A novel method of frequency variation using 555
Electronic hobbyists are very much familiar with the functioning of timer 555 in astable mode of operation. Traditionally, if at all there was a need to keep the duty factor constant and change the frequency continuously, the method was to use a variable capacitor, rated in picofarads. But in that case, changing the frequency in tens of hertz range would become tedious. On the other hand, sacrificing duty factor, change of frequency can be done by changing the values of R1 or R2.
Both of the above-mentioned problems can be overcome simply by using a variable resistor, as shown in the circuit diagram. Surprisingly, increase of resistance results in increase of frequency, instead of decrease. When the value of variable resistor is zero, frequency is given by
Now, when the value of variable resistor VR1 is increased, the frequency increases from the value of ‘f ’ as determined from above formula (with VR1=0). The circuit has been practically verified with two different values of timing capacitor C1 and the results obtained are tabulated in Table I.