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# 4. 555 timer

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### 4. 555 timer

1. 1. CHAPTER 4 555 TIMER
2. 2. 555 TIMER 555 timer is an integrated circuit that very stable. Use as monostable multivibrator, astable multivibrator, analog square wave signal generator, achometer frequency meter and others. Basically, 555 timer operate in 2 mode, ◦ Monostable ◦ Astable
3. 3. FUNCTION OF EACH PIN TIMERREFER TO MONOSTABLEMULTIVIBRATOR : GROUND  PIN 1 -all the measure voltage must refer to this pin  PIN 2 : TRIGGER - It is used to set the flip-flop. - Output is LOW if voltage at pin > 2/3 Vcc - When negative trigger pulse is 1/3 Vcc, output at comparator 2 cause the timer output HIGH.  PIN 3 : OUTPUT - Output can connect at 2 output pin, pin 3 and pin 1 or pin 3 and pin 8. - When output LOW, current will flow through load that connected between pin 3 and pin 8 to output terminal, know as SINK CURRENT. - Current flow through load that connected between pin 3 and pin 1, known as SOURCE CURRENT. - Maximum current for source current and sink current is 200mA.
4. 4. CONTINUE….. PIN 4 : RESET - Reset with negative pulse - When the reset pin is not used, the pin is connected to +Vs to avoid false trigger. PIN 5 : CONTROL VOLTAGE - Normally is connected to earth through 0.01µF capacitor - If output voltage is connected to pin 5, the output waveform bandwidth can be changed. - 0.01µF capacitor can avoid from noise problem. PIN 6 : THRESHOLD VOLTAGE - Input for inverting pin at comparator 1 - When voltage at this pin ≥2/3 Vcc, output at comparator 1 is HIGH, output timer LOW. PIN 7 : DISCHARGE - This pin is connected internally to collector at transistor Q1. - When output HIGH, Q1 OFF, circuit open - When output LOW, Q1 saturated capacitor C discharge through Q1.
5. 5. 555 TIMER INTEGRAL CIRCUIT
6. 6. MONOSTABLE MULTIVIBRATOR Known as shoot multivibrator Pulse generator circuit which the period is calculated from RC network and connected to external of 555 timer Stable when the output logic LOW (logic = 0) When a pulse is trigger at pin 2 (normally negative trigger pulse), timer output will change to HIGH (+Vs) for a while and change to LOW (stable condition). The condition will continue LOW until pulse is trigger again. The timing period is triggered (started) when trigger input (555 pin 2) is less than 1/3 Vs, this makes the output high (+Vs) and the capacitor C1 starts to charge through resistor R1. Once the time period has started further trigger pulses are ignored. The threshold input (555 pin 6) monitors the voltage across C1 and when this reaches 2/3 Vs the time period over and the output becomes LOW, At the same time discharge (555 pin 7) is connected to 0V, discharging the capacitor ready for the next trigger.
7. 7. CONTINUE….CONNECTION FOR MONOSTABLE MULTIVIBRATOR
8. 8. CONTINUE…. Operation for monostable multivibrator : ¤ Assume initial output is LOW, circuit at stable condition, transistor Q1 ON, capacitor is connected to ground. ¤ When negative pulse is triggered to PIN 2 , transistor Q1 OFF (Q1 open circuit), capacitor C start charging through resistor R1 and output is HIGH. ¤ When voltage at capacitor, C reach 2/3 Vs, output will change to LOW through flip flop. At the same time, flip flop output will make the Q1 ON. Capacitor will discharge through transistor Q1. ¤ Monostable output will remain LOW until another trigger pulse is triggered to pin 2.
9. 9. ASTABLE MULTIVIBRATORCONNECTION FOR ASTABLE MULTIVIBRATOR
10. 10. CONTINUE… Known as free running multivibrator. Do not have stable condition, the condition always change. Astable do not need trigger pulse for external to change the output. The period for LOW and HIGH can be calculated based on resistor and capacitor value that connected at outside of timer. Operation for astable multivibrator : i) Assume the initial is HIGH. Transistor Q1 OFF and capacitor is charging through resistor RA and RB. ii) When capacitor voltage reach 2/3 Vee, Comparator 1 will trigger flip flop and output change from change from HIGH to LOW. Resistor RB and transistor Q1. iii) When the capacitor voltage reach 1/3 Vee, comparator output 2 will trigger flip flop so the timer output is HIGH. The cycle is repeated. Period for capacitor charging from 1/3 Vcc to 2/3 Vcc same as period for HIGH output at timer. Period for capacitor discharging from 2/3Vcc to 1/3 Vcc same as period for LOW output at timer.
11. 11. FORMULA FOR TIMER TH = 0.693 (RA+RB) C TL = 0.693 (RB) C Period, T = TH + TL = 0.693 (RA + 2RB) C Frequency, f = 1/T = 1/ (TH + TL) = 1.44 / ((RA + 2RB) C) % Duty Cycle = [TH / (TH + TL) ]x 100 = [(RA + RB) / (RA + 2RB)] x100
12. 12. 555/556 Inverting Buffer(Schmitt trigger) or NOT gate  The buffer circuits input has a very high impedance (about 1MΩ) so it requires only a few µA, but the output can sink or source up to 200mA.  This enables a high impedance signal source (such as an LDR) to switch a low impedance output transducer (such as a lamp).555 inverting buffer or NOT gate  It is an inverting buffer or NOT gate because the output logic state (low/high) is the inverse of the input state: - Input low (< 1/3 Vs) makes NOT gate symbol output high, +Vs - Input high (> 2/3 Vs) makes output low, 0V
13. 13. CONTINUE… When the input voltage is between 1/3 and 2/3 Vs the output remains in its present state. This intermediate input region is a deadspace where there is no response, a property called hysteresis, it is like backlash in a mechanical linkage. This type of circuit is called a Schmitt trigger. If high sensitivity is required the hysteresis is a problem, but in many circuits it is a helpful property. It gives the input a high immunity to noise because once the circuit output has switched high or low the input must change back by at least 1/3 Vs to make the output switch back.
14. 14. 555/556 Bistable (flip-flop) - amemory circuit  The circuit is called a bistable because it is stable in two states: output high and output low. It is also known as a flip- flop.  It has two inputs:  Trigger (555 pin 2) makes the output high. Trigger is active low, it functions when < 1/3 Vs.  Reset (555 pin 4) makes the output low. Reset is active low, it resets when < 0.7V.  The power-on reset, power-on trigger and edge-triggering circuits can all be used as described above for the monostable.