schematic knowledge

A lot of guys requested a simple timer circuit so this is the circuit.This circuit runs with main IC NE555. you can change the frequencies of the circuit by changing the values of R1, R2,C .This circuit runs with 4.5V. Note : # Dont supply more than 4.5V # Build this on a PCB

A monostable multivibrator (MMV) often called a one-shot multivibrator, is a pulse generator circuit in which the duration of the pulse is determined by the R-C network,connected externally to the 555 timer. In such a vibrator, one state of output is stable while the other is quasi-stable (unstable). For auto-triggering of output from quasi-stable state to stable state energy is stored by an externally connected capaci­tor C to a reference level. The time taken in storage determines the pulse width. The transition of output from stable state to quasi-stable state is accom­plished by external triggering. The schematic of a 555 timer in monostable mode of operation is shown in figure.

Pin 1 is grounded. Trigger input is applied to pin 2. In quiescent condition of output this input is kept at + VCC. To obtain transition of output from stable state to quasi-stable state, a negative-going pulse of narrow width (a width smaller than expected pulse width of output waveform) and amplitude of greater than + 2/3 VCC is applied to pin 2. Output is taken from pin 3. Pin 4 is usually connected to + VCC to avoid accidental reset. Pin 5 is grounded through a 0.01 u F capacitor to avoid noise problem. Pin 6 (threshold) is shorted to pin 7. A resistor RA is connected between pins 6 and 8. At pins 7 a discharge capacitor is connected while pin 8 is connected to supply VCC.

555 IC Monostable Multivibrator Operation.
555 monostable-multivibrator-operation

For explain­ing the operation of timer 555 as a monostable multivibrator, necessary in­ternal circuitry with external connections are shown in figure.
The operation of the circuit is ex­plained below:

Initially, when the output at pin 3 is low i.e. the circuit is in a stable state, the transistor is on and capacitor- C is shorted to ground. When a negative pulse is applied to pin 2, the trigger input falls below +1/3 VCC, the output of comparator goes high which resets the flip-flop and consequently the transistor turns off and the output at pin 3 goes high. This is the transition of the output from stable to quasi-stable state, as shown in figure. As the discharge transistor is cut­off, the capacitor C begins charging toward +VCC through resistance RA with a time constant equal to RAC. When the increasing capacitor voltage becomes slightly greater than +2/3 VCC, the output of comparator 1 goes high, which sets the flip-flop. The transistor goes to saturation, thereby discharging the capacitor C and the output of the timer goes low, as illustrated in figure.

Thus the output returns back to stable state from quasi-stable state.

The output of the Monostable Multivibrator remains low until a trigger pulse is again applied. Then the cycle repeats. Trigger input, output voltage and capacitor voltage waveforms are shown in figure.
Monostable Multivibrator Design Using 555 timer IC

The capacitor C has to charge through resistance RA. The larger the time constant RAC, the longer it takes for the capacitor voltage to reach +2/3VCC.

In other words, the RC time constant controls the width of the output pulse. The time during which the timer output remains high is given as

tp = 1.0986 RAC
where RA is in ohms and C is in farads. The above relation is derived as below. Voltage across the capacitor at any instant during charging period is given as

vc = VCC (1- e-t/RAC)

Substituting vc = 2/3 VCC in above equation we get the time taken by the capacitor to charge from 0 to +2/3VCC.

So +2/3VCC. = VCC. (1 – e-t/RAC) or t – RAC loge 3 = 1.0986 RAC

So pulse width, tP = 1.0986 RAC s 1.1 RAC

The pulse width of the circuit may range from micro-seconds to many seconds. This circuit is widely used in industry for many different timing applications.

This egg timer, which is both simple and functional, shows once again that it is not essential to use a microcontroller for everything these days. The circuit consists of only two ICs from the standard 4000 logic family, a multi-position rotary switch and a few individual components. The combination of a 4040 oscillator/counter and a 4017 decimal counter is certainly not new, but it is an ideal combination for timers that are required to generate long intervals that can be programmed in steps. The circuit can be directly powered from a 9-V battery, without using a voltage regulator. The signalling device is a 12-V buzzer, which generally works quite well even at a much lower voltage.

We won’t explain the operation of the two ICs here; if you would like to know more about this, we recommend consulting the device data sheets. The RC configuration has been selected for the oscillator circuit of the 4060, since the frequencies of standard crystals and resonators would be too high (even 32.768 Hz is much too high), making it impossible to achieve the desired times. With an RC oscillator, it’s also easier to modify the times to suit our purposes. For instance, if the oscillator frequency is reduced by a factor of two, we obtain a range of 1 to 16 minutes in steps of 1 minute. The range is split into two by taking advantage of the fact that the 4017 has an AND gate at its input (with an inverted input).

The two ranges overlap by two steps. The oscillator has been dimensioned such that the 23 divider output (pin 14) has a period of 30 seconds, so IC2 receives a clock pulse every 30 seconds. This means that the oscillator frequency must be set to 8.5333 Hz. The first output of IC2 is active after a reset, so it cannot be used. If S1 is in position I, pin 14 of IC2 is connected to the positive supply line. This input is used as an enable input. Directly after the first pulse from the 4060, the second output of IC2 goes high (which means after exactly half a minute). The sub-sequent outputs become active in turn at intervals of one clock pulse, and thus generate the states for 1 to 4.5 minutes.

In the second range (II) of S1, the ‘enable’ pin of IC2 is connected to the 212 divider output of the 4060 (pin 1). This output goes high 4 minutes after the reset (which is why it is labelled ‘240 s’, instead of the period time of 480 s). Since the 4060 is an asynchronous counter, this output goes high a short time after the 23 output goes low. This delay provides the proper condition for an extra clock pulse for the 4017. The outputs of the 4017 will thus count upwards once. This means that the second output will become active after 4 minutes, with the rest of the outputs becoming active after 4.5 to 8 minutes. The desired timing interval is selected using switch S2.

The output of S2 is connected directly to emitter follower T1, which energizes the buzzer when the level on the wiper of the switch is high. At the same time, the counter of IC1 is disabled via diode D1 by forcing the oscillator input high. The buzzer thus remains active until the circuit is switched off. The first counter output of the 4060 is connected to an LED (D2), which indicates that the circuit is active and the battery not yet exhausted. The blinking rate is approximately 0.5 Hz. The current through the LED is set to a modest 1mA, since this current represents the majority of the current drawn by the circuit.

This ranges from 0.5 to 1.5 mA, with the average current consumption being approximately 1mA while the timer is running. The buzzer used in our prototype increases the current to around 13 mA when it is energized, but this naturally depends on the actual type used. In principle, the circuit will work with any supply voltage between 3 and 16 V. However, the actual supply voltage should be taken into account in selecting the buzzer. The value of the supply voltage also has a small effect on the time interval, but in practice, the deviation proved to be less than 5 percent - which is not likely to matter too much to the eggs.