One Transistor Radio

Here is a simple circuit
for a one transistor Audion type radio powered by a 1.5 V battery. It
employs a set of standard low-impedance headphones with the headphone
socket wired so that the two sides are connected in series thus giving
an impedance of 64 Ω. The supply to the circuit also passes through the
headphones so that unplugging the headphones turns off the supply. Using
an Audion configuration means that the single transistor performs both
demodulation and amplification of the signal.

The sensitivity of this receiver is such that a 2 m length of wire
is all that is needed as an antenna. The tap on the antenna coil is at
1/5th of the total winding on the ferrite rod. For details of the
antenna coil see the article Diode Radio for Low Impedance Headphones.
This circuit is suitable for reception of all AM transmissions from
long-wave through to shortwave.

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Transistorised Code Lock With Torch

This electronic lock for
domestic use opens only when you connect the right combination of five
switches. There are twelve switches in total. If you connect a wrong
combination, the lock remains closed. At night, flip switch S3 to ‘on’
position in order to enable the torch. In the daytime, flip it back to
‘off’ position. Fig. 1 shows the circuit of the transistorised code lock
with torch. For easy understanding, the entire circuit can be divided
in three sections: power supply, control and torch.

Circuit diagram:

The original image is sized 768×302px. The power supply section is
built around transformer X1, bridge rectifier comprising diodes D1
through D4 and regulator IC 7812 (IC1). The 230V AC, 50Hz AC mains is
stepped down by transformer X1 to deliver a secondary output of 15V, 250
mA. The transformer output is rectified by the bridge rectifier,
filtered by capacitor C1 and regulated by IC1. Capacitor C2 bypasses the
ripples present in the regulated supply. When mains power is available,
IC1 provides regulated 12V to the circuit and power-on LED1 glows to
indicate that the circuit is enabled.

The control section is built around switches S1 through S12,
transistor T1 and relay RL1. Relay driver transistor T1 is used to
energise/de-energise the relay.The torch section is built around six
white LEDs (LED2 through LED7) and resistors
R3 and R4.Working of the circuit is simple. To open the door, you should
know the connection code. Here the connection code is switches S1, S7,
S2, S11 and S9. This means you need to connect theses witches to each
other by flipping them to ‘on’ position. As the connection completes,
transistor T1 conducts and relay RL1 energises. As a result, the

door lock connected between the pole and normally-open contacts of
relay RL1 opens.If you connect a wrong combination, say, switches S4,
S10, S11 and S6, transistor T1 does not conduct and relay RL1 remains
de-energised. As a result, the door lock remains closed. Assemble the
circuit on a generalpurpose PCB and house it in a small cabinet. Fig. 2 shows the proposed cabinet arrangement for switches and LEDs. Install the cabinet at the front door of your house.

As mentioned in the beginning, switch S3 is used to enable the torch. When it is flipped to ‘on’ position, all the LEDs
(LED2 through LED7) glow. Switches S4, S5, S6, S8, S10 and S12 are used
just to confuse the intruders and play no role in opening the door. You
can also use a 12V battery to power the circuit. In that case, remove
transformer X1, diodes D1 through D4, capacitor C1 and regulator IC2
(7812) and connect the battery inside the cabinet with proper polarity.

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Mains Manager

Very often we forget to
switch off the peripherals like monitor, scanner, and printer while
switching off our PC. The problem is that there are separate power
switches to turn the peripherals off. Normally, the peripherals are
connected to a single of those four-way trailing sockets that are
plugged into a single wall socket. If that socket is accessible, all the
devices could be switched off from there and none of the equipment used
will require any modification. Here is a mains manager circuit that
allows you to turn all the equipment on or off by just operating the
switch on any one of the devices; for example, when you switch off your
PC, the monitor as well as other equipment will get powered down

You may choose the main equipment to control other gadgets. The main
equipment is to be directly plugged into the master socket, while all
other equipment are to be connected via the slave socket. The mains
supply from the wall socket is to be connected to the input of the mains
manager circuit. The unit operates by sensing the current drawn by the
control equipment/load from the master socket. On sensing that the
control equipment is on, it powers up the other (slave) sockets. The
load on the master socket can be anywhere between 20 VA and 500 VA,
while the load on the slave sockets can be 60 VA to 1200 VA. During the
positive half cycle of the mains AC supply, diodes D4, D5, and D6 have a
voltage drop of about 1.8 volts when current is drawn from the master

Diode D7 carries the current during negative half cycles. Capacitor
C3, in series with diode D3, is connected across the diode combination
of D4 through D6, in addition to diode D7 as well as resistor R10. Thus
current pulses during positive half-cycles, charge up the capacitor to
1.8 volts via diode D3. This voltage is sufficient to hold transistor T2
in forward biased condition for about 200 ms even after the controlling
load on the master socket is switched off. When transistor T2 is ‘on’,
transistor T1 gets forward biased and is switched on. This, in turn,
triggers Triac 1, which then powers the slave loads. Capacitor C4 and
resistor R9 form a snubber network to ensure that the triac turns off
cleanly with an inductive load.

Circuit diagram:

Mains Manager Circuit

Mains Manager Circuit Diagram

LED1 indicates that the unit is operating. Capacitor C1 and zener
ZD1 are effectively in series across the mains. The resulting 15V pulses
across ZD1 are rectified by diode D2 and smoothened by capacitor C2 to
provide the necessary DC supply for the circuit around transistors T1
and T2. Resistor R3 is used to limit the switching-on surge current,
while resistor R1 serves as a bleeder for rapidly discharging capacitor
C1 when the unit is unplugged. LED1 glows whenever the unit is plugged
into the mains. Diode D1, in anti-parallel to LED1, carries the current
during the opposite half cycles. Don’t plug anything into the master or
slave sockets without testing the unit.

If possible, plug the unit into the mains via an earth leakage
circuit breaker. The mains LED1 should glow and the slave LED2 should
remain off. Now connect a table lamp to the master socket and switch it
‘on’. The lamp should operate as usual. The slave LED
should turn ‘on’ whenever the lamp plugged into slave socket is
switched on. Both lamps should be at full brightness without any
flicker. If so, the unit is working correctly and can be put into use.

The device connected to the master socket must have its power switch on
the primary side of the internal transformer. Some electronic equipment
have the power switch on the secondary side and hence these devices
continue to draw a small current from the mains even when switched off.
Thus such devices, if connected as the master, will not control the
slave units correctly.

Though this unit removes the power from the equipment being
controlled, it doesn’t provide isolation from the mains. So, before
working inside any equipment connected to this unit, it must be
unplugged from the socket.


Bipolar Transistor Tester

This tester is primarily
meant to test bipolar transistors. It can indicate the type of the
transistor as well as identify its base, collector and emitter pins. The
circuit is very simple. The direction of current flow from the
terminals of the transistor under test (TUT) is indicated by a pair of LEDs
(green-red). An npn transistor produces a red-green-red glow, while a
pnp transistor produces a green-red-green glow, depending on the test
point that connects to the terminal of the transistor. Emitter and
collector are differentiated by pressing pushbutton switch S1 that
actually increases the supply voltage of the circuit by about 5.1V.

At the heart of the circuit is IC CD4069 (IC3), which oscillates and
produces pulses required to test a pair of transistor leads for
conduction in both the directions. Different combinations are selected
by an arrangement of counter CD4040 (IC1) and bilateral switch CD4016
(IC2). Fig. 1 shows the circuit of the bipolar transistor tester. A pair
of LEDs is connected to each test point through which current flows in both the directions. Each LED corresponds to a particular direction. In this manner, both junctions of the transistor can be tested. The LEDs are arranged to indicate the type of the semiconductor across the p-n junction.

The counter is clocked by the AC generator built around gates N5 and N6. This makes the LEDs glow continuously for easy observation, revealing the direction of current flow between different test points. So if the red LED
connected to certain point glows, it means that n-type of the junction
is connected to that test point, and vice versa. Thusared-green-red glow
indicates npn type of the transistor, while a green-red-green glow
indicates a pnp transistor. From this observation, you can easily detect
the base.

Circuit of bipolar transistor tester

Circuit of bipolar transistor tester

Collector and emitter are differentiated based on the principle that
the base-emitter junction breaks down under reverse bias much more
easily than the base-collector junction. Thus under increased AC
voltage, you can easily see that the emitter conducts more in the
reverse direction (associated LED glows significantly) than the collector. Use of transparent or semi-transparent LEDs is recommended.

Adjust preset VR1 (2-mega-ohm) to get equal glow when any two test
points are shorted. Unregulated 15V-18V is regulated by the
zener-transistor combination to power the circuit. The testing procedure
is simple. Normally, the transistors can be plugged in any orientation
as they come in a variety of possible arrangements of base, collector
and emitter pins, such as CEB, BEC and CBE. Simply plug the TUT
in the possible combinations of three points. A red-green-red glow
means that it is npn transistor and the pin associated with green LED is base. To identify the emitter and collector, simply press switch S1 and observe green LEDs adjacent to already glowing red LEDs. The green LED glowing with a high intensity indicates the emitter side, while the low-intensity LED indicates the collector side.

Similarly, a green-red-green glow means that the transistor is pnp type and the pin associated with the red LED is the base. To identify the emitter and collector, simply press switch S1 and observe red LEDs associated with the already glowing green LEDs on the sides. The LED glowing with a high intensity indicates the emitter side, while the low-intensity LED indicates the collector side. Assemble the circuit on a general-purpose PCB and enclose in a small box. Keep the preset knob in the middle. In order to make it easy to plug the TUT, you can increase the number of test points as shown in the author’s prototype in Fig. 2.


In Circuit Transistor Checker

This simple circuit has
helped me out on many occasions. It is able to check transistors, in the
circuit, down to 40 ohms across the collector-base or base-emitter
junctions. It can also check the output power transistors on amplifier
circuits. Circuit operation is as follows. The 555 timer ( IC1 ) is set
up as a 12hz multi vibrator. The output on pin 3 drives the 4027
flip-flop ( IC2). This flip-flop divides the input frequency by two and
delivers complementary voltage outputs to pin 15 and 14. The outputs are
connected to LED1 and LED2 through the current limiting resistor R3.

Circuit diagram:

The LED’s are arranged so that when the polarity across the circuit is one way only one LED will light and when the polarity reverses the other LED will light, therefore when no transistor is connected to the tester the LED’s
will alternately flash. The IC2 outputs are also connected to resistors
R4 and R5 with the junction of these two resistors connected to the
base of the transistor being tested. With a good transistor connected to
the tester, the transistor will turn on and produce a short across the LED pair. If a good NPN transistor is connected then LED1 will flash by itself and if a good PNP transistor is connected then LED2 will flash by itself. If the transistor is open both LED’s will flash and if the transistor is shorted then neither LED will flash.


Cheap And Cheerful Transistor Tester

By using a simple visual
indicating system, this small transistor tester allows you to run a
quick ‘go/non-go’ check on NPN as well as PNP transistors. If the device under test is a working NPN then the green LED (D1) will flash, while the red counterpart will flash for a functional PNP device. However if the transistor is shorted, both LEDs will flash, and an open-circuit device will cause the LEDs to remain off. The circuit is based on just one CD4011B quad NAND gate IC, four passive parts and two LEDs. The fourth gate in the IC is not used and its inputs should be grounded.

Alternatively, you may want to connect its inputs and output in
parallel with IC1.C to increase its drive power to the transistor test
circuit. IC1.A and IC1.B together with R2, R3 and C1 form an oscillator
circuit that generates a low-frequency square wave at pin 4. This signal
is applied to the emitter of the transistor under test as well as to
inverter IC1.C. The inverted signal from IC1.C and the oscillator output
then drive the test circuit (LEDs, device under test, R1) in such a away that the voltage across that part of the circuit is effectively reversed all the time.

Cheap And Cheerful Transistor Tester Circuit

Cheap And Cheerful Transistor Tester Circuit Diagram

For example, with an NPN transistor under test, when pin 10 is High and pin 4, Low, current flows through LED
D1 and the forward biased transistor. However, no current will flow when
pins 10 and 4 change states, since the transistor is then
reverse-biased. The green LED, D1, will therefore flash at the rate determined by the oscillator. As you would expect to happen, a PNP transistor will be forward biased when pin 10 is Low and 4, High, enabling current to flow through the red LED in that case.

A supply rail of around 3 V (two series connected 1.5-V batteries)
should be adequate. To prevent damage to the transistor under test,
supply voltages higher than 4.5 V should not be used. Because the LED
currents are effectively limited to a few mA by the output of IC1.C
(also slightly dependent on the supply voltage), it is recommended to
use high-efficiency devices for D1 and D2.

Author: R. J. Gorkhali
Copyright: Elektor Electronics


SiC Bipolar Junction Transistors for Improving Hybrid Car Cooling System

SiC Bipolar Junction Transistors for Improving Hybrid Car Cooling System: the test rig of silicon carbide sic bipolar junction transistors thumbjpg

Since Silicon Carbide (SiC) transistors can stand on higher temperatures, it is possible to use those devices into the SiC Bipolar Junction Transistors for Improving Hybrid Car Cooling System: the test rig of silicon carbide sic bipolar junction transistors thumbjpg
standard internal combustion engine (ICE) water cooling system of a hybrid car which needs to be separated from the main cooling system. The article below shows that SiC bipolar junction transistors (SiC BJT) can be used for improving Hybrid Car Exiting Cooling System.

The figure performs a test rig circuit diagram (click to enlarge), a high quality
laboratory test for testing the SiC transistors and extract its
behaviour. It will simulate a working environment for a transistor in a
motor drive for Belt driven Alternator Starter (BAS) in a hybrid car of medium size, and this model uses three phase inverter of PSpice model of SiC BJT to simulate an inverter for 5 kW electric machine.

You will be taken into another sections of the article such as BitSiC (The SiC BJT type from TranSic company), modelling a BJT component in PSpice including the calculations, SiC Bipolar Junction Transistors Efficiency Results (with and without driver for different PWM frequencies), and The SiC BJT thermal calculation.

See conclusion and complete article of SiC Bipolar Junction Transistors for Improving Hybrid Car Cooling System here in pdf filetype of datasheet application (source:


Low-Noise Microphone Amplifier (OP270E)

The signal from a
microphone is two weak for a standard line input. This low-noise
DC-coupled microphone amplifier provides a solution for anyone who wants
to connect a microphone to his or her hi-fi installation. As can be seen
from the schematic diagram, a good circuit does not have to be complex.
A differential amplifier is built around T1 (MAT-03E), which is a low-noise dual transistor. The combination of T2 and LED
D1 forms a constant-current source for the input stage. A low-noise
opamp (OP-270E) amplifies the difference signal that appears at the
collectors of the dual transistor. The result is an analogue signal at
line level.

Low Noise Microphone Amplifier

Low-Noise Microphone Amplifier

The bandwidth of the amplifier ranges from 1 Hz to 20 kHz. Within
the audio range (20 Hz to 20 kHz), the distortion is less than 0.005
percent. Since only half of the OP-270E is used, the remaining opamp
could be used in the output stage of a stereo version. The amplifier can
be powered from a stabilized, symmetrical supply with a voltage between
±12 V and ±15 V. Such supply voltages are already present in many


Economical Transistor Radio

The schematic diagram
shows an audio stage with a common-collector circuit. This does not damp
the tuned circuit, but instead actually increases its response. This
yields good sensitivity and selectivity. Due to the low supply voltage,
the subsequent audio amplifier needs three transistor stages. The volume
is adjusted using the potentiometer. This radio works well using an
internal ferrite rod (around 1 cm diameter and 10 cm long) with a
winding of around 50 turns of enameled copper wire. With a two-meter
external wire aerial, you can receive even more stations. This radio is
not only economical in terms of components, it also needs very little
‘juice’: since the current consumption is only 10mA, an alkaline AA cell
will easily last for around 200 hours of operation.

The specifications, very briefly stated, are:

  • medium-wave receiver with ferrite aerial
  • optional supplementary aerial
  • power supply 1.5 V/10 mA
  • 4 transistors
    l* oudspeaker output
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Practical Circuit Design Technique in Boosting Current Limit of CLDs by Using Transistor Current Gain

Practical Circuit Design Technique in Boosting Current Limit of CLDs by Using Transistor Current Gain: current booster with npn transistor circuit  thumbjpg

If you find that such an impractical way by just adding the Current Limiting Diodes (CLD) to obtain high current level, Practical Circuit Design Technique in Boosting Current Limit of CLDs by Using Transistor Current Gain: current booster with npn transistor circuit  thumbjpg
the following article may help you regarding the informations within.
This article will describe you with a simplified description of
practical circuit design technique in boosting current limit by using
transistor current gain.

What you will find in this aplication datasheet article are sections that discuss about the practical current booster circuit technique
(conventional circuit using CLD, current boosting circuit technique),
the analysis of booster circuit principle, the implementation of circuit
considerations, and an exemplary experimental result with ilustration
using NPN CZT3055/ PNPCZT2955 transistors.

Also you will find current booster circuit diagrams for both NPN and PNP transistors. Finally-according to the article,
a booster circuit can be comprised of only one transistor, one CLD, and
two resistors. This circuit can boost the current of CLDs to much
higher levels limited only by the current capacity and current gain of
the transistor used.

The article was well-written by the respectfull of Sze Chin, Senior Applications Engineer for Central Semiconductor Corp. in Hauppauge, NY. See complete read of Practical Circuit Design Technique in Boosting Current Limit of CLDs by Using Transistor Current Gain within this application datasheet article (source: