Innovative Ways Of Using Timer IC 555
Timer IC 555 is versatile general-purpose linear integrated circuits and is thus the most widely used one. It is indeed a favourite with hobbyists. It is mostly used either as a monostable or an astable multivibrator. In this project activity, we shall present some modified circuits for monostable and astable operation of timer IC 555 and see for ourselve show these changes simplify design. In two of the four different circuits given here, the circuits have been so modified as to yield very simple and design friendly expressions for the monostable and astable operations. In case of modified astable circuit, the expressions for output frequency becomes f = 1/RC where (R) is charge path resistance equal to the discharge path resistance and (C) is the capacitance. In a conventional astable circuit, this expression would be f = 1/(1.38RC). In case of monostable circuit, the suggested modification changes the expression for output pulse width from T = 1.1 RC to T = RC. Another circuit given here produces a 50% duty care free running output with the least component count. Yet another circuit transforms a conventional monostable circuit into are triggerable mononshot. Retriggerable mononshot circuits are available as digital ICs. 74123 in TTL and 4098 in CMOS are examples. However these ICs offer a very limited source current Capability. A retriggerable monoshot configured around IC 555 gives you a peak current sluicing capability of 200mA. The intention behind this project activity is to enable the experimenters experience new circuits and thus expand the horizon of their thinking.
Modified Astable Multivibrator
Refer to the astable part of Fig.6.1, The charge and discharge path resistances are respectively (R1) and (R2). The small modification to this circuit is in the form of (R3) connected from pin- 5 of the IC to ground. In the absence of this resistance, the HIGH and LOW times of the output waveform would respectively be (0.69Rl.Cl) and (0.69R2.C1 ). If R1 = R2 =R and C1 = C, the expression for frequency of output waveform would be f = 1/(1 .38RC), an expression very inconvenient to handle. It can be verified mathematically that a 5.6K resistance (R3 in this case) is connected from pin- 5 to ground, the HIGH and LOW times of the output waveform will be given by relationships T(high) = 0.31R1 .C1 and T(LOW) =0.69R2.C1 . The frequency of the output waveform would hen be given by f = 1/RC provided R1 = R2 = R and C1 = C. In all those applications of 555 astable circuits, where individual HIGH and LOW times are not important, this modified circuit offers a distinct advantage. Even otherwise, the circuit offers a lot of academic interest. We leave it to the readers to find out how this additional resistane modifies the expression for frequency. Remember that we are able to change only the HIGH time and also that HIGH time in a 555 Ratable circuit is the time taken by capacitor to charge from 1/3(Vcc) to 2/3(Vcc). Now 1/3(Vcc) and 2/3(Vcc) are reference levels for lower and upper comparators inside the IC. Does this additional resistance change those levels?
Modified Monostable Ultivlbrator
Refer to the monostable part of the circuit. A similar modification can be used forties circuit. It can be proved mathematically that connecting a 62K resistance from pin-5 to ground changes the output pulse width expression from T = 1.1R5.C5 to T = R5.C5. You can connectives a resistance of 68K without any noticeable error. Again we leave it to readers to find out for themselves reason for change in expression. Remember that in a 555 mononshot, the output pulse width is the time taken bathe capacitor to charge from 0 to 2/ 3(Vcc).
Astable Circuit with 50% Duty Cycle
Refer to the relevant part of the circuit shown in Fig.6.1. It is a simple circuit that makes use of only 3 external components, 2 resistors and 1 capacitor, to construct a 50% duty cycle astable circuit. The design idea takes advantage of the fact that the output is HIGH during the charging process and LOW during the discharge process of the capacitor. In the present circuit, the capacitor discharges through the resistor (R8) and the output transistor connected to pin-3 inside the timer IC. It does hot discharge through the discharge transistor connected to pin-7 of the IC, as is the case in conventional astable circuits. For best results (R8) should be 22K or greater. The frequency of the output waveform is given by f = 1/(1.38R8.C8).
555-monoshot circuit can be transformed to a retriggerable mononshot by including a few additional external components. The circuit consists of a bipolar transistor (Q1) , whose base is connected to the trigger input. The collector is connected to pins 6 and 7 of the IC and the emitter is grounded. The HIGH time of the monoshot in this case is larger than the time interval between the trigger pulses. With the leading edge of every trigger input pulse, the transistor goes into saturation and the capacitor
Figs.23.3 and 23.4 respectively show the PCB layout and components layout. This would help those who want to construct this project in a discharges through the transistor. The capacitor starts charging again with the trailing edge of the trigger pulse inputted the output remains HIGH. The voltage across pin-6 never reaches 2/3(Vcc) as long as there are trigger pulses at pin-2 of the IC with inter-pulse duration shorter than the expected output pulse width. The output goes expel LOW after a time equal to (1.1R10.C13) after the trailing edge of the last received trigger pulse input as shown in Fig.23.2. The design gives a retriggerable mononshot operation with an output current of 200mA, which is much higher than a few my (typical) sourcing capability of digital retriggerable mononshot ICs.Switch SW- 1 can be used to connect the battery voltage to any of the four circuits, one at a time. Compacting packaged form. However, the circuit is simple enough to be constructed on a general purpose PCB.
R 1, R2, R4, R5, R9 : 10K, 1/4 Watt
R3 : 5.6K, 1/4 Watt
R6 : 62K, 1/4 Watt
R7 : 1 K, 1/4 Watt
R8 : 22K, 1 /4Watt
R10 : 1M, 1/4Watt
R11 : 100K, 1/4 Watt
R12 : 33K, 1/4 Watt
C 1, C5, C8, C13 : lµF/25v(Tantalum)
C2, C3, C4, C6, C7, C9, C10, C11, C12, C14 : 0.01µF
Semiconductors and ICs
D1, D2, D3 (Diodes) : 1N4001
Q1 (Transistor) : 2N2222
IC-1 to lC-4 : Timer IC 555
Battery, B-1 : 9V Battery
SW-1 : SP4T Rotary switch
Multistrand Wires, Solder Metal etc.
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