Topic Astable Circuits. Recall that an astable circuit has two unstable states;

Transcription

1 Topic 2.2. Asable Circuis. Learning Objecives: A he end o his opic you will be able o; Recall ha an asable circui has wo unsable saes; Explain he operaion o a circui based on a Schmi inverer, and esimae he operaing requency using ; RC Draw he circui diagram or an asable using a 555 imer i.c.; Selec and use ormulae or o he ime he oupu is high: H 0.7( RA RB ) C o he ime he oupu is low: L 7 R B C o he requency: 7( RA 2RB ) C Calculae he mark space raio;

2 Module ET2 Elecronic Circuis and Componens. Asable Circui. In our previous secion we looked a he behaviour o a monosable circui, i.e. a circui wih jus one sable sae. In his secion we will be looking a he asable circui. The asable circui has no sable sae and is consanly swiching beween logic and logic Alernaive names or he asable are a pulse generaor or clock. The asable is a very useul circui in elecronics as i he circui responsible or causing lashing lighs, pulsing buzzers in alarm circuis and keeping couners running as we ound ou in module ET, even hough we didn look a how he clock was produced a ha ime. Once again we will be considering wo dieren mehods o producing an asable circui. The irs mehod involves he use o a special ype o NOT gae, called a Schmi NOT gae, or Schmi inverer. The symbol or he Schmi inverer is as ollows: A Q The Schmi NOT gae has a unique swiching characerisic which is very dieren o he sandard NOT gae. The ollowing diagrams illusrae he dierence beween o wo. V OUT V IN Swiching Characerisic or a sandard NOT Gae V OUT V IN Swiching Characerisic or a Schmi NOT Gae 2

3 Topic 2.2. Asable Circuis. Looking a he wo characerisics you should noice ha or a sandard NOT gae operaing on a 5V supply ha he swiching poin is a he midpoin o he supply volage or an increasing or decreasing inpu volage. Comparing his o he Schmi characerisic reveals a dieren siuaion alogeher. As V IN increases he volage has o increase above V beore he oupu volage changes. Once he oupu has changed however, i he inpu is hen decreased back o V, he oupu does no change back, as i would in he normal case, bu now he volage has o all o below 2V beore he oupu will go high again. We have hereore creaed some hyseresis in he NOT gae wih wo disinc swiching hresholds. We can use his o our advanage o make an asable imer wih he addiion o jus a resisor and a capacior. The circui required is shown below. This is a very simple circui, bu very reliable, requiring he minimum number o componens, and aking up very lile space on a circui board. The requency o he oupu is given by he approximaion RC

4 How does i work? Module ET2 Elecronic Circuis and Componens. Assume ha iniially here is no charge on he capacior, so he inpu o he NOT gae will be Logic 0, so he oupu is a Logic. The capacior begins o charge hrough he resisor R and so he volage a he inpu o he NOT gae sars o rise. When he volage a he inpu reaches he upper swiching hreshold, he oupu o he logic gae changes o Logic The capacior now sars o discharge hrough he resisor R, and he volage across he capacior begins o all. When he volage a he inpu reaches he lower swiching hreshold, he oupu o he logic gae changes o Logic again, The capacior sars o charge hrough R again and he whole process repeas as long as he power is swiched on. Noe : When selecing he resisor or he Schmi asable circui you should ensure ha he minimum value o resisance chosen is kω. This will limi he curren lowing o an accepably low value, and preven overheaing o he Schmi i.c. device. 4

5 Topic 2.2. Asable Circuis. The addiion o an oscilloscope o he circui, shows his happening, as shown below: The red race, shows he volage a he inpu o he Schmi NOT gae, he blue race shows he volage a he oupu o he Schmi NOT gae. Noes:. The irs cycle lass longer han subsequen pulses as he capacior has o charge up rom 0V, o he upper swiching hreshold. 2. Aer he irs cycle he capacior charges and discharges beween he upper and lower swiching hreshold o he Schmi NOT gae.. The On ime, and O ime are o he same duraion. 5

6 Module ET2 Elecronic Circuis and Componens. The Schmi NOT gae soluion is a very simple, nea and reliable soluion i a simple clock, or pulse generaor is required. However i you wan o have a dieren on and o ime hen his simple circui canno perorm his acion, and we need o consider a more complex soluion. The second mehod o making an asable imer is o use a amiliar device in he 555 imer we used or he monosable imer. This versaile device can also be conigured o run as an asable imer, by making he connecions shown below: R A R B C The circui has many similariies wih he monosable opion, and i is imporan o ensure ha you do no conuse he wo as you are expeced o be able o draw his circui in he examinaion. There are hree ormulae ha apply o his circui, all o which are provided on he Candidae Inormaion page a he ron o every examinaion paper so you do no have o remember hem. o he ime he oupu is high: H 0.7( RA RB ) C o he ime he oupu is low: L 7 R B C o he requency: 7( RA 2RB ) C From hese ormulae we can see ha he on ime is always greaer han he o ime, bu will be approximaely he same i R B >>R A. 6

7 Topic 2.2. Asable Circuis. We can conirm his by adding an oscilloscope o he oupu o he 555 asable circui o see wha he oupu looks like. Here we can see ha wih he wo resisors equal in value, he on ime is approximaely wice as long as he o ime. I R is changed o kω and R 2 changed o 00kΩ hen oupu becomes as shown below, where he on ime is virually equal o he o ime. 7

8 Module ET2 Elecronic Circuis and Componens. Noe : When selecing resisors or he 555 imer circui you should ensure ha he minimum value o resisance chosen is kω. This will limi he curren lowing o an accepably low value, and preven overheaing o he 555 i.c. device. The Mark-Space Raio. A common way o speciying he parameers o an asable circui are in erms o he Mark-Space raio. This sounds complicaed bu acually is quie sraighorward. The Mark reers o he On ime, he Space is simply he O ime. So i an asable is speciied as having a Mark-Space raio o :, hen he on ime mus be hree imes as long as he o ime. Graphically his would be shown as ollows: Mark Space ime 8

9 Topic 2.2. Asable Circuis. Suden Exercise :. The ollowing circui diagram shows a 555 imer conigured as an asable imer. (i) Calculae he duraion o he on pulse or he above circui. (ii) Calculae he duraion o he o pulse or he above circui. (iii) Hence, or oherwise calculae he requency o he oupu. 9

10 Module ET2 Elecronic Circuis and Componens. 2. (a) Use he grid below o draw an asable waveorm wih a mark-space raio o 2:. ime (b) Use he grid below o draw an asable waveorm wih a mark-space raio o 2:. ime. Complee he diagram below o show how an asable imer can be made rom a Schmi NOT gae, and choose componen values o give a requency o 2Hz. Calculaions:

11 Topic 2.2. Asable Circuis. The ollowing quesions have been aken rom recen examinaion papers covering he use o asable circuis. Some reerence is also made o monosable circuis as he wo opics are oen linked in examinaion quesions.. The ollowing diagram shows an asable buil wih a 555 imer used o clock an l.e.d. A daa shee or he 555 asable gives he ollowing inormaion (a) Calculae he values o T and T 2 or he asable. [2] (b) Explain how he circui could be modiied o provide a variable mark-space raio. [2] 2. The ollowing diagram shows a 555 imer being used as an asable.

12 Module ET2 Elecronic Circuis and Componens. A daa shee or he 555 asable gives he ollowing inormaion (a) Calculae he values o T and T 2 or he asable. [2] (b) Calculae he requency o he asable. [2] 2

13 Topic 2.2. Asable Circuis.. A Schmi inverer can be used as an asable circui. (a) Complee he circui diagram or he asable circui. (b) Draw a skech o illusrae he oupu waveorm produced by he asable. Your skech should show clearly how an asable waveorm diers rom a monosable waveorm. [2] [2]

14 Module ET2 Elecronic Circuis and Componens.. The ollowing alarm sysem ses o a buzzer when he monosable is riggered. The monosable oupu remains high or 8 seconds aer i is riggered. The asable has an equal mark-space raio and a period o 2 seconds. The oupu o he AND gae is high only when boh is inpu are high. (a) Describe wha he alarm does over he 8 second period aer he monosable is riggered. [] (b) Draw a skech o he waveorm produced a he oupu o he AND gae over he 0 second period aer he monosable is riggered. [2] 4

15 Topic 2.2. Asable Circuis. 4. The ollowing diagram shows a 555 imer being used as an asable. (a) The oupu signal has a mark:space raio o 4:. Skech wo cycles o he oupu signal. Label he mark T and he space T 2. (b) [] The space T 2 has a duraion o 0 ms. Calculae he value o resisor R B ha will produce his space when C =.μf. [2] (c) Deermine he value o resisor R A. [] 5

16 Suden Exercise : Module ET2 Elecronic Circuis and Componens. Soluions o Suden Exercises. (i) he ime he oupu is high: 0.7( R R C H H H H H 7( R A 7( ( s. 6s R H A B) B 0 0 ) C ) 0 ) (ii) he ime he oupu is low: R C L 7 B L L L L 7R B C s. 09s (iii) he requency: 7( RA 2RB ) C 7( R A 7( ( Hz 2R 0 B ) C ) ) Or H L Hz 6

17 Topic 2.2. Asable Circuis. 2. (a) ime (b) ime. When aced wih a design problem o his ype we have a dilemma o wheher o choose a capacior or resisor value o sar wih. As he minimum value o resisor we can use is kω hen his migh be a good value o sar wih, as he capacior can be o any value. 2 C RC 000 C F 500 F This is only one soluion, he ollowing combinaions are also accepable; 5kΩ and 00μF, 20kΩ and 250μF, 500kΩ and μf, 200kΩ and 25μF, Subsiue hem ino he ormula o check hem ou. Remember you may have come up wih anoher perecly accepable soluion. 7

18 Sel Evaluaion Review Module ET2 Elecronic Circuis and Componens. Learning Objecives Recall ha an asable circui has wo unsable saes; Explain he operaion o a circui based on a Schmi inverer, and esimae he operaing requency using ; RC Draw he circui diagram or an asable using a 555 imer i.c.; Selec and use ormulae or The ime he oupu is high: H 0.7( RA RB ) C The ime he oupu is low: L 7 R B C The requency: 7( RA 2RB ) C Calculae he mark space raio; My personal review o hese objecives: Targes:. 2. 8