Specifications

197
CONSTRUCTION
accomplished via relays RL1 - RL3 as
follows:
1. The output of timer IC4 energises
relay RL1. On energisation, con-
tacts RL1(a) of this relay extend
+12V to the coils of RL2 and RL3,
thereby energising both of them
(RL2 and RL3). RL1(b) contacts
of relay RL1 (on energisation)
switch on the DC supply to the
record player, whose play button
is supposed to be already de-
pressed. (Please note that DC
power supply for the player is not
catered to in the circuit. The sup-
ply voltage will depend on the
‘make’ of the player, and may
vary from one player to the other.
In many cases, battery supply
(provided inside the player itself)
could be routed via RL2(a) con-
tacts.) Thus, it will start playing
the prerecorded message, which
would get coupled to the telephone
lines, since the hook switch would
also be in the released state,
simultaneously via relay RL3
(energised) contacts, as described
later.
2. RL2(a) contacts of relay RL2,
on energisation, extend +12V to
reset pin 15 of counter IC2. Thus
the counter remains reset until
IC4 timer’s output goes low again.
RL2(b) contacts of relay RL2, on
energisation, switch on 230V AC
supply to the tape recorder and
thus it can record the message re-
ceived in the earpiece of the tele-
phone.
3. Relay RL3 performs the
functions of cradle switch for the
telephone in absence of the sub-
scriber (or even during his pres-
ence, if he fails to lift his handset
off the cradle during ringing).
Its contacts RL3(a) and RL3(b)
are wired in parallel with the
hook switch, as shown in Fig. 4.
(This application circuit is based
on Motorola single-chip telephone
set IC MC34010 or MC34011.
The latter chip does not provide
microprocessor interface; oth-
erwise both chips are identi-
cal.)
Important points
Normally, the player and re-
corder buttons are always in the
pressed or ‘on’ condition, so that these
are automatically switched ‘on’ by timer
IC4 via relays. If we use C90 cassette
tapes for the incoming calls, and as-
suming that pulse width of timer IC4
is set for 3 minutes, we can record/
answer 15 incoming calls, since we can
use only one side of the tape in this
set-up and hence we can play/record
the messages for 45 minutes only. How-
ever, one may use each 3-minute dura-
tion for message answering(playing) for
Fig. 7: Component layout for the PCB
Fig. 6: Actual-size, single-sided PCB for the circuit in Fig. 2
Fig. 5: Ringing tone timing waveform