Elektor - Circuits Collection 2000-2014.pdf

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SUMMER CIRCUITSCOLLECTION
001
T. Giesberts
Transmitter for Fibre-Optic IR Extender
This circuit restores the original modulation of the signal
received from the remote-control unit, which was demodulated
by the receiver unit at the other end of the extender (see
‘Receiver for fibre-optic IR extender’).
If no signal is received, the Toslink transmitter in the receiver
is active, so a High level is present at the output of the Toslink
receiver in this circuit. Buffer IC2a then indicates via LED D1
that the receiver unit is active. The received data are re-mod-
ulated using counter IC3, which is a 74HCT4040 since the
Toslink module has a TTL output. In the idle state, IC3 is held
continuously reset by IC1. The oscillator built around IC2c runs
free. When the output of the Toslink receiver goes Low, the
counter is allowed to count and a carrier frequency is gener-
ated. This frequency is determined by the oscillator frequency
and the selected division factor. Here, as with the receiver, we
assume the use of RC5 coding, so a combination has been cho-
sen that yields exactly 36 kHz. The oscillator frequency is
divided by 2
9
on pin 12 of the counter, and 18.432 MHz ÷ 2
9
=
36 kHz. The circuit board layout has a double row of contacts
to allow various division factors to be selected, in order to
make the circuit universal. You can thus select a suitable com-
bination for other standards, possibly along with using a dif-
ferent crystal frequency. The selected output is connected to
four inverters wired in parallel, which together deliver the
D3
H1
D4
R4
IC2
R3D2
C1
C2
H2
R1
1M
IC3
CTR12
6
C3
X1
15p
CT
10
0
1
9
7
6
5
3
2
4
13
12
14
15
1
13
L1
15
13
11
9
7
5
3
1
IC2b
36kHz: 5 - 6
3
5V
4
D3
D1
C6
5
IC2c
1
X1
R1
1
IC2d
+
J1
16
14
12
10
8
6
4
2
11
9
8
R4
D4
2
3
4
5
6
7
8
R2
J1
C3
1
IC2e
1
IC2f
1
C5
C7
IC3
IC1
004021-1
L1
C8 C4 IC4
ROTKELE )C(
C1
33p
18.432MHz
C2
33p
11
CT=0
39Ω
10
1-120400
9
10
11
5V
D5
H4
R5
12
+
0 D6
74HCT4040
5V
D1
1k
R3
H3
IC1
TORX173
3
47µH
IC2 = 74HCU04
D2
IC2a
1
1
C4
2
4
5
6
100n
1
2
R2
1k5
RECEIVER
ON
TRANSMITTING
1k
004021-1
(C) ELEKTOR
IC4
> 9V
D6
R5
14
16
7805
5V
1N4002
C8
220µ
25V
C7
D5
4µ7
63V
C6
C5
IC2
7
100n
IC3
8
100n
POWER
004021 - 11
18
Elektor Electronics
7-8/2000
SUMMER CIRCUITSCOLLECTION
drive current for the IR LEDs D3 and D4 (around 50 mA). A sig-
nal from the counter is also indicate that data are being trans-
mitted, via LED D2. This has essentially the opposite function
of LED D1, which goes out when D2 is blinking.
In the oscillator, capacitor C3 is used instead of the usual resis-
tor to compensate for the delay in IC2c. As a rule, this capac-
itor is needed above 6 MHz. It should have the same value as
C
load
of the crystal, or in other words 0.5C1 (where C1 = C2).
At lower frequencies, a 1kΩ to 2kΩ2 resistor can be used in
place of C3.
A yellow LED is used for the power-on indicator D5. The cur-
rent through this LED is somewhat higher than that of the
other LEDs. If you use a red high-efficiency LED instead, R5
can be increased to around 3kΩ3.
The circuit draws approximately 41 mA in the idle state when
the receiver is on. If the receiver is switched off, the transmit-
ter emits light continuously, and the current consumption rises
to around 67 mA.
The PCB shown here is unfortunately not available ready-
made through the Publishers’ Readers Services.
(004021-1)
COMPONENTS LIST
Resistors:
R1 = 1MΩ
R2 = 1kΩ5
R3,R5 = 1kΩ
R4 = 39
Ω
Capacitors:
C1,C2 = 33pF
C3 = 15pF
C4,C5,C6 = 100nF ceramic
C7 = 4µF7 63V radial
C8 = 220µF 25V radial
Inductors:
L1 = 47µH
Semiconductors:
D1 = high-efficiency LED, green
D2 = high-efficiency LED, red
D3,D4 = LD271
D5 = high-efficiency LED, yellow
D6 = 1N4002
IC1 = TORX173 (Toshiba)
IC2 = 74HCU04
IC3 = 74HCT4040
IC4 = 7805
Miscellaneous:
J1 = 16-way double contact row,
plus jumper
X1 = 18.432MHz quartz crystal
002
1...2V
ESR Measurements
T1 = BUZ10,
BUZ100,
BUK455
R1
1k
D
R2
1
Ω
5W
D
G
S
T1
G
D
S
C
Capacitor
Under Test
004069 - 11
K. Walraven
In a switch-mode power supply, the quality of the output volt-
age depends strongly on the quality of the electrolytic capac-
itors that are used. Here the effective series resistance (ESR),
which is the internal ‘ohmic’ impedance of the capacitor, is one
of the most important factors, since large currents are
involved. It is rather difficult to
exactly
measure the ESR, but if
you only want a quick idea, or you want to compare different
families of capacitors, the illustrated simple measurement
setup is very suitable.
The capacitor under test is charged via the 1-kΩ resistor (R1)
until its voltage is the same as the applied supply voltage. You
can then calculate the ESR using the formula (U1/U2) – 1 (see
trace 1 of the oscillogram). This formula is very simple, since
we have chosen a value of 1
Ω
for R2. The values of the sup-
ply voltage and R1 are not critical, since the measurement is
relative (as shown by the formula). The capacitor is discharged
via a 1-Ω resistor and a power FET. You would expect the dis-
charge to occur according to the well-known exponential for-
mula, but as you can see from the oscillogram, the voltage first
drops quickly, after which the expected exponential curve
appears. This fast initial drop is due to the ESR of the capaci-
tor, which produces a voltage drop equal to the product of the
resistance and the discharge current. The greater the voltage
drop, the poorer the capacitor.
If the initial voltage drop is approximately equal to half of the
charge voltage, as in this case, then we can deduce that the
ESR is approximately equal to the discharge resistance of 1
Ω.
This is actually a relatively good value for a small, inexpensive
10-µF electrolytic capacitor. As a general rule, you can assume
that the ESR decreases proportionally as the capacitance
increases, and that it also decreases slightly as the voltage on
the capacitor increases.
You can drive the FET directly from a pulse waveform genera-
tor, if it can deliver a short positive pulse to the gate with an
amplitude of at least 6 V (see trace 2). The repetition interval
must be 100 to 1000 times as long as the width of the positive
pulse, since the capacitor will otherwise not have enough time
7-8/2000
Elektor Electronics
19
SUMMER CIRCUITSCOLLECTION
to recharge. In most cases, you will only be able to obtain a
stable image on the oscilloscope if it has a storage function.
Almost any type of FET can be used, as long as the total dis-
charge resistance (the sum of R2 and R
ds
of the FET) is as
close as possible to 1
Ω.
(004069-1)
003
B. Kainka
Electronic Stethoscope
R1
470k
In order to listen to your heartbeat you would normally use a lis-
tening tube or stethoscope. This circuit uses a piezo sounder
from a musical greetings card or melody generator, as a micro-
phone. This transducer has an output signal in the order of
100 mV and its low frequency response is governed by the
input impedance of the amplifier. For this reason we have cho-
sen to use an emitter follower transistor amplifier. This has a
high input impedance and ensures that the transducer will
have a very low frequency response. At the output you just
need to connect a set of low impedance headphones to be able
to listen to your heartbeat.
Replacing the emitter follower with a Darlington transistor
configuration will further increase the input impedance of the
amplifier.
(004038)
T1
BC548
C1
BT1
100µ
Bz1
R2
470k
R3
1k
4V5
2x 32Ω
004038- 11

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