RadioButton switch (design idea)

françois · Posted: Mon Dec 08, 2008 6:09 am Post subject: RadioButton switch (design idea)

Hello all,

Some tiem ago (and it was probably not even on this forum), someone asked for an electronic version of a Radio Switch. You may have known this kind of thing, on pocket radio receivers: four buttons to select wavelength (LW/MW/SW/FM) plus one (usually red) to turn the whole beast off.

So here is what I came to after a bit of thought (not too much, admittedly). Suppose all channels (SW0 to SW3) are initially OFF (BTW, some kind of power-up reset would be needed). If I depress key K2 (say), then channel #2 will be turned ON and all other channels OFF, so OUT = IN#2. If I now depress key K1, channel #1 will be turned ON and OUT = IN#1. And now if I press key K1 again ? Then channel #1 will be toggled, because the "K" inputs of the FF3...FF0 flip-flops are wired to logic "High" level. So channel #1 will be turned OFF and so will all other channels. This removes the need for a dedicated OFF button.

The keys K0...K3 are buffered through Schmitt triggers for debouncing. The flip-flops are clocked whenever any key is depressed ; I used a 4098 to generate a delayed clock pulse but anything else will do. The 4-input OR gate could of course be simply a wired-OR.

Obviously the whole functionality is contained in U1, which I call a "priority encoder" although it is not really one in the strict sense. Its goal is to prevent any malfunction if you accidentally (because you have big fingers) or on purpose (just to see what happens) press more than one key. In any case, at most one output of U1 will be high (the one with the highest "weight"). Unfortunately, such a circuit is not available as such in CMOS, so I have drawn it just to show its functionality, but there are many ways to achieve the same thing.

I love using "DigiTast" keys because they are reliable and have a nice touch. Moreover, some of them come with a built-in LED, which is certainly useful here. I have drawn the LEDs directly wired to the flip-flops outputs, as if a CMOS circuit could actually drive directly a LED -- in real life some trannies would probably be needed.

Now what does this circuit bring new? Fairly little I think.

Well, it has no glitches when changing channel (but I never saw any glitch on a rotary switch, except if it was very dirty). The thing may be cascaded to any number of signals (have a look at U1's "internal structure"). And everything may be "voltage controlled", I mean it can be driven through a µC, that is, it an be made programmable. OTOH, the keys use a fair amount of front panel space as compared to a conventional rotary switch (see front panel idea on picture). And for a 4-channel switch you need four wires between the keys and the main board, plus four from the board to the LEDS, plus the wiring for the jack sockets, plus the power supplies... Much of a hassle.

I think this could be useful for selecting, say, modulation sources. But I would certainly not use this beyond 8 channels. Had I a breadboard at hand, I would have tested it, but no.

If any one of you can see a reasonable use for this thing...

-- François

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BananaPlug · Posted: Mon Dec 08, 2008 7:14 am Post subject:

I think you could simplify the circuit with a CD4514B, some diodes, and some kind of monostable (one-shot) circuit. What I'm thinking is have each switch connected by diodes to the address lines of the latch and also to the trigger of the one shot. The one shot duration should be just long enough for any switch bouncing settle and it's output should strobe the latch. The outputs of the decoder can drive the 4066 (up to 16 of them).

One shot from resistor, capacitor, diode and inverter: