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Capacitors and Timers

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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 the capacitor by connecting it to the battery. ...read more.


The circuit on the previous page lets you charge up the capacitor by connecting it to the battery through the push switch. We could discharge the capacitor directly through the LED - but it would be too quick. Instead, slow down the discharge by using a 1k resistor and a transistor. That small current turns the transistor on so that the LED shines. If you try the circuit in Crocodile Clips you can plot the charge and discharge on the oscilloscope. Put the oscilloscope probe at the collector lead of the transistor. Press the switch and the capacitor quickly charges up to the battery voltage. Release the switch and it discharges slowly through the resistor. The graph is shown below. The formula that can be used to work out the Time Constant for this kind of circuit is T= RxC; the units are Ohms, Farads and Seconds. Since Farads are rather large you can use MegaOhms (M), MicroFarads (uF) ...read more.


Here's the circuit: Here is the waveform. There is a drop to zero as the switch is pressed. This triggers the monostable and the output goes high - the LED turns on. The voltage on the capacitor slowly rises as shown in the graph. After a short time the monostable turns off again. The time period is given by the formula T=1.1xRxC. Here we have a 100k resistor and a 100uF capacitor so T=1.1x0.1Mx100uF=11s. The output voltage of the 555 is nearly 9v, the battery voltage, so a 470R resistor will allow a good current to flow through the LED. The 555 has 8 legs with a marker to indicate pin 1. Pin 1 is 0v, pin 2 Trigger, 3 is Output, 4 Reset, 5 Control, 6 Threshold, 7 Discharge, 8 +9v. The left-hand circuit above is switched by making the ldr briefly dark; switch over the ldr and the resistor and the circuit will switch with the ldr lit briefly. The right-hand circuit will automatically trigger when the power goes on so that the led will light after the preset time. 6a Capacitors and Timers page ...read more.

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