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Readers’ Circuits
Our regular round-up of readers’ own circuits
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Universal Ding-Dong – No more chimes blues?
found those chimes at an antiques
S
fair, and would like to use them as a
ding-dong doorbell. Or you bought those
prevents switch bounce, which might cause
solenoid L2 to trigger prematurely (L2 is
likely to activate briefly on power-up). IC1a
and IC1b form a positive-edge-triggered
monostable timer, so that when pins 1 and
2 go high, TR1 conducts for a fraction of
a second, activating solenoid L1. Diode D1
suppresses back-EMF, which could destroy
IC1 in particular. When pushbutton S1 is released, C2 discharges through R1.
IC1c and IC1d, with TR2, form a positive-edge-triggered monostable timer, so
O YOU
gongs at a market in Asia, but can’t work
out how to wire them to the pushbutton on
your patio. Here’s the solution, in Fig.1. The
circuit activates two solenoids in sequence,
to strike two chimes or gongs – one when
your doorbell is pressed, the other when it
is released.
When pushbutton S1 (the front doorbell)
is first pressed, C2 instantly charges. This
that when pin 8 goes low, TR2 ceases to
conduct. This means that TR3’s gate goes
high, and TR3 conducts, thus activating solenoid L2 for a fraction of a second. D2 is
again provided to suppress back-EMF.
Unless a large battery is used for B1, C1
is needed to provide the ‘whack’ required
for solenoids L1 and L2. Non-polarised
capacitors are recommended for C2 to
C4, to set aside worries about polarity.
However, polarity is shown in the circuit
in case the constructor is only able to
SOL1
S1
SOL2
k
R2
150k
D1
1N4007
k
R4
1M
L1
a
D2
1N4007
L2
a
PTM
B1
12V
IC1a
+
*
C3
470n
14 4093
C1
10000µ
1
3
2
+
IC1b
4093
5
6
TR1
BUZ11A
4
IC1c
d
4093
8
g
s
9
*
C4
470n
10 +
TR2
IC1d
2N7000
4093
12
13
11
TR3
d
g
BUZ11A
d
g
s
s
7
R1
150k
*
C2
470n
+
*SEE TEXT
R3
150k
Fig.1. Circuit diagram for the Universal Ding-Dong
52
Everyday Practical Electronics, August 2008