INTRODUCTION

In the previous project, the Light-Controlled Tone Generator, a 555 timer integrated circuit was used in a circuit which generated tones whose pitch depended on the amount of light striking a photoresistor. This kind of circuit is called an "oscillator"; it generates a signal which keeps repeating itself indefinitely.

There is another quite different way that the 555 timer is commonly used. Rather than generating a repeating waveform, when it is asked to, it generates one single "pulse" having a desired width. The signal that starts the pulse is called a "trigger".

The interval timer circuit you will be using is shown in Figure 1. Don't be frightened; the circuit is not as complicated as it looks at first glance.

When the trigger pushbutton is pressed (applying the trigger to the 555 timer), the led lights up for a certain time interval. This interval depends on the values of the resistor R and the capacitor C. The width of the pulse (the time that the led lights up) can be varied, by adjusting these values, from a very short time (on the order of 10-6 seconds) to a very long time (hours).

In this project, you will construct an interval timer that produces a 3 second pulse using components provided to you. An equation which predicts the pulse width is derived in Appendix A so that you can check the width for the given values of R and C.

Later, in the laboratory, you will measure the values of the pulse width, the resistance of R, and the capacitance of C and compare your values with those of your colleagues. You will try to explain what causes the variation in the time intervals that is observed. This problem will be studied both analytically and using a computer method called "Monte Carlo".

Finally you will look at "design" methods in which appropriate values of R and C can be rapidly determined for a given required time interval. This will be done using a simple graph and with a computer program that selects the values automatically from lists of available resistors and capacitors. A variety of resistors and capacitors will be given to you so that you can check these design methods by constructing timers with various intervals.