[SOLVED] Extremely simple question about how ribbon wire works on this circuit board

Hi guys,

I just started getting into the hobby, and I'm a complete and total noob. Please excuse my ignorance! I hope this is the right place to put this question.

Anyway, I was looking at someone building his own replica of the infamous alarm clock "bomb" brought to school by Ahmed Mohamed. Turns out it didn't actually require any building. But I was looking at the insides of this alarm clock, and I wondered: what is this ribbon cable doing?

I'm thinking that each individual wire inside the ribbon cable is running into a corresponding one of the pathways I have an arrow pointing to. If you press a button on the first board, a circuit is completed (depending on the button), and one(?) of the pathways on the main board gets current through one of the wires in the cable.

Am I completely off track? If not, could you actually manually run power to each pathway at the interface between ribbon cable and circuit board, one by one, and simulate pressing buttons?

Thanks for reading, and my apologies if this is a really dumb question.

The schematic will show you what the ribbon cable connects to on each circuit board.

Ribbon cable works exactly the same as individual wires, they are just fixed parallel to each other for convenience and extra strength. You could connect the two ends with ordinary insulated wire but it would be extra work and more importantly, it would cost more.

Brian.

I obviously can't make any guarantees without the schematic but you are likely correct. It looks like that ribbon cable interfaces the main circuitry to the PCB with the buttons and what not. The push buttons likely work pretty much how you describe, by completing the circuit and either allowing current to flow through a load or by connecting an IO pin on some chip to a known voltage.

My guess would be that each of those wires is connected to a different button on one side and an IO pin on the other. The "pathways" you mentioned are called traces and most of the time can be thought of as wires on the board. This feels like a messy answer, let me know if anything needs more clarity.

A big thank-you to everyone who responded, and especially KD494 for the detail! If I can bother you further (and no worries if you're done with this noob topic, heh), what exactly do you mean by "connecting an IO pin on some chip to a known voltage"? I might get the basic idea, but I'm not sure: the completed circuit connects an IO pin (are these pins visible on the picture?) to the power source, and the pin can be configured to respond in different ways (e.g., output high or low)? What role do the traces play here: a pin might be attached to a particular trace leading away from the interface with the ribbon cable, or vice versa?

My apologies, again, for the confused ignorance I display here; I'm sure the books I ordered will cover this stuff sooner or later, so no worries if you'd rather leave me to it. Thanks again!

Don't worry about 'noob' questions, that's why we try to assist.

Yes, basically, the ribbon and any copper traces are just an electrical connection between two places. The switches may be a little more complicated than just providing high and low but they do close a circuit between two of the wires.

The complication may be that some circuits like that use a matrix key arrangement, it's difficult to tell from the photograph. If you think of something like a calculator keypad, if each key had two wires to it, there would be so many wires it would be impractical to manufacture and would need many solder joints. A trick is used to reduce the number of wires, instead of each switch being individually wired, they are connected in a 'matrix' where one side of the switch goes to a common column connection and the other side to a common row connection. Then all you need is one connecting wire per row and one per column. If you think of a standard telephone keypad, there are 12 keys, 0-9 , # and *, so you would expect to need 24 wires, two per switch. By using a matrix it can be reduced down to three columns and four rows = 7 wires. The saving is even more as the matrix gets bigger. Instead of monitoring all the wires all the time, each row is sent a brief pulse of signal and all the columns are checked to see if any of them have that signal on them. If they have, it means the keyswitch where that row and column intersect has been pushed. Knowing which row had the signal at the time and which column connection responded lets the device work out which key it was.

It means you have to be a little cautious when working out how the switches and wires are used, it may be more complicated than each being wired individually.

Brian.