Rock around the clock…

31 October 2007 at 11:24 pm (Circuit Bending Workshop, Mechatronics, School of Art)

Today, we started the day rocking some toys’ circuit’s clocks. Electronic circuits have clocks inside in order to have a time base for the rest of the circuit to work. If you change this time base, the other components of the circuit will run faster or slow down. There are two ways to build a clock in electronics (in fact there are more ways but this two are the most common and simple ones): you can use a crystal or you can use a resistor and a capacitor. The resistor and the capacitor is a really cheap solution and we can find theme in many cheap games; the good thing is that they are easy to find and to change. To find theme we only need to use wet fingers and patience: you touch the components and you listen to see if you get some slow motion effects. Then we have to change the resistor into some other kinds of variable resistors (photo resistor, variable resistor, pressure resistor, etc.). Each kind of variable resistor can be seen as a single interface with its own characteristics, a little like instruments: a saxophone and a guitar can play the same notes but the musicians don’t play a guitar like he plays a saxophone. A little anecdote about the title of this post, the first time I understood that the speed reading a record can change its pitch was with the song Rock around the clock, I played it in a 45 rotation mode and the record was in 33, something like this… And since we are playing with clocks and pitch, I remembered this funny experience.

After the interface exploration, we started to learn something really technical: building oscillators. Building an oscillator starts with some basic logics. Using NOT doors (that is a logical function), we generate a signal that is the opposite of the entry: when the entry is low the output is high, when the input is high the output is low. So if you take the output and plug it back to the input, the signal oscillate from low to high and low and high, and so on…This generates a square shape wave.

Circuit Bending - Atelier Mecatronique - Day 2

But the problem is that the oscillation happens really fast and we don’t have a way to control its frequency. So we have to put something to slow down the signal changes. We do it simply with a capacitor and a resistor. The capacitor loads itself slowly and unloads fast; the speed that the capacitor does this is linked to its capacity and the resistance of the resistor.
Circuit Bending - Atelier Mecatronique - Day 2

When we plug this to the NOT door, we build an oscillator. If we use a variable resistor we can control oscillation frequency. That’s how we build a square signal generator. This square signal has amplitude set between 0V and the battery voltage (in our case 9V), so we can control a speaker or LEDs, fans, etc. When the signal is low the component controlled is off, when its high the component switches on.

Circuit Bending - Atelier Mecatronique - Day 2

Since the chip has 6 NOT doors, we can build a system that generates six different signals, with six capacitors and resistors. The problem is, if you add all six signals you will have a gain too high, so when you link the signals, you do it in parallel and you add a resistance to control the signal gain. The resulting sound is an addition of all signals together. Each “channel” can have its own indepedent interface.

Circuit Bending - Atelier Mecatronique - Day 2

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