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bell transformer supply
for wave-file player
016
H. Bonekamp
If the wave-file player described in the Feb-
ruary 1999 issue of this magazine is used as a
programmable doorbell, it is, of course,
handy if its supply is derived from the door-
bell transformer. In the design of the circuit
in the diagram, it was borne in mind that as
little as possible should be changed in the
existing wiring of the doorbell and that it
would be convenient to be able to switch the
doorbell back on when the wave-file player is
removed to program it with a new sound.
The alternating voltage at the sec-
ondary of the doorbell transformer is recti-
fied by diode bridge D 2 –D 5 , and smoothed by
capacitor C 2 . Zener diode D 6 , a fast type,
serves to suppress transients. The resulting
direct voltage is stabilized by regulator IC 1
which is set for 6.8 V. When the drop across
the protection diode in the wave-file player is
deducted from this, a direct voltage of about
6 V remains, which is used to supply the
amplifier IC
The need of modifying the existing
wiring is precluded by the circuit around tran-
K1
T1
D1
R1
12k
S b
1N4148
R2
BC547B
C1
S1
ON/OFF
BELL
µ
25V
WAVE FILE PLAYER
S1
F1
500mA T
K3
Tr1
IC1
K2
SW
D5
D3
6V8
LM317
8V
R3
D4
D2
4x 1N4001
R4
D6
C2
C3
C4
2200
100n
1
63V
1W3
15V
16V
994080 - 11
994080-1
C1
R1
R2
H1
T1
S1
D1
H2
R4
K1
C2
R3
D6
C3
D2
D3
D4
D5
K3
K2
F1
H3
0.5AT
H4
C4
IC1
50
Elektor Electronics
7-8/99
10
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sistor T 1 , which converts the alternating voltage arriving from the
doorbell into a switching signal that is applied to, and processed
by, the wave-file player. Switch S 1 enables the doorbell to be used
as normal when the wave-file player is removed for any reason.
Remember to open the switch again when the wave-file player is
replaced, otherwise the player does not work and it is set to the
wrong speed of 9600 baud via the S 1 input.
If desired, the unit can be coupled to a computer via a screened
cable to make it possible for the program of the wave-file player
to be altered without it being necessary to remove the unit.
The circuit is best built on the printed-circuit board shown,
which is available through our Readers’ Services (see towards the
end of this issue). Note that the voltage regulator should be
mounted on a small appropriate heat sink of 24 K W –1 .
Parts list
Resistors:
R 1 = 12 k Ω
R 2 = 1.8 k Ω
R 3 = 270
D 6 = BZT03, 15 V, 1.3 W
T 1 = BC547B
Ω
R 4 = 1.2 k
Integrated circuits:
IC 1 = LM317T
Miscellaneous:
K 1 , K 2 = 2-way terminal strip,
pitch 5 mm
K 3 = 3-way terminal strip,
pitch 5 mm
S 1 = slide switch
F 1 = fuse holder with
500 mAT fuse
Enclosure, e.g. Bopla E410
Heat sink for IC 1 : 24 K W –1
Capacitors:
C 1 = 10 µF, 25 V, radial
C 2 = 2200 µF, 16 V
C 3 = 0.1 µF, ceramic
C 4 = 1 µF, 63 V, radial
Semiconductors:
D 1 = 1N4148
D 2 –D 5 = 1N4001
[994080]
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