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You are here: Home / Mini Projects / Build Simple Transistor Circuits

Build Simple Transistor Circuits

Last Updated on September 1, 2020 by Swagatam 31 Comments

A compilation of important assorted transistor simple circuits to build has been included here.

Simple Transistor Circuits for New Hobbyists

Many simple transistor configurations like, rain alarm, delay timer, set reset latch, crystal tester, light sensitive switch and many more have been discussed in this article.

In this compilation of simple transistor circuits (schematics) you will come across many small very important transistor configurations, especially designed and compiled for new budding electronic enthusiasts.

The simple circuits to build (schematics) shown below have very useful applications and are yet easy to build even for new electronic enthusiasts. Let’s begin discussing them:

Adjustable DC power supply:

Adjustable DC power supply

A very nice adjustable power supply unit may be built using just a couple of transistors and a few other passive components.

The circuit provides good load regulation, its maximum current being not more than 500mA, sufficient for most applications.

Rain Alarm

Rain Alarm circuit

This circuit is built around just two transistors as the main active components.

The configuration is in the form of a standard Darlington pair, which increases its current amplification capacity hugely.

Rain drops or water drops falling and bridging the base with the positive supply is enough to trigger the alarm.

Hum free power supply:

Hum free power supply circuit

For many audio amplifier circuits hum pick-ups can become a big nuisance, even proper grounding sometimes are unable to rectify this problem.

However, a high-power transistor and a few capacitors when connected as shown can definitely curb this problem and provide the required hum free and ripple free power to the entire circuit.

Set-Reset Latch:

 Set-Reset Latch circuit

This circuit also utilizes a very few components and will faithfully set and reset the relay and the output load according to the input commands.

Pressing the upper push switch energizes the circuit and the load, whereas it is deactivated by pressing the lower push button.

Simple Delay Timer

A very simple yet very effective timer circuit can be designed by incorporating just two transistors and other handful of components.

Pressing the push ON switch instantly charges the 1000uF capacitor and switching ON the transistors and the relay.
Even after releasing the switch the circuit holds on the position until C1 is completely discharged. The time delay is determined by the values of R1 and C1. In the present design it’s around 1 minute.

Crystal Tester:

Crystal Tester circuit

Crystals can be quite unfamiliar components especially with the electronic novices.

The shown circuit is basically a standard Colpitts oscillator incorporating a crystal to initiate its oscillations.

If the connected crystal is a good one, will be indicated through the illuminated bulb, a faulty crystal will keep the lamp shut.

Water Level Warning Indicator:

water level indicator circuit

No more peeping and nervous apprehensions with overflowing water tanks.

This circuit will produce a nice little buzzing sound well before you tank spills over.

Nothing can be as simple as this one. Keep watching for more of these little giants, I mean simple circuits to build with huge potentials.

Hand Stability Tester:

 Hand Stability Tester circuit

Pretty confident regarding your hand dexterity? The present circuit can definitely challenge you.

Build this circuit and just try sliding a constricted metal ring over the positive supply terminal without touching it.
A buzzing sound from the speaker will entitle you with “antsy hands”.

Light Sensitive Switch:

Light Sensitive Switch circuit
Parts List is Given Here

If you are interested to build a low-cost light dependent switch, then this circuit is just for you.

The idea is simple, a presence of light switches OFF the relay and the connected load, absence of light does exactly the opposite.

Need more explanations or help? Just keep posting your valuable comments (comments need moderation, may take time to appear).

Simple Tester Circuit

Passive testing of an electronic circuit appears pretty straightforward job. All you want is really a Ohm meter.

Sadly, still, working with this type of device for semiconductors is not really advisable. The output currents will probably harm semiconductor junctions.

The tester explained in this write-up is simple to construct and possesses the benefit that a maximum of around 50 µA can only be delivered in the circuit under test.

Therefore it may be used for the majority of standard IC's and semiconductors which includes MOS based elements. The indication is implemented through a little loudspeaker, to ensure that in the course of testing, it isn't required to keep on referring to the testing device rather than the concentrating on the test points.

The transistor T1 and T2 constitute a basic voltage controlled LF-oscillator, with a loudspeaker working like a load. The oscillator frequency is formed by C1, R1, R4 and the external resistance between the measuring leads. Resistor R3 is the collector resistance of T2; C2 behaves like a low frequency decoupling of this particular resistor.

As previously mentioned, the tester never will cause any sort of harm to the circuit under check; alternatively, it is best to include diodes D1 and D2 in order that the circuit under test is no way able to counter damage the tester parts. So long as you don't have an electrical interconnection between the testing prods, the circuit pulls absolutely no current. Battery-life can then be approximately same as the shelf life of the battery.

Car Fused Tail Lamp Indicator

For those who would like to be assured that the lamps on their automobile are in excellent order, this circuit is probably the remedy. It is quite basic and offers an honest indication any time a specific light fuses or stops working. With respect to the current drawn by the lamp L, a voltage drop develops around resistance Rx.

This voltage drop should result in being around 400 mV, which can help determine the value of R.. For instance, if it is the tail lights, where a pair of lamps of 10 W 12 V may be parallel, Rx may be worked out as given below:

The current may be expressed as P/V = 20/12 = 1.7 amps

Then Rx can be calculated as V / I = 0.4 / 1.67 = 0.24 Ohms

T2 can be a BC557

Due to the fact the 400 mV drop develops across RX, T1 is typically switched ON leading to T2 getting cut off. In case one of the tail lights blows out, the current by means of Rx is lowered by one half, which is 0.84 Amp. The voltage drop across Rx at this point results in being 0.84 x 0.24 = 0.2 V.

This voltage looks appreciably minimal to activate T1, which means this T2 now gets base current via R1, and the LED illuminates. To get a well-performing indication on lamps failure, it is suggested to make use of a single detector circuit for may be only a couple of lamps.

It is rather allowable, nonetheless, to use a single LED for a number of detectors: D1 and R3 work commonly to all sensors, and the collectors of all T2 transistors may be wired up with each other. R3 must be 470 Ohms for a 12 V circuitry and 220 Ohms for a 6 V procedure.

Simple Regulated Variable Power Supply

A very simple variable power supply with stabilized output can e built with just a couple of transistors as shown below:

Transistors T1 and T2 form a high current gain Darlington pair for controlling the output voltage. Since the design is basically an emitter follower, the emitter output follows the base voltage, which means varying the base voltage proportionately varies the emitter output voltage.

R1, along with the zener diode determines the base voltage of the Darlington which in turn provides the equivalent emitter output voltage.

R1 and the zener can be fixed as desired, by selecting the values as per the following date:

PCB Design for the above transistorized stabilized power supply can be seen in the following figure.

Simple 30 Watt Power Amplifier Circuit

This simple 30 watt fully transistorized amplifier circuit can be used for powering small speaker systems from USB or from mobile, Ipod music sources. The unit will provide great sounding amplified music output sufficient for any small room.

The distortion level for this 30 watt transistor amplifier circuit is highly reduced and the stability is awesome.

Capacitor C7 is positioned to make up for the phase shift from the output transistors. The value of R1 is decreased to 56 k, and supplemental decoupling, by means of a 47 k resistor and a I0 µF capacitor are placed in series with high potential side of R1 and power supply positive.

The output impedance is rather minimal, since T5/T7 and T6/T8 work like power darlingtons. The control amplifier stage is effectively competent at delivering the 1-V RMS input voltage.

Due to the reduced input sensitivity, the amplifier provides excellent stability and its level of sensitivity to hum is minimal. Significant negative feedback through R4 and R5 guarantees reduced distortion. Optimum allowable supply voltage is 42 V.

The power supply circuit must be designed as a stabilized power supply unit for the amplifier. Besides the heat sinks presented the 3nos 2N3055 transistors needs to be cooled down by clamping them on the metal cabinet using mica insulating washers. The power supply table is designed for stereo.

Electrical Specifications for the 30 watt amplifier circuit is given below:

Full parts list for the the above amplifier circuit

Car Interior Lights Delay OFF

When a vehicle trip begins after sunset, it is helpful to provide a system that can keep the interior lights on sometime after the doors have been locked, making it easy for the drivers to strap seat belts and turn the ignition key. A simple delay OFF circuit shown below can be used for implementing this function perfectly.

When the doors are shut, the door contact is opened, disconnecting the transistor base from the ground line vi D3. This breaks the ground bias for the pnp transistor. However, the relay still holds for sometime due to C1, which allows the BC557 base current to conduct via C1 and the relay coil, until eventually the C1 charges fully and shuts off the transistors and the relay.

7-Segment Display Light controller Circuit

Typical 7 Segment display currents should be restricted to approximately 25 mA, which is normally carried out through series resistors. When fitted with resistors, the display illumination cannot be any further altered. The circuit demonstrated here, alternatively, supplies the display from an adjustable voltage source built with an emitter follower circuit.

Display's LED illumination varies according to the adjustments of voltage controls P1 (coarse) and P2 (fine), approximately within 0 and 43 volts, the precise setting being somewhat crucial because of the diode characteristic of the LED.

While adjusting the display light, the voltage output is initially fixed at the minimum point, after that steadily increased attain the proper brightness.

The overall current for any 7-digit display must not go over around 1 amp to get a safe and sound segment current of 25 mA (7 segments at 25 mA for 6 digits). The selection of the series transistor (T1) is determined through its recommended dissipation spec.

Operating Relay with Lower Supply voltage

Once a relay is operated with the rated voltage, it is actually able to hold the activation even if the driving voltage is reduced considerably. With reduced voltage it allows the relay to perform optimally yet save power.

However, the initial voltage has to be close to the relay's specified voltage, otherwise the relay may not activate.

The circuit explained below allows the relay to switch ON from a lower than the rated supply by ensuring that at the switch ON the voltage is boosted through a diode/capacitor voltage doubler network. This boosted voltage provides the relay with the required higher initial supply. Once the activation is accomplished, the voltage drops to the lower value, enabling the relay to hold and work with a reduced economical power




Previous: 2 Simple Capacitance Meter Circuits Explained – Using IC 555 and IC 74121
Next: How to Understand IC 4017 Pinouts

About Swagatam

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials.
If you have any circuit related query, you may interact through comments, I'll be most happy to help!

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  1. Search Related Posts for Commenting

  2. Andrew says

    Please sir I really want to know the software you use to design and draw your circuit diagrams. They always look so clear and beautiful

    Reply
  3. George says

    Hi
    So i can only achieve the boosted using the PNP, my BC109 wont do this operation.
    please help
    Thank You in advance
    Kind Regards
    George

    Reply
    • Swagatam says

      that’s correct, you must build it exactly as given in the schematic to get proper results.

      Reply
  4. George says

    Hi
    iam trying to boost a 3.43V to 5V i want to use the 5v to trigger a relay in my circuit, i have tried using transitor BC 109 B but iam failing to get the desired result instead iam getting 0,2V when i switch on the 3.43V when i switch off iam getting 9V which is the voltage iam supplying to the transistor

    Reply
    • Swagatam says

      Hi, in the circuit under “Operating Relay with Lower Supply voltage” the shown transistor is a PNP, while BC109 is an NpN.

      Reply
  5. sureka says

    about adjustable dc supply i want a detailed explanation onWorking of the circuit and the Justification for the design used.

    Reply
    • Swagatam says

      https://www.homemade-circuits.com/simple-voltage-regulator-circuits-using-transistor-and-zener-diode/

      Reply
  6. username me says

    can u give a detailed explanation on the light sensitive switch and proper values of resistances to be used.

    Reply
    • Swagatam says

      You can find it here:

      https://www.homemade-circuits.com/how-to-make-light-activated-day-night/

      Reply
  7. Thomas Ferko says

    I have constructed a all permanent magnet motor. Four huge N 52 power
    block magnets rotate around and around. I would like to find a VERY SIMPLE way to light up a 3 volt led bulb with the setup. Any ideas using the rotary motion of the motor??

    Reply
    • Swagatam says

      Connect the bulb across the motor wires and rotate the motor manually or through a windmill..

      Reply
  8. Sriram says

    Hi, I required a circuit like applying a positive trigger at the base and getting the positive output from the collector. please help me with the circuit. (Required output amp is max 1Amp)

    Reply
    • Swagatam says

      Hi, you can do it by using the set-reset circuit example…ignore and eliminate the reset switch and R4 stages from the circuit.

      Reply
      • sriram says

        Could you please me how to calculate the R1, R2, R3 in that for a particular load like LED or relay?

        Reply
        • Swagatam says

          The formula is

          R = (Us – 0.6)hFE / Collector current,

          Where R = base resistor of the transistor,
          Us = Source or the trigger voltage to the base resistor,
          hFE = Forward current gain of the transistor,
          Collector current is the current consumed by the collector load

          But this is not critical you can simply use 10K for all the resistors, for 12V supply.

          Reply
  9. Mayank Padm Bhushan says

    Hello Sir,
    I want to make a tesla coil and I need its driver circuit. Can you please help me.

    Reply
    • Swagatam says

      Hello Mayank,

      It is available all over net, here’s an example diagram:

      https://www.homemade-circuits.com/tesla-coil/

      Reply
      • Mayank Padm Bhushan says

        But Sir, the transistor gets too hot in few seconds and if I use some other transistors like TIP122 and MJE3055 then it does not work. What can I do sir

        Reply
        • Swagatam says

          Hi Mayank, Are you getting the Tesla effect at the output? If you are getting the effect, then you can try reducing the 9V to 5V to reduce the heat, or try increasing the 22K to 56K

          Reply
          • Mayank Padm Bhushan says

            Sir, I am getting the effect when I use BC547 but the effect is very low and if I replace the transistor with TIP122 then there is no effect at all. TIP122 gets burned.

            Reply
            • Swagatam says

              Mayank, TIP122 is Darlington, so it may be getting burned due to high gain….
              BC547 is only 100mA so it will get hot. Please try 2n2222 which is rated at 800mA

  10. kalanda hassan says

    hi sir I want to build a remote control project for my university project can u please help me with simple circuit

    Reply
    • Swagatam says

      Hi Kalanda, is it an infrared remote control you are looking for? or is it an FM remote??

      Reply
  11. Unknown says

    Hello
    first of all thank you for the great efforts . i want to make printed circuit that have one application of transistor i don't want it to be not so simple and not so hard i am thinking about a circuit have (transistors -dc motor – leds – resistors ….) but i don't know how to ? can you help me applying my idea and if you have better idea that would also be great
    thanks in advance

    Reply
    • Swagatam says

      Hi, you can try the second design from this post

      https://homemade-circuits.com/2012/01/how-to-make-any-light-strobe-light.html

      replace the bulb with a DC motor

      Reply
  12. Dilipkumar THAKAR says

    Sir,can you pl.design SMPS of output 100A DC current and 52 V DC in three phase.

    Reply
    • Swagatam says

      Dilip, you can refer to the following concept

      https://homemade-circuits.com/2014/07/smps-welding-inverter-circuit.html

      Reply
  13. surya smart says

    hi swagatam, can u explain the water level warning indicator circuit.pls sir i am going to do project in my school exhibition

    Reply
    • Swagatam says

      Hi Surya,

      yo can try the following circuit:

      https://homemade-circuits.com/2012/04/make-this-water-level-indicator-circuit.html

      Reply
  14. Vianey Martinez says

    Hello,
    I am interested in building the Rain Alarm for my science project, and I was wondering if you could please elaborate in regards to how to build it. My partner and I will appreciate it immensely.

    Reply
    • Swagatam says

      Hello, please refer to the following post which explains everything in details:

      https://homemade-circuits.com/2012/01/how-to-build-simple-rain-sensor-circuit.html

      Reply


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