Q Many solid-state timers function by producing a logic “high” at the output within a specified time after the timer is triggered. For my application, however, I need a timer that “goes high” as soon as it is triggered and remains high for about 60 seconds before dropping back to zero. Can you steer me in the right direction?
A How about trying the one-shot multivibrator shown in Figure 1? This one uses a garden-variety 555 timer chip and a couple of components. The trigger pulse causes the output (Q) to go positive and capacitor C to charge through resistor R. When the voltage across capacitor C reaches 2/3 of Vcc, the capacitor discharges to ground and the output returns to zero. You can calculate the values of R and C with the equation T = 1.1(RC) where T is the duration of the output pulse in seconds, R is resistance in ohms and C is capacitance in farads. For a 60-second pulse, you’ll need a 56-kΩ resistor and a 1000 μF capacitor. This works out to be about 61.6 seconds. Of course, you could use a potentiometer (a 100-kΩ pot, for instance) in place of R to tweak the pulse length and compensate for the tolerance range of the capacitor.
Figure 1—A simple one-shot multivibrator built around a common 555 timer IC.
From QST October 2000
