This circuit is the basis of electronic timers. Left to itself it never leaves its stable state (figure 8.1). The output remains indefinitely very close to zero volts. If you connect the input briefly to a positive supply the output immediately shoots up to plus + 12 volts, stays there for a pre-determined period of time, and then returns sharply to zero volts. The length of the predetermined time interval is set only by the values of the resistor and capacitor marked R and C
where t is measured in milliseconds, R in kilohms and C in microfarads. Hence for the circuit shown t = 0.7 × 50 × 0.1 = 3.5 milliseconds. The circuit works best for times between 100 milliseconds and 100 microseconds, but, by careful design, it is possible to make versions which operate for periods up to 1 minute, and down to 100 nanoseconds.
t = 0.7 RC
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© Stephen Young 1973