Short duration timer Circuit using IC 555

We use lot of short duration timers to alert when a predefined time elapses. This can come in extremely handy for applications such as Cooking, exercise, sports and so on. We use stopwatches, alarms and all sorts of things to do that. Similar to these, the circuit we see below will alert you of time elapse and you can alter the amount of time as per your requirement. This circuit is quite simple and uses IC 555 and few discrete components to operate.

Working of timer circuit using IC 555:

In this circuit IC 555 is configured as monostable mode which means a external trigger is necessary to get high output from its 3rd pin. If you are not aware of IC 555, learn about 555 working to understand this circuit better. External trigger to this circuit serves as a start button ( S1) for this timer. When not pressed this pin is pulled high using 10k resistor and therefore output of IC555 remains low. When S1 is pressed it feeds low pulse signal to trigger pin of IC 555. This toggles the output from low to high for a fixed amount of time. 

The timing of high signal from IC 555 is influenced by three components. C3, R4, 1M POT in the above circuit. Here the combination of Resistor R4 and 1M pot charges the Capacitor C3 now. The output of IC 555 will stay high until the capacitor reaches +2/3 Vcc. This timing can be calculated by the formula 

T = 1.1 x R x C

Applying to our circuit 

Maximum time limit = 1.1 x (500k + 1M ) x 100uF     ( when POT is in maximum position )

= 165 seconds

Minimum time limit = 1.1 x 500k x 100uF  ( when POT is in minimum position so the value is neglected )

= 55 seconds

Once this timing elapsed the output of IC 555 goes back to low state indicating that timing you have set is elapsed. Here is the operation in a nutshell, 

• Adjust the POT to set your preferred time between maximum and minimum values
• Switch ON the circuit using switch S2 and you should see Red LED lights up
• Pressing the switch S1 will initiate the timer, Red LED goes off and Green LED lights up instead.
• Once the timing elapses Green LED goes off and Red turns back on indicating the time has elapsed.

Note:

• Alter the values of R and C in the circuit to change the timing as per your need in reference to the above formula.

Hope this circuit is useful to you. Check out our Electronics circuit library for more circuits. If you have any questions and queries with this circuit do post them in the below comment box.