555 Timers. We are going to look at using the 555 to create two very different types of timer. A MONOSTABLE timer and an ASTABLE timer.

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1 555 Timers We are now going to concentrate on specific IC s and how they can be used to perform useful functions. In turn we shall be looking at Timers, Amplifiers, Counters, Microprocessors and finally Logic Gates. But lets start at the beginning with Timers. We do not have to think for very long to come up with many uses for a circuit that can measure the passing of time. The 555 timer IC is a widely used device for creating time delays. The diagram below shows what a 555 timer IC looks like, and as you can see it is contained in an 8 pin DIL package. You will also be able to see that the number LM 555CN is printed on top of the package. This tells us exactly what the IC is, as all 8 pin DIL packages look the same, without a number printed on them we couldn t tell one from another. We are going to look at using the 555 to create two very different types of timer. A MONOSTABLE timer and an ASTABLE timer. Monostable Timer The monostable timer is used to create time delays, for example we might want to make a circuit that will time for 3 minutes whilst we brush our teeth. To solve this problem we could use a monostable timer. The circuit diagram for this is shown below. 4.5 to 1V 10K R 2 Rled Start Switch C 7 1 The first thing to notice is that the IC is not drawn like the DIL packages we looked at on the previous page, the pin numbers are all over the place. This is often done to make the circuit diagram less confusing to look at and understand and you need to be aware that this is done in virtually all circuit diagrams using IC s. Page 1

2 Creating the Time Delay From our work on Capacitors you will remember that we can create a time delay using a capacitor and a resistor in series. However, just relying on a resistor and capacitor to create the time delay is not very accurate due to the tolerance of these components. The 555 timer circuit uses a capacitor C in series with a resistor R but the actual 555 integrated circuit contains extra components to make the time delay much more accurate. The values of the resistor and capacitor can be changed to set the length of time delay that we require by using the following formula. In this formula T stands for Time (Measured in Seconds) C stands for Capacitance (Measured in Farads) R stands for Resistance (Measured in Ohms) So, continuing our example of making a 3 minute tooth brushing timer, we need T = 180 seconds, but that leaves us with two unknowns in the formula C and R. We need to decide on a value for one of them and then work out the other. As capacitors are available in many less values than resistors, we always decide on a value of capacitor. The maximum value that should be chosen is 1000uF (micro farads) and this is suitable for very long delays (10 minutes or more). For most applications using a 470uF capacitor will suffice. So, inserting these numbers in the equation gives us the following. 180 = 1.1 x x R (note is the same as 470uF as u equals 1/1,000,000) R = 180 / (1.1 x ) R = 348,12.5 Ohms T = 1.1 x C x R But we cannot get this value of resistor, as you will remember that resistors are only available in preferred values and 348,12.5 Ohms is not a preferred value. The nearest preferred value is 330,000 Ohms or 330KOhms. The circuit diagram for the complete timer is shown below. I have chosen to use a V battery which as you remember will consist of 4 1.5V cells in series. I will leave you to satisfy yourself that the value of Rled is 220 Ohms by using the work already covered on Current Limiting Resistors Used With LED s. V 10K Start Switch 330K 470uF Page 2

3 So, what is happening in the circuit when we push the start switch? This is best explained using a diagram which is shown below. Time Delay (i.e. 180 seconds) Output High LED OFF LED ON LED OFF Output Low Start Switch Pushed When the push button start switch is pressed the LED switches on as the output has gone from low to high (off to on). The LED stays on until the time delay has finished when it switches off. So when using this circuit to time brushing our teeth we would switch it on by pushing the switch, then continue brushing our teeth until the LED turned off. If we wanted to switch on an output other than an LED we would need to use a transistor. Page 3

4 Astable Timer The astable timer is used to create a square wave output, which you will remember from the work on Waveforms. Two of the most common uses of an astable timer are to create clock pulses for use in counting circuits and to drive LED s or a loudspeaker to produce a tone, for example for an alarm. So, why is this astable output often called a clock pulse? Consider the drawing of a square wave below. Voltage Mark Space Time (secs) If we connected this square wave through a counter to a display, such as a LCD we have already looked at, we would see the numbers on the display increase by 1 each second. This is because the pulses (the name given to the square parts of the waveform) arrive once every second. What we have created is a kind of clock that displays the elapsed time in seconds, hence the name clock pulse. The circuit diagram to create an astable timer is shown below. 4.5 to 1V R1 7 R2 Output (clock pulse) C 2 1 One thing that you will notice is that there is no start switch. This is because that as soon as the circuit is connected to a battery the clock pulses begin. They will not stop until the batteries are removed. The time in between successive clock pulses is, as you know, called the frequency and is set by choosing appropriate values for R1, R2 and C according to the following formula. F = 1.44 (R1 2R2) x C In this formula F stands for Frequency (Measured in Hertz) R stands for Resistance (Measured in Ohms) C stands for Capacitance (Measured in Farads) Page 4

5 So, continuing our example of making a circuit to measure elapsed time in seconds we again have to fix a value for C as we did when working out time delays for the monostable circuit. This time a suitable value is 1uF. The frequency of the clock pulses we need is 1 Hertz, this is because frequency is the number of pulses each second, which in this case is 1. So, inserting these numbers in the equation gives us the following. 1 = 1.44 / (R1 2R2) x (note is the same as 1uF as u equals 1/1,000,000) R1 2R2 = 1.44 / R1 2R2 = 1,440,000 We now need to choose preferred values of resistors which combine to give these required values. R1 = 47,000 Ohms and 33,000 Ohms in series and R2 = 80,000 Ohms By keeping R2 very much greater than R1 we ensure that our clock pulse will be almost a perfect square wave. So the circuit diagram for the complete astable timer is shown below. V 47K 33K 7 80K Output (clock pulse) 1uF 2 1 But we will only have the LCD show exact seconds if these components are totally accurate. As you know this is certainly not the case. The resistors all have tolerance which means they are highly unlikely to be the values shown in the circuit diagram. So how do we obtain exact 1 second clock pulses? What we need to do is to replace one of the resistors (usually R2) with a variable resistor. We can then alter the resistance until the clock pulses are exactly one second apart by comparing the output of our circuit with, for example, a digital watch. Altering the values of components to achieve the desired output is called CALIBRATING the circuit. Page 5