GCSE Electronics: Component 2 Timing Circuits PDF

GCSE Electronics: Component 2 Unit 2: Timing circuits (Page 1) Capacitors 2. Discharging capacitor An improved buffered time delay circuit There are two main types of capacitors –...

GCSE Electronics: Component 2 Unit 2: Timing circuits (Page 1) Capacitors 2. Discharging capacitor An improved buffered time delay circuit There are two main types of capacitors – polarised and non- The discharging process can also be slowed down by 6V polarised. Polarised capacitors include electrolytic capacitors. discharging the capacitor through a large resistance. 1k + 1k 2200μF Polarised Non-Polarised Capacitor Capacitor 0V Units of capacitance Monostable Circuits The charge storage capability of a capacitor is measured in Practical timer circuits units called farads. The farad (F) is a very large unit and is not In this application of timer circuits, we are trying to produce a normally used in electronics. Capacitor values are usually given The simple RC timer circuit is satisfactory for demonstrating single pulse of fixed duration when the circuit is provided with a in micro-farads (µF). the idea of a timer circuit, but it is of little use for practical trigger pulse, as illustrated in the following diagram. circuits since it has three main limitations: i) The output changes gradually as the capacitor gradually discharges from 9V to 0V, resulting in a poorly defined timing period. Using capacitors as timing elements ii) The circuit can only supply a very small current which is 1. Charging capacitor barely sufficient to drive a LED. When a capacitor is charged directly from a voltage supply, iii) The timing circuit has to supply the current to drive the load it very quickly becomes fully charged. We can slow down the which affects the predictability of the timing. Monostable timer charging process by including a series resistor in the circuit. The signal produced by the resistor-capacitor network has to be The 555 timer IC is a very popular timing IC that can be VR processed to overcome the limitations stated above. configured as a monostable timer. A monostable has only one stable output state. Normally, it is in this stable state D.C. Supply VS VC Timing (output 0V) but can be triggered into the other state (output approximately equal to supply voltage) where it stays for a predetermined time. This time is determined by two external The capacitor will always charge up in a predictable way in a A processing sub-system is required between the timing unit components – a resistor and a capacitor. fixed length of time until it approximately reaches the power and the load. It must draw very little current from the timing unit supply voltage VS. The way the capacitor charges up is shown and supply sufficient current to drive a load. Such a sub-system in the graph below. The time taken is dependent on the value of is often referred to as a buffer. the capacitor and resistor used. A buffer is in fact any sub-system connected between two other sub-systems in order to strengthen a signal. 6V + 1k 2200μF 0V  GCSE Electronics: Component 2 Unit 1: Timing circuits (Page 2) When switch S is pressed momentarily, the monostable is Interpreting an oscilloscope trace The mark and space timings and therefore the frequency of triggered by the falling edge of the trigger pulse produced. The the output is determined by three external components – a output of the 555 timer, (pin 3), goes high and remains high for You will need to be able to calculate the period and amplitude capacitor and two resistors. a time given by the formula: of the output of an astable (or pulse generator) from an oscilloscope trace. The theoretical value for the mark, space and frequency are given by the following equations: where T is in seconds, if R is in ohms, and C is in farads. : : In practice, the capacitor value will usually be in µF and the : resistor values in either kΩ or MΩ which allows us to use one of the following rules: : If R is in kΩ, and C is in µF, then T will be in ms (milliseconds). If R is in MΩ, and C is in µF, then T will be in seconds. We can use similar rules as for the monostable: Varying the time delay If R1 & R2 are in kΩ, and C is in µF, then T1 and T2 will be in ms The squares on an oscilloscope screen are 1cm x 1cm. (milliseconds). The actual and theoretical times are usually within one or two The amplitude is the vertical distance from the lowest to seconds of each other because of the tolerances of individual If R1 & R2 are in MΩ, and C is in µF, then T1 and T2 will be in highest point, which in this case is 2cm. The voltage gain is seconds. components; however, this is usually acceptable. For some set to 5V/cm making the amplitude of this astable output 2 x 5 applications, we need very accurate timings, and for other =10V. If the period is in seconds, the frequency will be in Hertz (Hz). applications, we need adjustable timings. The frequency is found by calculating the sum of mark and If the period is in ms (milliseconds), the frequency will be in The 1MΩ fixed resistor could be replaced with a 1MΩ variable space, i.e. the time for one complete cycle. kilohertz (kHz). resistor in series with a 1kΩ fixed resistor. The fixed resistor is required to limit the current flowing into pin 7 when the variable In this example the mark is 2cm, which is 2 x 2ms = 4ms, and resistor is set to zero. the space is 1cm, which is 1 x 2ms = 2ms, giving the complete cycle time to be 6ms. Mark/space ratio Astable circuits Timing resistors and capacitors in the circuit control when the The frequency can then be calculated from: An astable has no stable output state. The output will time output is high (the mark time) and when the time output is continually switch between 0V (‘low’) and the supply voltage low (the space time). (‘high’), producing a ‘square’ wave output. Astable circuit based on a 555 timer The circuit diagram for a 555 astable is shown below. The astable is sometimes called a pulse generator. The time We can also define a mark/space ratio of an astable as being when the output is on is referred to as the ‘mark’, and the off the on time (mark) or off time (space). time is usually referred to as the ‘space’. The frequency of the output pulse can be calculated using the formula: