VARIABLE OFF TIMER
hello friends today I am gonna telling you about some easy and valuable electronics projects for school and college practical
Here is a reliable , low - power , versatile and simple - to - wire preset off ' timer built around IC NE555 . The specifications of this timer are :
1. When the connected load is not in use , the timer automatically turns it ' off ' after the preset time , thus saving power.
2. As the timer switches of the mains , it would not restart and run the load even if there is a power supply surge or power interruption .
3. It's fail - safe because it won't switch on in the event of power resumption .
Transformer X1 is used to step down the mains power supply from 220V AC to 12V AC , 300 mA. Rectifier diodes D1 through D4 form the full wave bridge rectifier, which converts the 12V AC transformer output into DC. Capacitor C1 removes ripples. IC 7812 ( IC1 ) regulates the filtered supply to 12V DC
IC 555 ( IC2 ) is wired as a one - shot timer whose time period can be easily calculated by T = 1.1xR1xC3 , where T ' is in seconds , R1 ' is in ohms and ' C3 is in Farads . Select a low - tolerance resistor R1 and low - leakage capacitor C3 if you want the timer to be accurate , Here , we have selected value of resistor R1 as 1 mega - ohm and value of capacitor C3 as 1000 pF , which amounts to a time period ( T ) equal to 18 minutes approximately .
Output of timer IC2 at pin 3 is connected to relay RL1 with the free wheeling diode D5 connected as shown in the circuit diagram . The re- lay rating will depend on the load cur rent but it would be taken care of by keeping the relay coil current within 200 mA . If it exceeds 200 mA , a driver transistor should be used to drive the relay .
The operation of the circuit is simple. To start the timer, press switch S1 momentarily. Relay RL1 gets energized immediately and switches on the load . At the same time , LED1 glows to indicate that the timer is ' on . ' At the end of 18 minutes , the output of the timer will go low and de-energize the relay , thus switching off the supply to load and timer.
This timer is useful for switching off water pumps used in houses to pump water from sump to the over head water tank , which depends on delivery rating of pump and the capacity of over - head tank . In most of the cases, it takes approximately 18 minutes, hence the design for 18 minute fixed time. The timer can be made variable by having fixed - value C3 and varying R1 , or vice versa . Replacing R1 with a potmeter to set different switch - off timings can be one of the ways of implementing the application .
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