Specifications

CIRCUIT IDEAS
sumption is extremely low. This makes it
possible to power the circuit from a bat-
tery. The input sensing current through
the fluid (with all the four probes im-
mersed in water) is of the order of 70 µA,
which results in low rate of probe dete-
rioration due to oxidation as also low lev-
els of electrolysis in the fluid.
Note: This circuit should not be
used with inflammable or highly reactive
fluids.
T
his is an effective and useful
project for educational institu-
tions. In most schools and col-
leges, the peon rings the bell after every
period (usually of a 40-minute duration).
The peon has to depend on his wrist watch
or clock, and sometimes he can forget to
ring the bell in time. In the present sys-
tem, the human error has been elimi-
nated. Every morning, when the school
starts, someone has to just switch on the
system and it thereafter work automati-
cally.
The automatic microprocessor con-
trolled school bell system presented here
has been tested by the author on a
Vinytics’ microprocessor-8085 kit
( VMC -
8506). The kit displays
the period number on
two most significant
digits of address field
and minutes of the
period elapsed on the
next two digits of the
address field. The
data field of the kit
displays seconds con-
tinuously.
The idea used
here is very simple.
The programmable peripheral interfacing
( PPI ) Intel-8255-I chip present in the micro-
processor kit has been used. It has three
8-bit wide input/output ports (port
A , port
B , and port C ). Control word 80 (hex) is
used to initialise all ports of
8255-I as out-
put ports. Bit 0 of port
A ( PA
0
) is connected
to the base of transistor
BC 107 through a
10-kilo-ohm resistor as shown in the fig-
ure. It is used to energise the relay when
PA
0
pin of 8255- I is high. A siren, hooter,
or any bell sound system with an audio
amplifier of proper wattage (along with 2
or 3 loudspeakers) may be installed in
the school campus. The relay would get
energised after every 40 minutes for a
Dr D.K. KAUSHIK
AUTOMATIC SCHOOL
BELL SYSTEM
RUPANJANA
PA
O
21