Design Ideas: March 28, 1996
You can easily implement a long-interval digital timer using the popular 4060 oscillator-divider IC. However, in repetitive-timer applications, this IC produces equal on and off times. The circuit in Figure 1 allows you to set different on and off times; the state of the timer’s Q14 output automatically selects one of two independent RC time constants.On reset, Q14 goes low, opening analog switch S1, which acts as an inverter and closes S2, thereby selecting the R1C1 time constant. After the R1C1 time interval, Q14 goes high, S2 opens, and S3 closes. Now, R2 takes over for R1, and a new time constant of R2C1 determines the timer’s output high time.
With the component values in Figure 1, the adjustable output low and high times are 30 and 10 minutes, respectively. Because most manufacturers of the 4060 recommend a minimum value of 10 kOhms for the time-constant resistor, small variations in the resistance of the analog switches over temperature changes do not significantly affect the timing accuracy. However, you should select a resistor value of about 50 to 500 kOhms whenever possible.