Electronic Reminder
OBJECTIVE
An easy to build gadget that could give you a reminder after the preset time after you pressure start button is the project under discussion here.
It is particularly useful in situations where you want to do something urgently but not immediately as you are busy doing something else and you would like to be reminded of that aftersome time.
For instance, you have put something for cooking in the kitchen and you know that the next operation in cooking that dish is full 20 minutes away. So, you move to your bedroom and start reading some magazine. If you come across something engrossing to read, you would take your eyes off the magazine only after you get the burning smell of the dish being cooked. There can be many more similar situations. All you have to do in this reminder is to set the time in minutes with the help of two thumb wheet switches provided for the purpose and press the START button. Precisely after the time set by you is over, there is an audio as well as visual indication to remind you that the time you had set had elapsed. The gadget is portable and operates from a 9V battery. If should attract both the House-wives as well as busy professionals
CIRCUIT DESCRIPTION
The functional principle of this gadget is more easy to follow if we understand well different ICs being used here and the function of various terminals on each one of these ICs. The two counter ICs (designated lC-5 and IC-6 in Fig.
12.1) are the heart of this eledronic reminder gadget. These are programmable UP/DOWN 4- bit Binary/Decade counters belonging to CMOS family of digital integrated circuits. These are programmable as the information present on the program inputs P0 to P3 is parallels loaded into the counter when the PL input is HIGH independently the CP?(clock pulse) input. These
can be made to count either in the Up-Mode or the DOWN-Mode when the Up/Dbwn input is HIGH or LOW. These can be wired as 4-bit binary
counters (when BINARY/DEC input is HIGH) Or decade counters (when BINARY/DEC input is LOW). The counter advances by one count on
every LOW to HIGH transition of the clock pulse.The terminal count (TC) output is usually HIGH. It goes LOW when the counter reaches its maximum count (if wired in the UP-Mode) or minimum count (if wired in the DOWN-Mode), and then HIGH again with the immediately following clock transition. The Clock Enable (CE) input is an active low input as indicated by the bar i.e. the clock pulses will be enabled only when this input is LOW. Fig. 12.2 shows the internal schematic of counter IC CD4029B (lC-5 and IC-6) to facilitate better appreciation of What has been described above.IC-2 (CD4013B) is a dual D-type flip flop IC with each D-flip flop inside CD4013B having independent DATA, CLOCK, SET and RESET inputs. The data bit (LOW or HIGH) on the Dinput is transferred to the output on the LOW to HIGH transition of the clock input. SET and reset are asynchronous inputs and are activated with a HIGH enthuse lines.This implies that when the SET input is at logic HIGH, the Qoutput is HIGH irrespective of logic status of Dinput and clock transitions. Similarly when RESET is HIGH, it overrides all other information and forces the Q-output to go to the LOW state.lC-3 (a 555 timer IC) is wired as an astable multivibrator with the output waveform having a time period of one minute. For timing accuracy and stability, resistors (R5) and (R6) should preferably be metal film resistors and (C2) a tantalum capacitor. The output waveform’s time period can be expressed as: T = 0.69C2 (R5+R6+R7+R8+P1) Initially, microswitches (S1) and (S2) are in a position that both MASTER RESET (MR) and START outputs are LOW. The first thing to do is to set the timing (in minutes) with the help of two BCD switches (also known as Thumbwheel switches) (S3) and (S4). If you select ’5′ in (S3) and ’6′ in (S4), the time delay is set at 65 minutes.The next step is to press and release the microswitch (S1). When this is done, a positive going pulse resets the flip flop IC, lC-2, and also loads the programmed time delay information into the counter ICs IC-5 and IC-6. Since the output of the.lC-2A is initially LOW, the AND-gate is disabled and the clock pulses appearing at the output of timer IC are not allowed to reach the counter ICs. When we press and release (S2), IC-2A is clocked, 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 thetime delay begins. To summarise, in order to use the gadget, set the time delay after which you would like to be reminded, press and release (S1) to be followed by a similar operation with (S2).
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 LOW and then HIGH again 65 minutes (if the time delay is set at 65 minutes) after the start. It i basically 65 clock cycles. Remember that the clock period here has been chosen to be equal to1 minute. This pulse appearing at the (TC) output of IC-6 clocks IC2-B whose output goes from LOW and HIGH and drives both the LED and the microbuzzer ON. The output of IC-2B also forces Clock Enable (CE) input if lC-5 to go HIGH and disable the clock. The LED and the Buzzer remain ON unless you reset the system. After the systemis reset, the gadget is ready again for a fresh time setting. The counters have been Wired in DOWN COUNT mode as it is Only then that the counter IC in LSB position completes its count cycle in a number of clock cycle equal to the number set in the LSB BCD switch and the counter connected in the MSB position completes its count cycle in a number of clock cycles equal to ten times the number set in MSB BCD switch. The time can be set in steps of one minute in this gadget. If the clock period is changed to say 2 minutes, the time resolution becomes 2 minutes whereas the maximum settable time delay increases from 99 to 198 minutes. In essence, maximum time delay that is achievable With this circuit is 99 clock cycles with a resolution that is equal to period of one clock cycle.
CONSTRUCTION GUIDELINES
Figs. 12.3 and 12.4 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 : 4.7 K 1/4 Watt
P1 : 100K multi-turn trimmer potentiometer
C1 : 0.1µF (Ceramic Disc)
C2 : 10µF 16V (SolidTantalum)
C3 : 0.01µF (Ceramic Disc)
Semiconductor Devices and ICs
D1 : 1N4001 or equivalent
LED-1 : LED (any colour you Iike) with holder
IC-1: CD4011B
IC-2 :CD4013B
IC-3 :555
IC-4 : CD408IB
IC-5, lC-6 : CD4029B
Other Components
S1, S2 : Miniature push button microswitches
S3, S4 : BCD switches (Thumbwheel switches) Fig. 12.5
S5 : ON/OFF switch
Microbuzzer, 9V Battery
Miscellaneous
IC Bases(8-pin, 14-pin, l6-pin), Multistrand wires, solder metal, suitable mounting cabinet.
CLOCK PERIOD ADJUSTMENT
For the given component values, the clock waveform at the output of 555 has a high time of 20 seconds and a low time of 40 seconds. In fact, for a high time of 20 seconds, the total charging resistance (R5+R6+P1) is required to be 2.9
mega-ohms. P1 can be adjusted to get (R5+R6+P1) equal to 2.9 mega-ohms. Total discharge resistance required for a 40 seconds low time is 5.8 mega-ohms. The total discharge resistance connected in the circuit is 5.82 megaohms which should be fairly accurate.
Discrepancy If any should be corrected by readjusting P1 to get an overall time of 1 minute. That is what is important. The high and low times Individually are not important, it is the total time period that must precisely be one minute.
PIN CONNECTION DIAGRAMS OF ICS
Pin connection diagrams of CD4011B, CD4013B,
CD408lB and CD4029B are shown in Figs. 12.6(a),
(b), (c) and (d) respectively.
Written by kannan on January 4th, 2010 with
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