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  Integrated Circuit Review LM 555         Preparation of equipment       Footer

Integrated Circuit Review LM 555 :

The first part of this practice will allow you to finish editing the Digilab. For this, you will install a speaker inside the cabinet. This speaker will be connected to a jack mounted on the metal panel.

You will then perform a series of experiments, highlighting the versatility of a timer integrated circuit. Due to its electrical characteristics, the latter lends itself to a wide variety of applications.


Among the materials in your possession, take the following items :

The speaker of 12 Ω.

The two speaker mounting springs, (depending on your mounting).

Four self-tapping screws of 2.2 x 9.5 mm.

Four washers Ø 6 x 3,2 mm.

A miniature female jack.

A miniature male jack plug.

A twist of soft red and black wire of section 0,25 mm²

You can now proceed to the editing phase, following the instructions below :

a) First, remove from the Digilab all the links and components related to the last experiment.

Then unplug the Digilab from the power outlet and unscrew the four self-tapping screws fixing the metal front and place it next to the Digilab without disconnecting the various connections.

b) Place the female jack into the hole in the front panel marked SPK. You will insert the washer between the nut and the front as shown in Figure 1-a. Observe the position of the jack as shown in Figure 1-b.


c) Cut two pieces of flexible wire of 0.25 mm² section, one red and the other black, about 30 cm long.

Weld one end of the black wire to tab 1 of the jack and one end of the red wire to tab 2, as shown in Figure 2.


d) Twist the two red and black wires together and weld the two free ends to the speaker ends as shown in Figure 2.

e) Referring to Figure 3, place the speaker in the slot in the Digilab box (depending on the model). Lock the speaker using two springs and four 2.2 x 9.5 mm self-tapping screws, intersecting four Ø 6 x 3.2 mm washers.


f) Place the metal panel on the cabinet without attaching it, then plug the plug into the AC outlet.

You will now prepare a cord with a male jack plug.

This cord will allow you, at first, to check the installation of the speaker, then later connect the speaker to an electronic circuit.

The male jack plug consists of two parts, as shown in Figure 4-a.


To mount the jack, proceed as follows :

g) Unscrew the insulating sleeve from the male plug and insert it into the jack on the Digilab panel as shown in Figure 4-b. This will facilitate the welding work on the male jack plug.

h) Fold out the end of the ground tab 1 of the plug, as shown in Figure 4-b.

Etch center contact 2 by depositing enough solder so tin flows into contact hole 2.

i) Take the two pieces of red and black thread about 20 cm long. Strip the ends about 5 mm ; Twist the strands of the conductor and tin the ends.

j) Reduce to 2 mm one of the tinned ends of the red conductor and solder it to the central contact 2 of the jack plug. Ensure that the red insulating sleeve is in contact with the weld spot (Figure 4-b).

k) Tin tab 1 of the jack plug and solder one end of the black flexible wire (Figure 4-b).

l) Straighten tab 1 as shown in Figure 4-c and screw the sleeve back onto the jack plug.

m) Twist the red and black conductors of the formed cord together and after reducing their free end to 2 mm, weld on each one a small piece of tinned bare wire approximately 1 cm long.

In this way, both ends of the cord can be introduced into the holes of the matrix.

When you have finished the cord, it must be as shown in Figure 5.



Prepare the controller for the resistance measurement on the Ω x 10 gauge and with the plug inserted in the corresponding socket put the test tips in contact with the free ends of the cord.

You must find a value between 10 and 14 Ω.

If you find a null value, there is a short circuit, presumably at the jack.

You will need to check the welds at the latter.

If you find an infinite value, check that the welds are well done or that the speaker is not cut.

When this check is complete, you can permanently attach the metal panel to the Digilab with the four self-tapping screws.


This integrated circuit has been specifically designed to function as a timer. He is preaching to a wide variety of uses.

It can, in particular, operate in monostable or astable, and deliver pulses ranging from a few µs to several minutes.

This circuit can be used in alarms, turn signals, light sets for illuminated signs and in general in many circuits that provide timing functions.

Its supply voltage can vary from + 4.5 volts to + 16 volts. Therefore, it can be used in circuits based on TTL components (5 volts supply voltage), or based on CMOS components.

Its output can provide a maximum current of 200 mA, which allows it to control loads requiring a large current.

The duration of the output pulse from the LM 555 is virtually independent of the supply voltage.

If this voltage varies between 5 volts and 15 volts, the variation of the pulse duration does not exceed 1%. It will therefore not be necessary to supply this circuit with a stabilized or regulated power supply.

Its operation is also very little dependent on the temperature, at least in the range of ambient temperatures where most electronic assemblies operate.

This circuit also exists in a dual version : it is the NE 556.

It should be noted that this circuit is not really a digital component but rather a hybrid component like the monostable or astable circuits previously seen in theory 6.

During the following experiments, you will see some applications of this circuit and in particular a device allowing the measurement of the frequencies.


We will return to the operation of this circuit already seen in theory 6. The Figure 6 shows its block diagram.


This circuit is composed of two comparators (one is the threshold comparator, the other the trigger comparator) and a flip-flop R.S.

The output Q_barre.gif of the rocker controls a transistor and a buffer ; the latter inverts the output signal and can supply a current of 200 mA.

The R.S. flip-flop is controlled by the outputs of the two comparators.

      That of the trigger comparator is connected to the SET input of the flip-flop.

When the voltage on the tripping input becomes less than 1 / 3 Vcc, the comparator output goes to H, so the SET input is activated and the Q_barre.gif output of the flip-flop goes to the L level.

The discharge transistor is then blocked (equivalent to an open switch).

      For the threshold comparator, the operation is similar.

When the voltage on the threshold input (non-inverting input of the comparator) exceeds 2 / 3 Vcc, the comparator output goes to level H. In this case, the output Q_barre.gif of the flip-flop goes to level H and the transistor discharge is saturated (closed switch), while the output of the buffer goes to level L.

There is a control voltage input for varying the two reference thresholds, on the one hand on the inverting input of the threshold comparator and on the other hand on the non-inverting input of the trigger comparator.

The RESET input of the integrated circuit is active at a level L and makes it possible to force the output of the LM 555 to the level L.

The Figure 7 shows the pin assignment of the LM 555.



Take from the stock of equipment in your possession the following components :

  • 1 integrated circuit LM 555

  • 1 integrated circuit MM 74C193

  • 1 integrated circuit MM 74C04

  • 1 integrated circuit MM 74C74

  • 1 integrated circuit MM 74C08

  • 1 integrated circuit CD 4040

  • 2 diodes 1N 4148

  • 1 resistor of 1 MW - 1 / 4 W - tolerance 5 %

  • 1 resistor of 120 KW - 1 / 4 W - tolerance 5 %

  • 1 resistor of 47 KW - 1 / 4 W - tolerance 5 %

  • 1 resistor of 12 KW - 1 / 4 W - tolerance 5 %

  • 1 resistor of 10 KW - 1 / 4 W - tolerance 5 %

  • 2 resistors of 6,8 KW - 1 / 4 W - tolerance 5%

  • 2 resistors of 1 KW - 1 / 4 W - tolerance 5 %

  • 1 resistor of 4,7 KW - 1 / 4 W - tolerance 5 %

  • 1 resistor of 68 KW - 1 / 4 W - tolerance 5 %

  • 1 trimmer potentiometer of 10 KW.

  • 2 tantalum electrolytic capacitors of 10 µF - 12 V

  • 1 tantalum electrolytic capacitor of 1 µF - 12 V

  • 1 capacitor of 0,1 µF

  • 2 capacitors of 0,01 µF

  • 1 capacitor of 330 pF

  • 1 twist of isolated rigid wire (red, black, green).

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