Logic IC yields simple, wide-range timer
Harry Bissell Jr, Robotron Corp, Southfield, MI -- EDN, August 1, 1997
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You can use a 4060B binary counter/oscillator to make a simple, accurate, long-delay timer (Figure 1). The circuit offers longer times (milliseconds to hours) than you can achieve with common IC timers. A positive logic level at the input holds the counters in the reset state and the output in the low state. When the input goes low, the output goes high, forward-biasing D1 and "stalling" the oscillator. The output then latches in its present state, as the timing diagram shows. The oscillator stops halfway through the count period (when the output switches from low to high), so the turn-off time is a function of the clock period times Q(n1). Note that the 4060B does not provide a Q11 output. C1, R1, and R2 form a classic two-inverter oscillator. R1 should have a minimum value of 5 kilohms, with R2 two to 10 times greater than R1. Manufacturers' data sheets contain recommendations on minimum and maximum component values. JP1 is a jumper like those used on computer boards. You can use other methods, but don't allow two output terminals to short together. The state of the counter is not defined at power-up, so the output is usually on. The circuit counts out of this state, or you may use the broken-line optional circuit to prevent an output from occurring until after a valid input appears. As another option, you can capacitively couple the input to form a retriggerable one-shot multivibrator. (DI #2068) |
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