The two popular multivibrator modes of operation that’s associated with this IC are the astable mode, and the monostable mode. Both of these are useful configurations and have plenty of different applications.
Using the IC 555 in Monostable Mode
For the present adjustable IC 555 timer circuit design we incorporate the second mode of operation, which is the monostable mode.
In this mode of operation the IC is configured to receive a trigger externally, so that it’s output changes state, meaning if with reference to the ground if the output of the IC is zero, then it would become positive as soon as the trigger (momentary) is received at its input terminal.
This change in its output is sustained for a certain period if time, depending upon the external time determining components. Normally the time determining components are in the form of a resistor and a capacitor which together determine or fix the time period for which the IC output would hold its “high” position.
By changing either the value of the capacitor or the resistor, the timing can be altered as desired. The above time fixing components are termed as the RC component.
Note: Please connect the buzzer or the load between pin#3 and ground, and not between pin#3 and positive as incorrectly shown in the above diagram.
How the Circuit Functions
The 555 IC timer circuit above shows a very straightforward design where the IC 555 forms the central controlling part of the circuit. As discussed in the above section, the IC is in its standard monostable mode.
Pin #2 receives the external timing trigger from a push-to-ON switch. Once this switch is pushed, the circuit pulls its output to a positive potential and holds it until the predetermined time delay lapses.
The entire circuit can be built over a small piece of general PCB and housed inside a neat looking plastic enclosure along with the battery.
The output may be ideally connected to a buzzer for receiving the warning alarm after the set time lapses.
R1, R4 = 4K7,
R2 = 10K,
R3 = 1M pot,
C1 = 0.47uF,
C2 = 1000uF/25V,
C3 = 0.01uF,
IC1 = 555,
Bz1 = Piezo Buzzer,
Push Button = push to ON switchA circuit design requested by Mr.Bourgeoisie: