Timers were used in many applications in our day to day life.One can see the timers in washing machines,micro ovens etc. These devices uses timer to switch the loads for particular amount of time.Traditionally, various loads would have been manually controlled, i.e., the operator would turn ON the loads and after desired conditions met, the loads again would have been turned off by the operator.
Here i am going to explain different ways of building adjustable timer circuits. However, these methods are cost ineffective.Three circuits are explained here are 1)Simple adjustable timer using 555 IC,2)A cyclic on/off timer using 555 IC,3)Adjustable timer using arduino. (40+ Simple 555 Timer Circuits & Projects)
Simple Adjustable Timer Circuit with 555 IC
- 555 timer
- Electrolytic capacitor – 470 uf
- ceramic capacitor – 0.1nF
- 120k ohm
- 10k ohm
- Relay -12v
- Push button
- Here 555 timer is operated in monostable mode.
- When the trigger input is applied,555 timer produces a pulse. This pulse width depends on R and c values.
- The above proposed circuit is a 1-10 minute timer.When Pot is minimum it gives 1 minute delay,where maximum value of pot can produce 10 minutes.
- Time period can be calculated using formula
- When Pot is maximum R is 120K+1.1M ≈ 1.2M (approximately) and C1=470uf
T= 1.2M*470uF = 620 seconds≈10 minutes.This is the maximum time.
- For minimum time place the pot in least position.Then R= 120k
- Hence time T=120k*470uf=6 2 seconds~1 minute (approximately).
- A 12v relay is used to drive the ac load connected at the output.
- Thus relay will be on for required amount of time set by the user using pot and then it is switched of automatically.
- This circuit is used in such applications where the load is switched on for sometime and is off for rest of the time.
- To prevent 555 timer from flyback current in the relay use a diode before the relay.
- Some 555 versions may get damaged because of this.
[Also Read: 12V Time Delay Relay]
Adjustable ON OFF Timer(using 555 astable mode)
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.
- R1 and R2 – 47 KΩ
- R3 – 15K Ω
- VR1 – 1M Ω
- C1 100µF
- C2 0.01µF
- C3 0.1µF
- 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
- 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.
Adjustable Timer(Using Arduino)
Arduino adjustable timer is simple circuit to generate timer for required time. This is used to switch on the loads for the certain time period and then they are automatically switched off.
Here arduino plays a key role in setting this time period.
Here a relay is used to switch the load for certain amount of time.
- Arduino Board
- Push buttons
- Initially when the circuit is switched LCD will display ” adjustable timer”.
- Now using the two buttons set the timer.Button connected to 8th pin is used to set the timer in minutes and button connected to 10th pin is used to set the timer in hours.
- Set the time by pressing these buttons.When the button is pressed , time is incremented every time.
- Now click on start button to switch the load.
- When the time is completed load is switched off automatically.
- To set the timer next time click the reset button on the arduino and set the timer again.
Applications of Adjustable Timer
There are numerous real time operations which require time scale switching loads. Some of these are listed below.
2.Irrigation pump control
3.Exhaust fan switching
4.Industrial repetitive switching of loads
5.Load shedding and control
6.Automatic lubrication tools
7.Traffic lights control
8.Printing applications, etc.