0 Appreciate Project

Adjustable Timer

Have you ever made a circuit that automatically power ON and OFF after a desirable or set amount of time? Traditionally, various loads would have been manually controlled, i.e., the operator would turn ON the loads and after a set or desired conditions met, the loads again would have been turned off by the operator.

Step 1: Adjustable Timer

Today, most of the load control operations are automated by intelligent devices and automated technologies. However, these methods are cost ineffective.

Here you can operate the loads by switching them for particular or set time using cost-effective unit. Now we will build the circuit which can switch off the load after a predefined time period. This ON and OFF timer can be adjusted for desired time duration over the total duty cycle offered by the circuit.

Some of the critical applications where you come across of switching the loads for small duration include automatic pumps ON/OFF, automatic turn off of soldering iron, exhaust fan control, etc.

Adjustable Timer Circuit with 555 IC

The main aim of this project is to reduce the wastage of power during unnecessary operation of the loads. And also to achieve the automatic operation of loads for definite time periods, this project will be very helpful.

This project uses very basic components like 555 timer and 4017 counter to produce a very long time period. This circuit is extended version of 555 astable multivibrator circuit which can provide timer up to 10 minutes.

Here in this circuit, a binary counter is used along with a timer circuit to produce long duration. A cyclic ON and OFF operation for a set timings can be achieved by properly connecting the counter outputs to the load.

These time periods can be adjusted by varying the 555 timer output and number of counter outputs. Let us discuss in detail about this circuit.

Circuit Components

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

Circuit Connections

Choose the components listed above and select their appropriate values to achieve exact timings.
Take bread board or PCB or zero board to build this circuit. First place the 555 timer IC in the middle of board and try to assemble selected components to timer in order to achieve astable multivibrator operation.
Install CD4017 IC right to the 555 IC and connect the timer output pin 3 to the clock input of binary counter.
Based on the time of operation for the loads, connect decoder output pins to the transistor in OR fashion using diodes.
Connect the relay coil inputs between power supply and collector terminal of transistor. And connect the load terminal appropriately to the relay contacts.
Connect the power supply terminal and a ground terminal to their respective positions as shown in figure, but make sure that it should be turned OFF initially.

Circuit Diagram

Circuit Operation

When the power supply is given to the circuit, 555 timer produces a square output at the pin 3 and this output duty cycle depends on the RC time constant obtained from R1, VR 1 and C1 components. CD4017 IC decade counter has 10 outputs which are activated sequentially upon a given clock input.

Thus the timer output fed at the clock input of decade counter produces an active high signal at each output pin sequentially. Once the time elapses in 555 timer circuit, it automatically turns off the decade counter input and thus load switching time is varied accordingly.

Here the length of ON-time is a multiple of 555 timer period output and number of outputs used in CD4017. Similarly the OFF-time period is a multiple of 555 timer period output and number of unused outputs of CD4017. Therefore, ON and OFF can be varied for desired duty cycles by appropriately connecting the pins of decade counter.

Here in this arrangement, the output is connected such that the load is turned ON for three to four minutes and turned OFF for seven to eight minutes (which can be varied by adjusting pot connected to the timer). The outputs of the decade counter drive the transistor into active mode so that relay coil will be energized.

Further the relay closes the load circuit contacts. And after a desired time period, load will be disconnected by the relay, once the decade counter drives the transistor into cutoff mode.

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.

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 cycling will be continued by the 555 timer by generating delay cycles. In this way load switching will be performed for desired time periods.

Advantages

Long time-period is achieved as compared with 555 timer based astable multivibrator configuration.
Precision in timing is maintained by two ICs, thus increases the reliability.
Adjustable ON and OFF time setting for meeting desired time durations with timer output and decade counter output configuration.
Extremely ease of building the circuit as it uses less number of components.