Electronics_For_You_July_2017

Do-it-yourself

http://www.EFymag.com ElEctronics For you | July 2017 123

Fig. 3: Actual-size PCB layout of the countdown timer

Fig. 4: Components layout for the PCB

Parts List

Semiconductors:

IC1 - ATmega8A microcontroller

T1, T2 - BC547 npn transistor

LED1 - 5mm LED

D1 - 1N4007 rectifier diode

Resistors (all 1/4-watt, ±5% carbon):

R1-R7 - 10-kilo-ohm

R8, R9, R15 - 1-kilo-ohm

R10, R12 - 3.3-kilo-ohm

R11, R13 - 4.7-kilo-ohm

R14 - 1.5-kilo-ohm

Capacitors:

C1, C4, C5 - 0.1μF ceramic disk

C2, C3 - 22pF ceramic disk

Miscellaneous:

CON1, CON2,

CON5, CON6 - 2-pin terminal connector

CON3 - 2-pin berg strip male connector

CON4 - 6-pin berg strip male connector

XTAL1 - 3.6864MHz crystal oscillator

PZ1 - Piezobuzzer

S1-S7 - Tactile switch

LCD1 - 16×2 LCD module

RL1 - 12V, 1C/O relay

• 5V DC regulated power supply
  SJ1 - Shorting jumper


• 12V DC regulated power supply


• 28-pin IC socket

the input switches (S3 through
S6) are enabled for countdown
timer settings.
When start switch (S2) is
pressed, relay RL1 energises and the
countdown starts. The timer stops
automatically when the countdown
reaches 000 or stop button (S1) is
pressed. The status of countdown is
shown on LCD1. A beep is gener-
ated by the piezobuzzer when
the countdown reaches below ten
seconds and stops when the count-
down reaches 000. Jumper J1 can
be disconnected if the buzzer is not
required. The power supply to relay
RL1 depends on the coil voltage
of the relay (like 5V, 6V or 12V).

Here, we used 12V DC at CON1 for

the relay.

In stopped or setting mode, LED1

blinks every ten seconds. When start

(S2) button is pressed, LED1 blinks

every second.

Software

The software is written in ‘C’ lan-

guage and compiled using an AVR

Studio 4 software. You can use any

suitable software to burn the hex code

into the microcontroller through ISP

port. At EFY Lab, we used ProgISP

programmer for the purpose.

Before programming, set fuse bits

for the 3.6864MHz crystal option as

shown in Fig. 2.

Construction and testing

An actual-size, PCB layout for the

countdown timer is shown in Fig. 3

and its components layout in Fig. 4.

After assembling the circuit, enclose it

in a suitable box.

Insert the programmed MCU in

the IC socket on the PCB. Connect

an electrical load (say, 100W bulb)

across CON6, and 230V AC mains

across CON5.

Now, power on the circuit by

connecting a 5V DC supply at CON2.

LED1 will glow and the buzzer will

sound for about one second, followed

by display of ‘countdown timer’ mes-

sage on LCD. The system is now ready

to use, waiting for the user input.

The source code of

this project is

included in this

month’s EFY DVD and

is also available for free

download at source.

efymag.com

efy Note

Fayaz Hassan is a manager

at Visakhapatnam Steel

Plant, Andhra Pradesh. His

interests include MCU projects,

mechatronics and robotics