OBJECTIVE

Believe it or not, there are numerous occasions when you have switched. on your TV set to watch a specific program of your interest and you found out that there are still a few more minutes to go. The options before you are to either remain glued on to the TV get and wait for your program or switch off the TV set and then switch it on again after sometime. The first option is boring and with the second, you always run the risk of missing a part of the program if you get busy in something else and forget to switch it on in time. Here is a very simple and interesting gadget that could be very easily mounted on your electrical switchboard and which can be used to perform the auto switch on function for your TV set (or fortran matter any other mains operated system) at a time programmed by you. With this gadget, you could set a timing of 99 minutes maximum with a resolution of one minute. For instance, if you want that your TV set should automatically switch on fifteen minutes from now, all you have got to do is to set l5 in the BCD switches, reset the gadget and press start. Remember that you have to keep the ON/OFF switch on your TV set in the On-position. The gadget can also be bypassed lf you so desired with the help of a switch provided on the gadget itself.

CIRCUIT DESCRIPTION

The circuit shown in Fig. 13.1 is identical to the one described in case of Electronic Reminder gadgetry the previous project except for addition of a power supply section that generates +l2V DC for circuit operation and the output of lC-2B driving an SCR to energise a relay rather than a microbuzzer and an LED as in case of reminder gadget. The ICs used are the same. IC-1 is a Quad 2-input NAND wired as two independent denouncing circuits, 1C-2 is a dual D-type flip flop, IC-3 is a 555 timer generating clock pulses at a rate of one clock pulse per minute, IC-5 and IC6 are programmable Binary/Decade UP/DOWN counters, IC-4 is a Quad 2-input AND gate. All these ICs along with their pin designations and functions have already been discussed at length in project-l2 and shall not be repeated here. The auto switch on gadget operates as follows: Initially, both MASTER RESET (MR) and START outputs are LOW. Set the time in minutes from the BCD switches S4 and S5 after which you want your gadget (TV, VCR etc.) to switch on.

Having programmed the time delay, press and release (Sl) to apply the reset pulse. The positive going reset pulse resets flip flop IC (IC-2) and also loads the programmed time delay information into counter ICs, IC-5 and IC-6. Since the output of IC-2A is initially LOW, the AND-gate is disabled and the clock pulses are not allowed to reach the clock input terminal of counter lC-5 when we press and release (S2), IC-2A is clocked and its output goes HIGH, the AND-gate is enabled and the clock pulses are allowed to the clock input of IC-5, This is the time when the time delay begins. Also, as the power to the TV set or VCR is through the normally open relay contact, the system remains off. We shall not go into the detailed operation of counter ICs but it would suffice to say that (TC) output of IC-6 is normally HIGH and that it would go to LOW state and then HIGH again when the preset time delay has elapsed. This pulse appearing at (TC) output of IC-6 clocks IC2B whose output goes from LOW to HIGH. This LOW to HIGH transition triggers the SCR thus energising relay coil RL-1. The normally open relay contact closes and the system is switched on. The output of IC-2B also forces the Clock Enable (CE) input of IC-5 to go HIGH and disable the clock. The system remains on unless you reset the system.The counters have been wired in the DOWN count mode as it is only then that the counter IC in LSB position completes its count cycle in a number of clock pulses equal to the number set in the LSB BCD switch. The counter connected in MSB position then completes its count cycle in a number of clock pulses equal to ten times the number set in the MSB BCD switch. This gives a total time delay equal to the time period of (N) clock pulses where (N) here is the decimal number preset with the help of BCD switches (S4) and (S5). As the clock period is one minute, the time here can be set in steps of one minute. In essence, maximum time delay that is achievable from the gadget is 99 clock cycles’ Period with a time resolution equal to period of one clock cycle.The power supply section is a conventional step- down transformer, two-diode fpll wave rectifier (using a transformer With a center tapped secondary winding) and a capacitor filter configuration followed by a 12V output three     terminal regulator.

CONSTRUCTION GUIDELINES

Figs. 13.2 and 13.3 respectively show the PCB layout and the components layout.

PARTS LIST

Resistors and Capacitors

R1, R2, R3, R4 : 22K 1/4 Watt

R5 : 2.2M 1/4 Watt (Metal film)

R6 : 680K, 1/4 Watt (Metal film)

R7 : 5.6M, 1/4 Watt (Metal film)

R8 : 220K 1/4 Watt (Metal film)

R9 :  1.5K, 1/4 Watt

R10 : 220K, 1/4 Watt

P1 : 100K multitude trimmer potentiometer

C1 :  (Ceramic Disc)

C2 :  16V tantalum)

C3:  (Ceramic Disc)

C4:  Rev (electrolytic)

C5,C7:  (Ceramic Disc)

C6 :  16V (Electrolytic)

Semiconductor Device: and ICs

D1 to D4 : 1N4001 or equivalent

IC-1 : CD4011B

IC-2 : CD4013B

lC-3 : 555

IC-4 : CD4081B

IC-5, IC-6 : CD4029B

SCR-1   : OE101 or equivalent

VR-1 : Three terminal Regulator, type 7812

OTHER COMPONENTS

S1 , S2 : Miniature push button microswitches

S3 : Mains power ON/OFF switch

S4, S5 : BCD switches (Thumb- wheel switches)

S6 : ON/OFF switch (230VAC, 1A)

RL-1 : 12DC Relay with at least one normally open contact

T-1 : Mains transformer (Primary: 230VAC Secondary: 14-0-14 250mA)

F-1 : Fuse (0.5A) with holder

Miscellaneous

IC Bases (8 pin, 14pin, 16pin), Multistrand wires, Solder metal, Suitable mounting cabinet, etc.

CLOCK PERIOD ADJUSTMENT, PINCONNECTION DIAGRAMS

Clock period should be precisely adjusted to one minute. The adjustment guidelines are the same as explained in case of Project: 12. The pin connection diagrams of different ICs being used have already beeh given in Project: 12. Pin connections for the SCR are shown in Fin. 13.4.

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