Here is a versatile on-off (or off-on) timer circuit. Its time delay can be set from seconds to hours. The time for which the circuit will remain on (or off) can be selected with the help of switch S2. This will be followed by off (or on) state lasting for a period which can be selected with the help of switch S3. Thus the load or device will be initially on (or off) for certain selected duration after which the intial state will toggle to off (or on) state for another selected duration. Thereafter the cycle will repeat. Switch S1 in conjunction with potentiometer VR1 can be used for varying the overall cycle duration (on and off periods or the other way around) without disturbing the on to off (or off to on) ratio determined by switches S2 and S3. IC1 (4060) functions as an oscillator and 14-stage binary counter. The oscillator timing is so adjusted with the help of VR1 that a pulse repitition rate of one hour is obtained at its final output pin 3 (Q14 output). The outputs Q1 through Q14 are square waves, i.e. their on and off periods are equal. The pulse repetition rates of Q13, Q12, Q11....
will accordingly be 1/2, 1/4, 1/8..... (hour) respectively.
One of the outputs of IC1 selected with the help of switch S1 serves as clock for IC2 and IC3 which are both Johnson ring counters (4017). IC2 is used for the initial on or off time setting. After it counts the number of pulses it is set for, it produces a low-to-high going output. If you want a device or load operated via relay RL1 to be initially off then flip switch S5 to position marked ‘off’. If the relay is to be initially in on position, flip switch S5 to on position as shown in the circuit. Transistors T1 and T2 form a darlington pair for driving the relay. On power-on, a reset pulse developed across resistor R4 resets IC1, IC2, and IC3. Also Q output at pin 1 of IC4 flip-flop is low which enables IC2, while Q output at pin 2 being high keeps IC3 inhibited. Assuming switch S5 to be in ‘on’ postion, the output of inverter N1 is initially high and the relay gets energised. At the end of the selected period (through switch S2), output of IC2 goes high and that of inverter N1 goes low. As a result relay gets de-energised and simultaneously J-K flip-flop toggles enabling IC3 and inhibiting IC2.
Thus IC3 starts advancing and when the period selected via switch S3 is completed, the output at pole of switch S3 goes high. This low-to-high input is converted into high-to-low going voltage at the output of inverter N2. This triggers IC5 (555) which is configured to operate as monostable multivibrator. The high output pulse from pin 3 of IC5 is used as reset pulse for IC1 through IC3. As a result the circuit returns to its initial conditions and the entire operation repeats. Manual resetting of the circuit is possible by momentary depression of switch S4. Important points concerning the operation of the circuit are:
1. On/off sequence can be changed to off/on sequence using switch S5.
2 Maximum ‘on’ and ‘off’ time selectable is 10 hours, unless values of timing components ie preset VR1 and capacitor C1 are altered.
Switch S1 in fact determines the timer resolution while switches S2 and S3 determine “how many times?” If S1 is connected to Q14 (pin 1) of IC1 and if S2 is in position 5, and S3 is in position 7, the initial on (or off) period is 5 hours followed by off (or on) period of 7 hours. When S1 is connected to Q8, the resolution is approx. one minute.
Download the Report of this project
password: be
0 comments:
Post a Comment
Thanks for your Valuable comment