Electronics_For_You_July_2017

Do-it-yourself

122 July 2017 | ElEctronics For you http://www.EFymag.com

Countdown Timer

Fayaz Hassan

S.C.

Dwive

Di

S

ometimes there is a need to

switch on or off electrical

appliances or gadgets after a

predetermined time. The circuit pre-

sented here can switch on/off any

electrical appliance (load) through a

relay switch after the specified time

set in minutes or seconds.

The timer can be easily set with

four switches (S3 through S6) to

select minutes/seconds, hundredths,

tenths and units. Two more switches

are used to start (S2) and stop (S1)

the timer.

Circuit and working

Circuit diagram of the countdown

timer is shown in Fig. 1. ATmega8A

(IC1) from Atmel is at the heart of

the circuit. Associated components

include a 16×2 liquid crystal dis-

play (LCD1), two BC547 transistors

(T1 and T2), a piezobuzzer (PZ1), a

12V single-changeover (1C/O) relay

and a few other components.

The system works in three

modes: started, stopped and setting.

Started means the countdown is

under process and settings are not

allowed. Stopped means the count-

down has stopped and the user can

change settings to start the count-

down timer afresh. In setting mode,

GND

4

RS

5

R/W

6

EN

7

D0

8

D1

9

D2

10

D3

11

D4

12

D5

13

D6

14

D7

1 Vss

3 VO

2 VDD A/VEE 15

LCD1 K^16

16x2 LCD

R9

1K

S1

STOP

S4

HUNDS

S5

TENS

S6

UNITS

S2

START

S3

MIN/SEC

R3

10K

R1

10K

R2

10K R410K

R5

10K

R6

10K

S7

R8 10KR7RESET

1K

T2

BC547

PZ1

PZ1 = PIEZO BUZZER

R13

4.7K

R12

3.3K

CON2

5V

CON3

FOR SJ1

CON4

FOR

R14

1.5K

LED1

BLINKER

T1

BC547 R11

4.7K

R10

3.3K

N/O

N/C

RL1

RL1 = 12V, 1C/O RELAY

CON1

12V

CON6

FOR LOAD

D1

1N4007

GND

5V

XTAL1

3.6864MHz

C1

0.1u

C2

22p

C3

22p

C5

0.1u

C4

0.1u

MOSI

MISO

230V AC

SCK

RST

ISP PORT

R15

1K

(^2) PD0/RXD
(^3) PD1/TXD
(^4) PD2
(^6) PD4
(^8) GND
(^10) PB7/XTAL2
(^1) PC6/RESET
(^5) PD3
(^7) Vcc
(^11) PD5
(^9) PB6/XTAL1
(^12) PD6
(^13) PD7
(^14) PB0
PC4/SDA^27
PC3^26
PC2^25
PC0^23
AVREF^21
PB5/SCK^19
PC5/SCL^28
PC1^24
AGND^22
PB4/MISO^18
AVcc^20
PB3/MOSI^17
PB2^16
PB1^15
IC1
ATMEGA8A
CON5
FOR
FOR
SJ1 = SHORTING JUMPER
Fig. 1: Circuit diagram of countdown timer
Fig. 2: Screenshot of the fuse bits settings; ‘1’ means ‘unprogrammed’ and ‘0’ means ‘programmed’