|
Design Ideas |
The circuit in 
Figure
1 gives an application for an earlier Design Idea (Reference 1). In this design, you can adjust as many
as eight time intervals, and repeated operation is optional. You can use the
timer to control the external load either serially or sequentially, depending
on the application. The basic timer uses a 4060 (IC4) with a fixed
timing capacitor. The analog switches connect the timing resistors to the
circuit. A five-stage Johnson counter, IC1, controls the analog
switches.
At power-on, R1/C1 resets IC1 and IC4. The O0 output of IC1 switches high, thereby closing the first analog switch and starting the first time interval. When Q14 of IC4 goes high, the counter advances, making O1 high and O0 low. The second analog switch then closes, and a new time interval starts. When the number of timing cycles selected by switch S1 are complete, the next output of the counter drives the f1 pin of IC4 low, thereby, stopping the timer. Note that, although the serial output comes from Q14 of IC4, Q13 advances the counter because the counter can advance either on the rising or the falling edge of the clock, but not on both.
Here, the counter is configured to count on falling edges because the internal flip-flops of the 4060 trigger on the falling edges of the output of preceding flip-flops. To select repeated-operation mode, close switch S2. The counter IC receives a reset pulse from the 4047 monostable multivibrator (IC6) for starting the next sequence. The signal that disables the oscillator of the timer triggers IC6. In the nonrepetitive mode, you must select an odd number of time intervals; in repetitive mode, select an even number. At the completion of any time interval, the one-shot IC5 and the piezoelectric buzzer generate a short audible beep.
With the values shown in Figure 1, you can adjust each timing
cycle to approximately 3 to 30 minutes. You can obtain much longer intervals
(hours) by cascading ICs, such as a 4020 or 4040, with the timer. If you need to
control only a single load, you can connect the load-control circuitry of
Figure
2a to the serial output. However, for controlling multiple
processes, you need to control different loads sequentially. In that case, you
should connect relay drivers to each of the outputs (O0 to O7)
of IC1(Figure 2b).
For the drivers connected to O0 to O7, Q1 in
Figure 1 can turn the base
drive off. You need this base-drive control because, when the required number
of cycles in the nonrepetitive mode are complete, the IC1 output
corresponding to the next cycle remains high, and the corresponding relay
driver would remain on indefinitely until the circuit is reset. (DI #1917)
1. Chendvankar, Sanjay R, "Digital timer has independent on/off periods," EDN, March 28, 1996, pg 112.