PRESETTABLE TIMER

PRESETTABLE TIMER

ABSTRACT
Most of you may be quite familiar with ‘ON’/’OFF’ timer circuits constructed using thumbwheel switches,rotary switches,etc. It is a novel DELAYED-‘ON’/DELAYED-‘OFF’ presettable timer wherein delays are preselected using secret-coded punch cards.
The circuit can be easily configured for secured operation of sensitive appliances and equipment for a specific time duration using the secret- coded punch cards and will thwart such attempts and not allow any unauthorized person to operate. In DELAYED-‘ON’ mode, the power to the device is connected continuously after a specific delay, its is selected by using an appropriate punched card. In DELAYED-‘OFF’ mode, the power to the device is connected immediately and disconnected after a specific delay.
The circuit consists of a monostable multivibrator built around timer NE 555. Time duration of it is valued by values of timing resistors R 14 through R 19, presets VR 1 through VR 6 and capacitor C1. The timing resistor that determines the selective time duration is selected through the output of 1-of-8 digital demultiplexer IC 4(CD4051). The circuit is designed to operate by using the hanging position of changeover switch S3. It consists of 3 interrupter module MOC 7811 that acts as a sensor for circuit. It consists of an infra-emitting diode facing a phototransistor with a slot in the housing between the IR LED and the phototransistor. To operate interrupter module in digital mode by applying +12v. The time duration of the timer is theoretically determined by T=1.1RC. Resistor value can be determined by R=T/(1.1*100*1O^-6) and C=100µF has the tolerance of 1 percent.
The negative-triggering pulse for pin 2 of NE 555 is generated with the help of NOR gate CD 4078(IC5), capacitor C3, resistor R10 and D3. The high output from pin 3 of NE 555 drives transistor T 3 into saturation and relay RL 1 energies. 230V AC mains is stepped down by transformer X 1 to deliver the secondary output of 15V-0-15V,500mA and regulated by IC 7812(IC7). At that time, the buzzer beeps continuously indicating that an unauthorized person is trying to operate the device.