SMALL CIRCUITSCOLLECTION

Twilight Switch 002

thing that exactly matches your own taste and wishes.
Building a twilight switch is certainly not difficult, as is illus-
trated by the design presented here. It uses the well-known
555 IC (here in the economical CMOS version), which has
a trigger input with hysteresis (‘on’ at 2/3 of the supply volt-
age, ‘off’ at 1/3) and an internal flip-flop as a memory ele-
ment. The 555 output can switch up to 100 mA, which is
adequate for driving a relay. When it becomes dark, the
resistance of the LDR (R1) increases and the voltage on
pins 2 and 6 of the 555 rises. When it reaches 2/3 of the sup-
ply voltage level (which depends on the setting of P1), the
relay is engaged and the load is switched on.
Since relay contacts are used for switching, the load can
also be inductive (which means that you can also connect
an energy-saving (PL) lamp or a halogen-lamp trans-

COMPONENTS LIST
Resistors: Semiconductors:
R1 = LDR D1,D3 = zener diode 12V
Nowadays, you do not actually need to build twilight 1W
R2 = S10K250 varistor
switches yourself, since they can be purchased quite inex- R3,R4 = 470Ω D2,D4 = 1N4004
pensively. However, if you enjoy doing so and you perhaps R5 = 10Ω 1W D5 = 1N4001
already have the parts on hand (cannibalised from a ready- P1 = 100 k preset H IC1 = 555C
made unit?), then who’s to stop you? In addition, if you do it
Capacitors: Miscellaneous:
yourself you have the unique opportunity to make some- Re1 = 12-V-relay, e.g.,
C1 = 100nF
C2 = 100µF 16 V radial Siemens V23057-A2-A101
C3 = 470nF 250VAC, K1,K2 = 2-way PCB
D4 class X2 terminal block, lead pitch
C4 = 470µF 16V radial 7.5 mm
1N4004
D3

U CC 12V 400mW
former). Although the relay can switch up to 4 A, it is a good
R3 R4 K1 idea to keep the load current under 1 A, since the tracks on
470k 470k
230V
the circuit board cannot handle more than this. The circuit
D2 C3 R5
X2
10Ω
board layout shown here is anyway intended primarily as
1N4004 470n
250V
1W an example. Space for the lamp fitting has intentionally
D1
C4
been left in the middle of the circuit board.
470µ
16V 12V 400mW
Since the circuit is connected to the mains network, you
naturally have to give suitable regard to safety when fitting
U CC
it into a fixture. With an earthed fixture, ensure that the insu-
K2 lation clearance is at least 3 mm; for non-earthed fixtures it
RE1
must be at least 6 mm. Also fit a cover plate, to prevent
D5
P1 4 8 R2 accidental electrocution when replacing the bulb. Always
7
DIS
R
12V
S10K250
switch off the mains voltage before using P1 to adjust the
IC1 1N4001 light sensitivity!
100k 3
OUT
6
THR 555C In many cases it will be necessary to provide a small
2
TR
CV
(watertight!) window for the LDR, such that daylight can fall
R1
5 1 on it but it is screened from the light of the lamp. The shell
C2 C1
of a pilot lamp can often be used for this purpose. Use a
LDR
100µ
16V
100n medium-sized, encased LDR (approximately 1 cm diame-
014040 - 11
ter). Without a case, the LDR would not be able to achieve

24 Elektor Electronics 12/2001

 SMALL CIRCUITSCOLLECTION

1-040410 014040-1 014040-1

ROTKELE )C( (C) ELEKTOR
H1

H2

RE1 R2 K2 K1 ~ ~

R5
D5

R4
IC1

C3

C1

R3
C2
R1

C4
D3

P1
D1

D4
D2
H3

H4

high enough resistance in damp weather. voltage spikes when the load is switched off. Larger or
The relay is a Eurocard type with a coil voltage of 12 V. It smaller types can also be used, as long as they are suitable
draws around 37 mA. A 230-VAC VDR is connected across for 230-V operation. An S5K250 or S20K250 could thus be
the contacts. This is a voltage-dependent resistor that acts used instead.
somewhat like a bi-directional Zener diode and suppresses (014040-1)

Fundamental/Overtone
Crystal Oscillator 003
+5V +5V
The schematics show two crystal oscillator circuits con-
structed using the high-speed LT1394 comparator from Lin-
C1 C1
ear Technology (www.linear-tech.com). In each case the
100n 100n
crystal is in the positive feedback path, while in the negative
feedback path an RC network provides the necessary R1
X1
1...10MHz R1
X1
10...25MHz
phase lag.
2k2

2k2

Circuit (a) is designed to use AT-cut crystal oscillating IC1 IC1

at its fundamental frequency and is suitable for operation 2 1 2 1

7 7
over a frequency range from 1 MHz to 10 MHz. LT1394 R4 LT1394
3 3
22Ω

Circuit (b) includes the RC network consisting of R4 and 4 4

C3, which reduces the gain as the frequency rises by R3 R3

2k2 2k2
increasing the loading on the positive feedback path cor- R2
C2 C3
R2
C2
2k2

2k2

respondingly. In this way oscillations at higher multiples of 68n 220p

820p
the fundamental frequency are suppressed, although the
wanted third harmonic must not be unduly damped. This 014110 - 11 014110 - 12

circuit is designed to use AT-cut crystals between 10 MHz

and 25 MHz. (a) Q1 = AT-cut crystal for (b) Q1 = AT-cut third-overtone
(014110-1)
operation at a fundamental crystal for operation in the range
frequency in the range 1-10 MHz 10-25 MHz

12/2001 Elektor Electronics 25