the cams to the rod; loosen it, tune to a station, press the button and the cam realigns itself, then tighten it up. Next time you push the button, the cam will rotate the shaft to your favorite station.
Keep in mind that this is just one simple-to-understand mechanism, and there are many ways to achieve this. Different manufacturers did it different ways. The “individual tuner per button” mechanism existed as well, but that wouldn’t have been practical in a 1970s car radio.
I have an old synth where buttons correspond to different sounds. Pressing one button will disengage previous one, but you can press more than one at the same time and mix the sounds. Even all 6 of them. To disengage all of them, you simply need to press any button a bit harder. Have no idea what’s the mechanics behind it.
