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Latching and Timers The circuits we have done so far react to the sensor immediately. This means that if the problem goes away the circuit turns off. For a burglar alarm and for many others we need the warning to stay on. We need a latching circuit. This will turn on in response to the sensor and then stay on until reset. We can use a capacitor for this purpose. A capacitor stores electric charge so we can let the capacitor charge up quickly and use it to turn a transistor on. Then we let the capacitor discharge slowly through the base resistor and eventually it will have lost so much charge that the transistor will turn off again. How Capacitors Work Time Constants The 555 integrated circuit forms the basis of lots of timing circuits. Let's look first at how Capacitors work. Try these two circuits: The first circuit lets you charge up...

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