Adjustable ON OFF Timer(using 555 astable mode)


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In this circuit a timer with cyclic on off operations is designed. This circuit uses very basic components like 555 timer and 4017 counter.
These on off intervals can be adjusted by varying the 555 timer output and number of counter outputs. Let us discuss in detail about this circuit.

Circuit Diagram

  • R1 and R2 – 47 KΩ
  • R3 – 15K Ω
  • VR1 – 1M Ω
  • C1 100µF
  • C2 0.01µF
  • C3 0.1µF
  • Diodes
  • 555 Timer IC
  • CD4017 IC
  • BC 148 B Transistor
  • 6 V/ 100 Ω SPST Relay
  • When the power supply is given ,555 timer produces square wave at pin 3 as it is in astable mode.
  • It gives a pulse width according to value of pot. It can be calculated as
T( high) =0.693*(R1+R2)*C 1

T(low) =0.693*R1*C1

  • This square wave is given to CD4017 IC decade counter which has 10 outputs activated sequentially upon a given clock input.
  • The outputs of the decade counter drive the transistor into active mode so that relay coil will be energized. (Instead of 6v relay one can use 12v relay also but relay should be applied with 12v instead of 6v.)
  • Here the length of ON-time of the load is a multiple of 555 timer period output and number of outputs used in CD4017.
  • Suppose in this circuit 3 outputs of CD4017 are used.So,On time of load is 3 times of T (high) and off time is 9 times of T(high).
  • Therefore, ON and OFF can be varied for desired duty cycles by appropriately connecting the pins of decade counter.
  • It is also possible to add a sensor or switch at reset input of decade counter for automatic turning off the load in emergency or needy (for an automatic operation) situations.
  • Let us understand the application of this circuit. For example in air coolers,there is a pump that pumps water to the mat.This need not be switched on continuously .
  • It can be ON until cooler mats are wet and then it can switched off.Again when they are dry it should pump the water.
  • Suppose if there is adequate water in the tank, the pump must be turned off automatically.
  • This can be achieved by adding a level sensor such that this sensor input drives the reset and inhibit pins towards the ground potential.
  • This circuit is used in such applications where cyclic operation is required.
If very long time delays are required it is not advisable to use 555 timer.Instead one can use a microcontroller. Here is timer using arduino which is user friendly.
Son düzenleme:
Hi....It is my agreement that that measure of time is beyond the realm of imagination utilizing only the 555timer. The issue is capacitor spillage. One route around this is to utilize a counter and have the light run off that and not simply the 555timer. You can utilize a CD4060 counter, I will investigate this more and give you a schematic on the off chance that you need.