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Anticipating timer switches before you push the button

-April 03, 2003

It happens to almost everyone that an apparatus or system should have been turned off a moment ago. The device in question could be the car heater, the air conditioner, the lights...

This Design Idea offers a solution to the challenge of turning devices on or off in the past. In Figure 1, IC2 is a 555-type timer (preferably CMOS) connected as a monostable one-shot multivibrator. The pushbutton switch, S1, triggers IC2. You can replace S1 with a transistor or an optocoupler, for example. You can connect VOUT to a relay or a transistor, if needed. You might need to adjust the values of R4 and R5, depending on the output load and the characteristics of S1. The interval during which VOUT remains high is T=1.1RC2. In Figure 1, you replace the resistor, R, that normally connects to C2 with the circuit inside the dashed line. This circuit comprises a 741 op amp, IC1, and three resistors: R1, R2, and R3. You could replace the war-horse 741 with a TL081 if your design needs longer time delays.

Taking into account the usual op-amp assumptions—equal voltage on both inputs and zero input current—you derive the following expressions: VO=V(R2+R1)/R1, and VO=V–R3IC, where VO is the op amp's output voltage, V is the voltage at the noninverting input, and IC is the current through R3.IC is also the current that charges C2. Combining the cited expressions, you can compute the value of resistor R that the op-amp circuit replaces: R=V/IC=–R3R1/R2. The timing interval of this timer is thus T=–1.1C2R3R1/R2. Using appropriate values, you can obtain long time delays that you can't attain with the basic 555 circuit. But the real innovation inherent in this circuit is that its output turns on at a defined time, T, before you press S1. To adjust interval T, use a potentiometer for R1. Because the wiper of the potentiometer connects to the power supply, adjusting R1 contributes minimal EMI and other insidious effects to the op amp's input. C1 is a power-supply bypass capacitor, and C3 stabilizes the 555's control voltage. With the values shown in Figure 1, the interval T is approximately 18 minutes.

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