Pressing the switch S1 momentarily enables the IC to start the timing sequence (pin 3 going high and LED turning ON), while pressing S2 reset button allows instant termination or resetting of the timing sequence so that the output pin 3 reverts to its original 0 V situation (LED turning OFF permanently)
#1HOUR 10 MIN TIMER SERIES#
The 10 K series resistor also ensures the minimum resistance value required for the correct working of the circuit at the minimum pot setting. Notice the series 10K resistor which is very important since it safeguards the IC from burning in case the pot is turned to its lowest value. In this design the output can set to produce time periods from 1.1 seconds to 120 seconds through different adjustments of the pot R1. The variable resistor can be in the form of a potentiometer as shown in the following figure 2. The LED illumination indicates the ON and OFF switching of the output pin 3 of the IC If R1 is replaced with a variable resistor, this output timing could be set as per user preference. The time period for which this rectangular pulse remains active is determined by the values of R1, and C1. This instantly causes a rectangular pulse to appear at pin 3 and simultaneously generates an exponential sawtooth at DISCHARGE pin 7. The actions as described in the waveform image initiates as soon as the TRIGGER pin 2 is grounded with the pressing of momentary START Switch S1. The adjoining figure shows the waveforms obtained across the indicated pinouts of the IC during the switching process. It is basically an IC 555 monostable design.
#1HOUR 10 MIN TIMER HOW TO#
The first figure below shows how to make a IC 555 timer having a fixed period output. It is possible to set delays ranging from 10 µseconds to 100 µseconds by selecting appropriate values of capacitors from 0.001 ♟ to 100 ♟ and resistors from 1 k Ω to 10 meg Ω. It shows a range of time delay curves and the linearly changing values with respect to the corresponding values of RT and C. Here tD is in milliseconds, R is in kilo Ω, and C in μfarads. resistance, and capacitance using the above the time delay formula. The following graph shows the plotting of time delay vs.
The formula for calculating this is: tD (time delay) = 1.1 (value of R x value of C) In other words the timing interval produced by IC 555 is directly proportional to the product of R and C. The timing pulse generated at the IC output is mostly in the form of a rectangular wave whose time interval is defined by the magnitudes of R and C. But if one wants to terminate the timing cycle, this can be done at any moment by applying a negative pulse or 0 V to the rest pin 4. And this is how the IC 555 completes a timing cycle.Īs per one of the characteristics the IC, once triggered it stops responding to any subsequent triggers, until the timing cycle is completed.
At this instant the output pin 3 returns back to its earlier low state yet again. This allows the capacitor CD to charge via the timing resistor RD until the voltage across CD reaches 2/3rd supply level or Vcc.Īs soon as this happens the R/S flip flop reverts to its previous state, switching ON Q6 and causing a quick discharge of CD. With Q6 switching OFF disconnects the short across CD. This 0V pulse being below the 1/3rd level of the DC supply voltage or the Vcc, forces the output of the trigger comparator to change state.ĭue to this, the R/S flip-flop also changes its output state, turning off Q6 and driving OUTPUT pin 3 high. The standard Timer action of the IC 555 is initiated by introducing a 0 V trigger pulse at pin 2. In this situation, Q6 remains saturated, which keeps the external timing capacitor CD shorted to ground, causing the OUTPUT pin 3 is to be at a low logic or 0 V level. When IC 555 is configured in the monostable timer mode, the TRIGGER pin 2 is held at the supply level potential through an external resistor RT. The output state of the flip flop can be also set by triggering the reset pin 4 of the IC.
#1HOUR 10 MIN TIMER DRIVER#
With these trigger inputs the two op amps control the R/S (reset/set) flip flop stage, which further control the ON/OFF conditions of the complementary output stage and the driver transistor Q6 The stage involving the three 5 kohm resistors work like a voltage divider stage which produces 1/3rd voltage level at the non-inverting input of the trigger comparator op amp and a 2/3 voltage division on the inverting input of the threshold comparator op amp.
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