www.joeknowselectronics.com
Joe Knows, Inc.
1930 Village Center Circle #3-8830
Las Vegas, NV 89134
R6
5.6 kΩ
C5
100 µF
Q1
2N3904
RESISTIVE TOUCH SENSOR................ 24
R5
10 kΩ
R4
10 kΩ
J1
2N7000G
V1
9V
C4
10 µF
R7
82 kΩ
R8
36 kΩ
POT1
1 kΩ
R9
1 kΩ
Q2
2N3904
C6
100 µF
How Circuits Work
© Copyright 2013
Joe Knows Electronics™
HOW
CIRCUITS
WORK
ASTABLE MULTIVIBRATOR.................. 27
CAPACITIVE TOUCH SENSOR.............. 31
FLIP-FLOP
R1
1 MΩ
R2
1 MΩ
R3
1 MΩ
C2
12-500pf
C3
12-500pf
C1
12-500pf
(BISTABLE MULTIVIBRATOR)............... 34
AUDIO AMPLIFIER............................... 38
AUDIO OSCILLATOR............................ 42
CONTENTS
CONTACT US........................................ 3
DATASHEET ACCESS............................. 4
V1
6 to 12 V DC power source
R4, R5
10 kohm resistors
R6
5.6 kohm resistor
R7
82 kohm resistor
R8
36 kohm resistor
R9
1 kohm resistor
VARIABLE VOLTAGE REGULATED
DC POWER SUPPLY.............................. 5
DIODE LIMITER................................... 10
VARIABLE CURRENT REGULATED
DC POWER SUPPLY............................ 13
VOLTAGE MULTIPLIER......................... 20
46
CONTACT US
Email
sales@joeknowselectronics.com
The best way to communicate with us is by
email. This allows us to have a record of your
correspondence and the ability to convey
information like technical specifications
more easily.
Toll Free Number
855-JOE-KNOW
(855-563-5669)
Someone is available 24/7 to take your call at our
toll free number. Your contact information will be
taken and a member of our management team
will contact you within normal business hours,
9am – 6pm EST M-F. Please note that our phone
associates don’t have the necessary information to
discuss products or orders but will direct your call
to the appropriate company representative.
AUDIO OSCILLATOR
Parts List
C1-C3
a triple gang variable capacitor where
each gang (segment) has the same
capacitance and that value can be varied
between about 10 and 500 pf. A search
on eBay usually brings up suitable units.
C4
10 uF 35V electrolytic capacitor
J1
2N7000G JFET transistor from kit
C5, C6
100 uF 35V electrolytic capacitors
Q1, Q2
2N3904 NPN bipolar transistor from kit
R1, R2, R3
1 Megohm resistors (1-5% tolerance)
www.joeknowselectronics.com
3
44
from the oscillator output (the loose wire
of C6), which when combined with the
C4-R5 integrator produces rather clean
sine waves at the output. Here clean
means that the output if primarily at
one frequency, with very few harmonics
or distortion. The oscillation frequency
can be adjusted from 100-5000 Hz by
adjusting the variable capacitor, but the
potentiometer will also provide a fine
adjustment to the frequency if you need
to set the frequency to better than 1% of
a desired value. The output voltage is a
2-4 volts peak to peak, depending on the
frequency and the load on the output.
DATASHEET
ACCESS
Access more information about this
product including datasheets by
scanning this QR code or visiting
wiki.joeknowselectronics.com
Find community driven content and
contribute your own projects at
wiki.joeknowselectronics.com
4
41
Wall Wart
12V
R1
10 kΩ
Q3
2N3904
C1
100 µF
Q1
2N3904
MJE182
R2
10 kΩ
R3
1 kΩ
D1
1N4148
C2
100 µF
R6
100 Ω
R5
100 Ω
R4, R5
2.2 ohm resistors
Q1,Q3
2N3904 NPN bipolar transistor from kit
R6
100 ohm potentiometer
Q2
2N3906 PNP bipolar transistor from kit
SPKR1
Low power 8 ohm speaker
R4
10 kΩ
V+
Gnd
43
VARIABLE VOLTAGE REGULATED
DC POWER SUPPLY
This power supply will be the foundation
for many of the projects described in this
kit. Powered by a 12 VDC wall wart, this
power supply will provide output voltages
ranging between 1.25 and 10.5 VDC as
R4 is adjusted. The maximum current that
can be supplied by the supply depends
mainly on the capacity of the wall wart,
but it is best not to draw more than about
one ampere. The voltage changes by less
than 0.2% as the driven load (attached to
V+ and Gnd) is changed from 10 to 1000
ohms. The output voltage also changes by
less than 0.5% for a 1 volt change in the
wall wart voltage.
5
The output voltage varies with the setting
of 10 kohm potentiometer R4. If the
lower potentiometer resistance is the
resistance between the potentiometer’s
wiper (tap) and the end closer to ground,
then the output voltage is given by the
graph opposite:
VARIABLE VOLTAGE REGULATED
DC POWER SUPPLY
Parts List
C1, C2
100 uF 20V electrolytic capacitors. The
capacitance is not critical, so feel free
to use what you have (within reason –
capacitors smaller than about 30 uF will
start to let ripple through.)
D1
1N4148 from kit
Q1, Q3
2N3904 transistors from kit
7
AUDIO OSCILLATOR
An audio oscillator is a very useful piece
of test equipment – and not only for
testing audio equipment. When combined
with the audio amplifier above, it can be
used to inject a signal into a wire, and the
amplifier used to determine if a circuit
still has continuity (or is shorted out),
or even just to identify wires that pass
through a cabinet or piece of equipment.
This is a phase shift oscillator, which
produces sine waves through positive
feedback through a tuned phase shifting
network consisting of C1-C3 and R1-R3.
Each stage of the network provides a 60
degree phase shift at a frequency that is
set by adjusting the triple gang variable
capacitor that contains C1, C2, and
C3. The three stages then produce 180
degrees of phase shift to the feedback
42
amp stage at the six volt level, which
increases the low-distortion dynamic
range of the amplifier by increasing the
symmetry of the circuit.
AUDIO AMPLIFIER
Parts List
C1, C3
470 uF 25V electrolytic capacitors
R1
1 kohm resistor
C2
220 uF 25V electrolytic capacitors
R2
4.7 kohm resistor
C4
1 uF 25V unpolarized capacitor
R3
180 ohm resistor
D1,D2
1N4148 from kit
39
D1, D2
5mm 20 mA red LEDs
(similar to LTL-307EE)
Q1, Q2
2N3904 transistors from kit
R1, R2
470 ohm 0.5 watt resistors
R3, R4
10 kohm resistors
R5
100 ohm resistor
S1
normally open pushbutton switch
(shorting a pair of wires works also)
V1
9 VDC power supply or battery
DIODE LIMITER
This useful and simple circuit can be used
in a number of situations in which a small
regulated AC voltage is required. It gives
a nearly constant output of about 0.7
VAC (the diode turn-on voltage: see “How
Diodes Work”) for input voltages between
2 and 15 VAC. The circuit can only drive
relatively high impedance (>1000 ohm)
circuits without degrading the voltage
regulation. As the circuit clips off the larger
voltages, the output is a bit more like a
square wave than a sine wave, a fact that
must be considered in some applications.
If need be, a low pass filter can be used to
remove some of the harmonics, producing
a wave closer to a pure sine wave.
You can make a limiter that produces a
larger voltage by using a pair of LEDs
37
10
MJE 182
MJE 182 transistor from kit
R1, R2
10 kohm 0.5 watt resistors
R3
1 kohm 0.5 watt resistor
R4
10 kohm potentiometer (linear is best,
but an audio taper pot can be used)
R5, R6
100 ohm 0.5 watt resistors (will get
warm when the R4 is adjusted near zero)
Voltmeter
(a DMM (Digital MultiMeter) will do) to
monitor the output voltage is useful, as
the voltage does not depend linearly on
the setting of R4.
9
instead of the silicon diodes shown
below. Red LEDs will produce about 2.2
VAC output, green LEDs about 2.3 VAC,
yellow LEDs about 2.4 VAC, and blue
LEDs as much as 3.3 VAC. If you use the
1N6263 Schottky diodes, the limiting
voltage will be about 0.35-0.4 volts,
owing to their smaller cut-on voltage
(see “How Diodes Work”). If you try this,
be careful not to draw too much current
(no more than a milliamp or so), as these
diodes are a bit delicate.
By replacing D1 and D2 with several
diodes (or LEDs in series) the circuit
allows a choice of voltages without
rebuilding. For example, placing red,
green, and blue LEDs where D1
is and blue, red, and green LEDs for D2;
the measured voltage between red and
11
AUDIO AMPLIFIER
Here is a (non hi-fi) audio amplifier to
work with. Designed to work with audio
signals on the millivolt level, the voltage
gain of the amplifier is about 100. It
has a lower end cutoff (-6dB power) of
about 40 Hz and an upper end cutoff of
about 50 kHz. The lower end cutoff is
established by the R1*C2 time constant,
while the upper end cutoff is controlled
by the R4C4/2 time constant.
Following a single stage of amplification
(Q1), the signal enters a complementary
push-pull amplifier stage. The diodes
seem odd. But their effect is to ensure
that the voltage difference between the
bases of Q2 and Q3 is about 1.5 volts –
twice the cut-on voltage of the diodes.
This essentially centers the push-pull
38
A flip-flop is a circuit that has two stable
states, usually called on and off, or 1 and
0. The flip-flop shown in this circuit is a
traditional transistor-based circuit. (Today,
a flip-flop is usually made of logic gates.) It
has a toggle (the pushbutton switch) which
causes the circuit to flip from one stable
state to another. The LEDs signal which
half of the flip-flop is switched on. One
of the LEDs can be exchanged for a green
LED, to indicate a logical “1” state, but the
value of the current limiting resistor R1 or
R2 would have to be changed to perhaps
330 ohms to accommodate the larger
voltage drop across the green LED.
green on the left to and green and red on
the left will be 5.5V (2.2 + 2.3).
DIODE LIMITER
Parts List
D1, D2
1N4148 diodes from kit
R1
220 ohm 0.5 watt resistor
V1
AC voltage source between 2 and 15 VAC
FLIP-FLOP
Parts List
C1, C2
22 uF 15V or greater
electrolytic capacitors
0.
00
1,
0m
0.
90
0m
0.
80
0.
0m
70
m
0.
0
60
0.
0m
FineAdj.K
50
0.
0m
40
0m
30
0.
m
0.
0
20
0m
10
0.
0m
0.
.0
m
00
-1
m
.0
00
-2
14.00mA
16.00mA
18.00mA
20.00mA
22.00mA
24.00mA
26.00mA
28.00mA
30.00mA
32.00mA
34.00mA
33
Amperes
D1
5mm 20 mA red LED
(similar to LTL-307EE)
J1
J310G n-channel JFET from kit
Q1
2N3904 from kit
Q2
2N3906 from kit
R1
10 Mohm resistor R2
47 kohm resistor
R3
120 kohm resistor
R4
470 ohm resistor
R5
470 ohm 0.5 watt resistor
V1
12 volt power supply
0m
1,
10
0.
0m
35
VARIABLE CURRENT
REGULATED DC POWER SUPPLY
This constant current regulated power
supply is the complement to the Variable
Voltage Regulated DC Power Supply.
The supply will provide regulated DC
current at values from about 1-30 mA,
when powered by a 12VDC wall wart (the
circuit itself will operate properly with
input voltages from 3 to 15 volts).
FLIP-FLOP
(BISTABLE MULTIVIBRATOR)
There are two adjusting potentiometers
and a range switch, which are used to
set the output current. When the switch
is in the high current position, the
CoarseAdj potentiometer is bypassed,
and the current is set using the FineAdj
potentiometer, with the following results
(see opposite graph) (K is proportional to
the rotation of the potentiometer knob,
The reason a circuit for a flip-flop is
included here is to make a point: there
are no digital circuits. A “digital” circuit
is simply an analog circuit tortured into
appearing to have one or two stable
states. A simple example would be a PA
system on the edge of feedback. If you
whisper into the microphone, it works
fine. But add more input sound, and that
horrible howl induces cringes across the
hall. However, if the PA system is just
on the verge of feedback and the hall
is large, a moderate increase in sound
level can require several seconds to build
up into the characteristic howl. Digital
circuits usually use feedback to establish
distinct states, and to make the transition
between those states as fast as possible.
13
34
so that the resistance of the FineAdj
potentiometer is 0 when K=0, and 50
ohms when K=1).
To obtain smaller currents, change the
position of the switch to the low current
position. Now the CoarseAdj pot and
the FineAdj pot are in series. In this
configuration, the CoarseAdj pot is the
main current control, but the current
value can be trimmed precisely using the
FineAdj pot (see graph opposite).
The current changes by less than 0.1%
when the load of the circuit it feeds
changes from 10 to 1000 ohms. Clearly,
however, once the power supply needs
more than 12 volts to supply the load
with enough current, it will no longer
run in a regulated mode. You may be
wondering what happens if the output
15
0m
0.
10
1,
0m
1,
00
0.
CAPACITIVE TOUCH SENSOR
0.
0m
20
0.
0m
30
0.
0m
R1, R3
270 ohm 0.5 watt resistors
R2, R4
10 kohm 0.5 watt resistors
V1
5V wall wart, batteries,
or the variable voltage supply
29
-5.00mA
0.00mA
5.00mA
15.00mA
20.00mA
10.00mA
Amperes
31
25.00mA
-2
00
.0
m
-1
00
.0
m
0.
0m
10
CAPACITIVE TOUCH SENSOR
Parts List
C1
10 uF 20V (or more)
electrolytic capacitor
FineAdj.K
40
0.
0m
50
0.
0m
60
0.
0m
70
0.
0m
80
0.
0m
90
0.
0m
This touch sensor detects the tiny flow of
current (tripping at about 25 femtoamps
– about 150,000 electrons per second)
that flows from the static electricity
stored by the capacitance of your body
when you touch the sensor. The sensor
can be a wire, a plate, or anything
metallic, but should be insulated from
ground (cardboard with a patch of tapedon aluminum foil works well). When you
touch the sensor, the LED goes on, but
many other circuits can be substituted
for the LED.
terminals on the power supply are
shorted together. The short answer is
that nothing happens – the same limited
amount of current flows.
The actual amount of current flowing
depends on the gain of the transistor
used in the circuit. The graphs above
use a gain of 140, a typical value for
your 2N3904 transistors. However, the
actual gain may be rather different, so
we suggest that you use an ammeter
with this power supply (for most uses an
external DMM can serve.)
VARIABLE CURRENT REGULATED
DC POWER SUPPLY
Parts List
D1
1N5221B zener diode from your kit
17
R1
1.5 kohm resistor
CoarseAdj
1 kohm potentiometer
R2
50 ohm resistor (can use 47 ohm)
FineAdj
50 ohm potentiometer
SW1
Single Pole Double Throw (SPDT) switch
Q1
2N3904 NPN transistor from your kit V1
12 VDC wall wart
19
be used to cycle other circuits on and
off, and (adjusted to higher frequencies)
makes a useful square wave generator for
use in analyzing audio circuitry.
ASTABLE MULTIVIBRATOR
Parts List
C1, C2
47 uF electrolytic capacitors (If you
want to adjust the operating frequency
into the audio regime, these capacitors
will have much smaller values, and need
not be electrolytic.)
D1, D2
5mm 20 mA red LED
(similar to LTL-307EE)
Q1, Q2
2N3904 transistors from kit
28
ASTABLE MULTIVIBRATOR
VOLTAGE MULTIPLIER
An astable multivibrator is essentially
a pair of amplifiers, whose outputs
are connected to the other’s outputs
through RC delays. When the output
voltage charges the RC circuit enough,
the transistor that was on turns off, and
the one that was off turns on. This cycle
repeats endlessly, making this a simple
oscillator. Each half of the oscillator has
an RC time constants of its own (R2C1
and R4C2). In the circuit diagram these
are set equal, and give an oscillation
frequency of about 1.5 seconds. Either
of these can be changed – there is no
need for the on periods of both LEDs
to be equal, and the circuit itself will
work fine at least up through audio
frequencies. As shown here the LEDs will
blink alternately, but the same circuit can
This is a simple version of a half-wave
series voltage multiplier, sometimes
called a Villard cascade. Its purpose in
life is to convert lower AC voltage into
higher DC voltage without requiring a
transformer or a rectifying bridge. Such
circuits are used today in copiers, laser
printers, and bug zappers, but were most
prevalent in color TVs that used cathode
ray tubes for display. They provided the
very high voltage needed to operate such
tubes. Voltage multipliers are also used
in particle accelerators to generate the
accelerating voltage.
27
20
The circuit uses the unidirectional flow
of current through standard diodes to
provide charging paths for each of the
capacitors. As the AC current changes
polarity, various capacitors are charged
during different parts of the cycle. C1,
3, and 5 are all charged to (nearly) 12
volts, and are wired in series so that
they provide +35 volts DC at the 70VDC
output terminal. C2, 4, and 6 are similarly
charged to 12 volts, and are also in
series, serving to float the upper string of
capacitors at a voltage of 35 volts. These
voltages added together provide the 70
VDC output. There is surprisingly little
voltage ripple in the output.
In use, the circuit below will function
provided that the output of the circuit
is not providing more than about 100
microamps. However, if you need a
small, lightweight source for a bit of high
voltage in a circuit, a voltage multiplier
may well serve the need.
21
VOLTAGE MULTIPLIER
Parts List
C1-C6
1 uF capacitors with voltage rating over
35V. The value can vary a bit (factor
of two or so) from 1 uF, so long as all
capacitors have the same value.
D1-D6
1N4148 diodes
AC source
This can be a simple step-down
transformer. There are also wall warts
(though rare nowadays) that provide AC
voltage in the 6-24V range.
Q1
2N3904 from kit
Q2
2N3906 from kit
R1
1K ohm 0.5 watt resistor
R2
470 ohm 0.5 watt resistor
V1
6 volt DC power source – either the
variable power supply circuit included
herein or a 6V wall wart would work fine,
as would batteries.
Touch Sensor
Anything that you can touch both
conducting parts of at the same time. A
pair of wires taped to a board works fine,
as would a pair of screws in a dowel to
make a grip sensor.
26
RESISTIVE TOUCH SENSOR
This resistive touch sensor will respond
to a touch on the touch sensor that
bridges the leads with a brightly burning
LED. The complementary pair of
transistors (meaning that one is NPN
and the other PNP) amplifies the tiny
current flowing through your fingers (a
few microamps) until it is large enough to
light the LED. An external circuit can be
controlled with this circuit by replacing
the LED with a connection to an SCR
switch as described a bit later on.
RESISTIVE TOUCH SENSOR
Parts List
D1
5mm 20 mA red LED
(similar to LTL-307EE)
23
24