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Joe Knows, Inc.1930 Village Center Circle #3-8830

Las Vegas, NV 89134

How Circuits Work © Copyright 2013

Joe Knows Electronics™

C3

12-5

00pf

V1

9 V C

410 µ

F

R7

82 k

ΩR

836 k

Ω

C5

100 µ

F

Q1

2N

3904

Q2

2N

3904

C6

100 µ

FJ1 2N

7000G

R3

1 M

ΩR

21 M

ΩR

11 M

ΩR

510 k

ΩR

410 k

ΩR

65.6

kΩ

PO

T1

1 k

ΩR

91 k

Ω

C2

12-5

00pf

C1

12-5

00pf

HOWCIRCUITS

WORK

RESISTIVE TOUCH SENSOR ............... 24

ASTABLE MULTIVIBRATOR ................. 27

CAPACITIVE TOUCH SENSOR ............. 31

FLIP-FLOP

(BISTABLE MULTIVIBRATOR) .............. 34

AUDIO AMPLIFIER .............................. 38

AUDIO OSCILLATOR ........................... 42

CONTENTSCONTACT US ....................................... 3

DATASHEET ACCESS ............................ 4

VARIABLE VOLTAGE REGULATED

DC POWER SUPPLY ............................. 5

DIODE LIMITER .................................. 10

VARIABLE CURRENT REGULATED

DC POWER SUPPLY ........................... 13

VOLTAGE MULTIPLIER ........................ 20

CONTACT USEmail 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.

www.joeknowselectronics.com

3

V16 to 12 V DC power sourceR4, R510 kohm resistorsR65.6 kohm resistorR782 kohm resistorR836 kohm resistorR91 kohm resistor

46

AUDIO OSCILLATORParts ListC1-C3a 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.C410 uF 35V electrolytic capacitor J12N7000G JFET transistor from kitC5, C6100 uF 35V electrolytic capacitors Q1, Q22N3904 NPN bipolar transistor from kitR1, R2, R31 Megohm resistors (1-5% tolerance)

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.

43

R4, R52.2 ohm resistorsQ1,Q32N3904 NPN bipolar transistor from kit R6100 ohm potentiometerQ22N3906 PNP bipolar transistor from kitSPKR1Low power 8 ohm speaker

41

DATASHEET ACCESSAccess 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

Wal

l W

art

12V

R1

10 k

ΩR

210 k

ΩD

1

1N

4148 Q

32N

3904

C1

100 µ

FQ1

2N

3904

MJE

182

R3

1 k

ΩR

5100 Ω

C2

100 µ

F

V+ Gnd

R6

100 Ω

R4

10 k

Ω

VARIABLE VOLTAGE REGULATED DC POWER SUPPLYThis 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 ListC1, C2100 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.) D11N4148 from kitQ1, Q32N3904 transistors from kit

7

AUDIO OSCILLATORAn 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 AMPLIFIERParts ListC1, C3470 uF 25V electrolytic capacitors R11 kohm resistorC2220 uF 25V electrolytic capacitors R24.7 kohm resistorC41 uF 25V unpolarized capacitor R3180 ohm resistorD1,D21N4148 from kit

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D1, D25mm 20 mA red LEDs (similar to LTL-307EE)Q1, Q22N3904 transistors from kitR1, R2470 ohm 0.5 watt resistors R3, R410 kohm resistorsR5100 ohm resistorS1normally open pushbutton switch (shorting a pair of wires works also)V19 VDC power supply or battery

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DIODE LIMITERThis 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

10

MJE 182MJE 182 transistor from kitR1, R210 kohm 0.5 watt resistorsR31 kohm 0.5 watt resistorR410 kohm potentiometer (linear is best, but an audio taper pot can be used)R5, R6100 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 withoutrebuilding. For example, placing red, green, and blue LEDs where D1is and blue, red, and green LEDs for D2; the measured voltage between red and

11

AUDIO AMPLIFIERHere 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

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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.

FLIP-FLOP Parts ListC1, C222 uF 15V or greater electrolytic capacitors

35

D15mm 20 mA red LED (similar to LTL-307EE)J1J310G n-channel JFET from kitQ12N3904 from kit Q22N3906 from kitR110 Mohm resistor R247 kohm resistorR3120 kohm resistor R4470 ohm resistorR5470 ohm 0.5 watt resistorV112 volt power supply

33

green on the left to and green and red on the left will be 5.5V (2.2 + 2.3).

DIODE LIMITERParts ListD1, D21N4148 diodes from kitR1220 ohm 0.5 watt resistorV1AC voltage source between 2 and 15 VAC

Amperes

34.0

0m

A

32.0

0m

A

30.0

0m

A

28.0

0m

A

26.0

0m

A

24.0

0m

A

22.0

0m

A

20.0

0m

A

18.0

0m

A

16.0

0m

A

14.0

0m

A-2

00.0

m

-100

.0m

0.0

m

100

.0m

200

.0m

300

.0m

500

.0m

600

.0m

700

.0m

800

.0m

900

.0m

1,0

00.0

m

1,1

00.0

m

Fine

Adj.

K

400

.0m

VARIABLE CURRENT REGULATED DC POWER SUPPLYThis 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).

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,

13

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

FLIP-FLOP (BISTABLE MULTIVIBRATOR)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.

34

CAPACITIVE TOUCH SENSORThis 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 taped-on aluminum foil works well). When you touch the sensor, the LED goes on, but many other circuits can be substituted for the LED.

CAPACITIVE TOUCH SENSORParts ListC110 uF 20V (or more) electrolytic capacitor

31

R1, R3270 ohm 0.5 watt resistorsR2, R410 kohm 0.5 watt resistorsV15V wall wart, batteries, or the variable voltage supply

29

Amperes

25.0

0m

A

20.0

0m

A

15.0

0m

A

10.0

0m

A

5.0

0m

A

0.0

0m

A

-5.0

0m

A -2

00.0

m

-100

.0m

0.0m

100.

0m

200.

0m

300.

0m

500.

0m

600.

0m

700.

0m

800.

0m

900.

0m

1,00

0.0m

1,10

0.0m

Fine

Adj.

K

400.

0m

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 SUPPLYParts ListD11N5221B zener diode from your kit

17

R11.5 kohm resistorCoarseAdj1 kohm potentiometer R250 ohm resistor (can use 47 ohm)FineAdj50 ohm potentiometer SW1Single Pole Double Throw (SPDT) switchQ12N3904 NPN transistor from your kit V112 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 MULTIVIBRATORParts ListC1, C247 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, D25mm 20 mA red LED (similar to LTL-307EE)Q1, Q22N3904 transistors from kit

28

ASTABLE MULTIVIBRATORAn 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

27

VOLTAGE MULTIPLIERThis 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.

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

20

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.

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Q12N3904 from kitQ22N3906 from kitR11K ohm 0.5 watt resistorR2470 ohm 0.5 watt resistorV16 volt DC power source – either the variable power supply circuit included herein or a 6V wall wart would work fine, as would batteries.Touch SensorAnything 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.

VOLTAGE MULTIPLIERParts ListC1-C61 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-D61N4148 diodes AC sourceThis can be a simple step-down transformer. There are also wall warts (though rare nowadays) that provide AC voltage in the 6-24V range.

23

RESISTIVE TOUCH SENSORThis 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 SENSORParts ListD15mm 20 mA red LED (similar to LTL-307EE)

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