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Digital electronics 6

The internal make-up of the timer can be found here.

The NE 555 is a so-called timer IC. If you've ever had any closer dealings with pulse-width modulation or you know why a function generator is needed, then you may be familiar with the workings of this component. A timer generates (by itself) voltage changes, whereby the voltage height is always the same, but the retension period at 0- or at 5 V is very variable.

Voltage changes are often the pulse-rate of the digital electronics. Pulse-rates are very important, e.g., for the buyers of computers, because this indicates a certain processing speed, and of course, it influences the price as well. Of course, our timer, the NE 555, is very far from being suitable for use in the computer, indeed, the basic function is similar. An advantage for the motor vehicle field is, that it can be operated, not only with a 5 V power supply, which is customary for these components, but can handle up to 15 V.

The circuit is relatively simple. Apart from the plus and earth connections, there are, in addition to the in- and output lines, also one resistance each and a condenser. Using these two, one can externally determine the voltage-impulses or the interruptions. To keep it simple, we've kept the condenser constant and only changed the respective resistance. The effect can be seen in the above diagram.

Here the permanently changing output voltage is plotted. The X-axis shows the timing in stages of one millisecond each. The so-called retension period is calculated using the formula:

t = 1,1 · R · C

Of course, the resistance must be shown in whole Ohms, in this case, 1000 Ohms. Thus, shown here for 'C' are 0,000001 F(arad). At 1 kOhm, the renension period is then 1,1 mlliseconds. This increases in the above pictures up to 20 kOhms and 22 milliseconds. Of course, you can extend this almost at will, e.g., with a resistance of 1 Mohm up to more than a second. This then, is even slower than an indicator flasher.

Under normal circumstances, we don't refer to the videos from youtube in the text. In this case, we've made an exception. Should the above video have been deleted, send us an E-Mail or click on the respective field at the bottom of the page. Although the clip is not in English, it does show nicely, how the RPM of a fan can be controlled by using an NE 555 circuit.

Now of course, someone could get the idea, of controlling this fan directly with a rotary potentiometer, without the circuitry, indeed, this would make a considerable difference from the aspect of energy use. Depending on the maximum current drawn by the fan, one could notice it by the temperature of the potentiometer. The electric current is simply converted into heat, giving a pretty low efficiency in the motor car where the fuel supplies the energy input. In this case, the circuit shown in the video is far more future-orientated. 10/13

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