TIMMER IC 555 AND ITS INTERNAL BLOCK DIAGRAM

 

Description:-

The timer IC 555 circuit consists of two comparator A1 and A2, an S-R flip-flop, and a transistor Q1 that operates as a switch. One power supply is required for operation with the supply voltage typically 5V. A resistive voltage divider, consisting of the three equal valued 5k resistors. is connected across Vcc and establishes the reference (threshold) voltages for the two comparators. These are Vth=2/3Vcc for comparator A1 and Vtl=1/3Vcc for comparator A2.

In this circuit, S-R flip-flop is a bistable circuit having complementary outputs, denoted Q and Q'. Inset state, the o/p at Q is "high" (approximately equal to Vcc) and that at Q' is "low"(approximately equal to 0v). In other Stable State, termed the reset state, the o/p at Q is "low" and that at Q' is "high". The flip-flop is set by applying a high level(Vcc) to its set input terminal, labelled as S. to reset the flip-flop, a high level is applied to the reset input terminal, labelled R.

The reset and set input terminal of a flip-flop in 555 timer IC circuit are connected to the outputs of comparator A1 and comparator A2 respectively.
The +ve i/p terminal of comparator A1 is brought out to an external terminal of the 555 packages, labelled threshold. Similarly, the -ve i/p terminal of comparator A2 is connected to an external terminal labelled trigger and the collector of transistor Q is connected to a terminal labelled discharge. finally, the Q o/p of a flip-flop is connected to the o/p terminal of the timer package labelled out. 

Music Box Circuit using a CD4017 and two 555

This Music Box Circuit attempts to emulate the operation of a music box. The maximum number of notes is 10, allowing us to make a simple melody. The circuit uses two well known integrated circuits: the 555 timers and the CD4017 decade counter.
How the Music Box Circuit works?

The first 555 is used as a clock for the 4017 decade counter. The operating frequency of the clock can be varied by changing the resistor R1 with a 50K or more potentiometer, in series with a 1K resistor.

The 1K resistor is needed to prevent a short circuit between the output of the 555 (pin 3) and pins 6 and 2 when the potentiometer has a value of 0 ohms and the 555 timers have an output voltage that is different from 0 volts.

When the operating speed of the first 555 changes, the frequency applied to the clock input of 4017 changes, and consequently the cadence of the music box tones. The output of 555 is applied to the clock input of CD4017. The decade counter delivers sequentially, on its 10 outputs a high voltage signal starting on pin “0” output and ending on pin “9”.

It is noted from the diagram that the outputs 3 and 6 are not used. These are to create spaces of silence in the melody. Each CD4017 output is connected to a diode which is in series with a resistor. When one output is high, the corresponding resistor is placed in series with the resistor R2 of the second 555. (“R”)

This set of resistors (“R”) and capacitor C2, causing the second 555 to oscillate at a specific frequency. Each time a CD4017 output goes high, the second 555 oscillates at a frequency established by the group of resistors “R” (resistors which are connected to the outputs of 4017), R2 and C2. Once the sequence of 10 tones is completed, the process is repeated.

To handle the speaker, a transistor (Q1) is used. The transistor goes from the cut-off region to the saturation region, according to the oscillation frequency of the second 555.

As you can see, this is a good project to experiment with and we can achieve a sequence of notes that we like. You can change the values of the resistors in series with the diodes to experiment with different sounds. You can also change the how many times and location of the “silences” leaving, the corresponding output pin, without connection and choosing a different output.

The circuit is connected to a 9 VDC voltage source. You can make a simple voltage source with an LM7809 voltage regulator or you can use a 9-volt battery.

List of components for the Music Box Circuit

    2  555 Timer IC (IC1, IC2).
    1  CD4017 Decade Counter (IC3).

    1  TIP29 NPN bipolar transistor.

    2  33K resistors (R1, R2).
    3  10K resistors (R3, R5, R9).
    3  15K resistors (R4, R7, R10).
    2  22K resistors (R6, R8).
    1  470 ohm resistor.
    1  10uF electrolytic capacitor (C1).
    1  10nF capacitor (C2).
    8  1N4148 diode (D1, D2, D3, D4, D5, D6, D7, D8).
    1  8 ohm miniature speaker (LS).