CIRCUIT IDEAS
ELECTRONICS FOR YOUMAY 2004
WATCHMAN WATCHER
S.C.
DWIVEDI
H
ere is a circuit that can be used in
offices, stores, warehouses, etc during
night to check whether the
watchman of your establishment is on duty.
For operation, it uses an existing telephone
(e.g. in office or store) closest to the
watchman's post. The watchman is given
an audio alert signal by just ringing the
office/store telephone once (minimum)
from your residence or any other place,
preferably using your mobile phone. The
ring is detected by the given circuit and the
watchman is also given a visual alert signal
by a glowing lamp. The lamp remains 'on'
for a duration of nearly 60 seconds soon
after the ringtone. The watchman is given
JAYAN A.R.
an instruction to register his presence by
simply pointing his torch-light beam towards
a wall-mounted LDR sensor unit
(without lifting the handset off-cradle of
the ringing telephone). This is to be done
within the time period during which the
alert lamp glows. If he fails to do it within
the permissible time, the circuit registers
his absence by incrementing a count. If he
does, the count remains unaltered.
Up to nine separate alert rings are considered
here. The count displayed is the
number of times the watchman failed to
register his presence. The mobile phone
records the called number and call time,
and it can be used with the displayed count
to get the timing details.
The telephone lines (TIP and RING) in
the circuit are connected across optocoupler
MCT2E (IC1) through a resistor-capacitor
(R1-C1) combination. The diode in the
optocoupler conducts only during
ring pulses. The collector of the optocoupler
transistor is normally off and a 5V signal is
available here. This signal is connected
to the trigger input of IC 555 (IC2) configured
in monostable mode. The time constant
of IC2 is set to nearly one minute
(1.1RxC). Its output pin 3 is low during
normal mode of operation and the relay is
de-energised.
When the phone rings, the internal
transistor of the optocoupler conducts to
cause a high-to-low transition at trigger
pin 2 of monostable IC2. Timer IC2 gets
triggered on this trailing edge to energise
CIRCUIT IDEAS
ELECTRONICS FOR YOU
MAY 2004
relay RL1. This relay is used to switch on
alert lamp L1. The circuit doesn't respond
to additional trigger inputs for the set duration
of the monostable. The caller may
cut the phone call after hearing ringback
tone from the called phone.
The sensor circuit formed using LDR1
activates another monostable 555 (IC6).
LDR1 has a resistance of 2.2 kilo-ohms in
daylight, which drops below 50 ohms when
torchlight beam falls on it. (An LDR of
nearly 2cm diameter has been used in this
circuit.) Comparator LM358 (IC5) compares
the level set at pin 3 (nearly 1V, set using a
10k pot) with the level at pin 2.
When no light is falling on LDR1, its
voltage is above 1V and IC5 has a low
output at its pin 1. When light is falling on
LDR1, its voltage drops below 1V and IC5
output at its pin 1 becomes high. This low-
to-high transition is NANDed with the output
of monostable IC2 (via inverters gates
N1 and N2) to form the trigger signal for
monostable IC6. So the trigger input is
normally high, which falls when torchlight
beam is focused on LDR1. It returns to high
state when torchlight is switched off. So
the torch is used as a remote
for triggering
monostable IC6 and this
triggering is enabled only
when alert lamp L1 is
'on.'
Monostable IC6 has
a time constant of nearly
one minute (1.1RxC). It
is used to form a down
clock signal for 4-bit up-/down-counter
74LS192 (IC7). Counter IC7 has two separate
clocks for up and down counts (refer
to the table). For correct counting, it needs
one clock line to be high during high-to-
low transition of the other clock line. Otherwise,
it counts erratically.
To operate counter IC7, the voltage
levels and timings of the two clock inputs
(up and down) are to be properly adjusted.
Both trigger inputs, i.e. up and down
clocks, are asynchronous.
The output of monostable IC2 is filtered
using capacitor C4 to remove unwanted
transitions and inverted using
Schmitt trigger inverter 74LS14 (IC3). This
forms a signal with correct rising and
falling edges. The inverted signal from
pin 6 of gate N3 is used as the up clock.
Counter 74LS192 (IC7) is reset to zero
state by making its reset pin 14 high through
reset switch S1. The 7-segment, common-
anode display DIS1 is driven through IC
74LS47 (IC8). When the phone rings, count
'1' is displayed after nearly one minute.
This happens if the watchman fails to focus
the torchlight beam on LDR1.
If LDR1 receives light from the torch
of the watchman within the allowed time
period, the down clock remains high until
the up clock is high. The counter counts
up and then down, so, in effect, the count
remains unchanged.
All components, except LDR1, are kept
in a sealed cabinet with locking arrangement.
Only LDR1 is wall-mounted and visible
outside. This is done to avoid manual
resetting of the counter. The circuit is to
be powered by a battery to avoid resetting
of the count during power failure.
The working procedure can be
summarised as follows:
1. Initially, when the power supply is
switched on, power-on-reset components
C8 and R13 reset counter IC7 and the display
shows '0.'
2. Now dial the telephone number
(where parallel system is installed) from
outside or from your mobile. For the first
ring, relay RL1 energises and alert lamp
L1 glows.
3.When alert lamp L1 is off, the
counter is incremented by '1.'
4. If the watchman focuses the torchlight
beam on LDR1 within the glowing
time of alert lamp L1, the counter first
counts up and then counts down and finally
the display shows 0. This indicates
that the watchman is present.
5. If the watchman focuses the torchlight
beam on LDR1 after alert lamp L1
goes off, up-counting takes place and the
display shows '1.' This indicates that the
watchman is absent.
Mode-Select Table of 74LS192
MRpin 14
PL pin 11
UPpin5 DNpin4 Mode
H
X
X
X
Reset
L
L
X
X
Preset
L
H
H
H
No change
L
H
H
Count up
L
H
H
Count down
Note: X = Don't care
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