555 Timer IC and Its Applications .pdf

# One alternative solution is to use a small value

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One alternative solution is to use a small value timing capacitor and a much larger value resistor up to about 20MΩ’s to produce the require time delay. Also by using one smaller value timing capacitor and different resistor values connected to it through a multi-position rotary switch, we can produce a Monostable 555 timer oscillator circuit that can produce different pulse widths at each switch rotation such as the switchable Monostable 555 timer circuit shown below.
555 Timer IC and Its Applications EEE311-NSU Rummana Rahman 11 A Switchable 555 Timer (example for self study. Didn’t cover in lecture) We can manually calculate the values of R and C for the individual components required as we did in the example above. However, the choice of components needed to obtain the desired time delay requires us to calculate with either kilohm’s, Megaohm’s, microfarad’s or picafarad’s and it is very easy to end up with a time delay that is out by a factor of ten or even a hundred. We can make our life a little easier by using a type of chart called a “Nomograph” that will help us to find the monostable multivibrators expected frequency output for different combinations or values of both the R and C . For example,
555 Timer IC and Its Applications EEE311-NSU Rummana Rahman 12 Monostable Nomograph So by selecting suitable values of C and R in the ranges of 0.001uF to 100uF and 1kΩ to 10MΩ’s respectively, we can read the expected output frequency directly from the nomograph graph thereby eliminating any error in the calculations. In practice the value of the timing resistor for a monostable 555 timer should not be less than 1kΩ or greater than 20MΩ
555 Timer IC and Its Applications EEE311-NSU Rummana Rahman 13 Voltage-controlled oscillator (VCO) using the timer 555 555-timer-voltage-controlled-oscillator The circuit is sometimes called a voltage-to-frequency converter because the output frequency can be changed by changing the input voltage. As discussed in previous blog posts, pin 5 terminal is voltage control terminal and its function is to control the threshold and trigger levels. Normally, the control voltage is ++2/3V CC because of the internal voltage divider. However, an external voltage can be applied to this terminal directly or through a pot, as illustrated in figure, and by adjusting the pot, control voltage can be varied. Voltage across the timing capacitor is depicted in figure, which varies between +V control and ½ V control . If control voltage is increased, the capacitor takes a longer to charge and discharge; the frequency, therefore, decreases. Thus the frequency can be changed by changing the control voltage. Incidentally, the control voltage may be made available through a pot, or it may be output of a transistor circuit, op- amp, or some other device.
555 Timer IC and Its Applications EEE311-NSU Rummana Rahman 14 Appendix I 555 Oscillator Applications

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