Simple Signal injector circuits using transistors

Do you know a signal injector circuit? It can be a tool or accessory for repairing television or radio receivers, amplifiers, etc. It will produce a small-frequency signal. It uses a 1.5V battery power supply.

This circuit is small, has few components, and is therefore easy to build.

For example, we are going to repair an amplifier when we feed it an audio signal with this simple circuit. If the amplifier is normal, we will immediately hear a loud sound coming out of the speaker.

Even checking signals at various points with an oscilloscope is easy. We can use it to check the vertical yoke driver of the old TV, or we can use it for many other applications.

Are you starting to get interested?

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How Signal Injector Circuit Works

Simple Signal injector circuits using transistors

In a simple signal injector circuit, as shown below, we used two transistors joined together to create an astable multivibrator form. The output is a pulse signal in a square waveform of about 1,000 Hz.

Read also: How does a transistor circuit work

The power supply voltage of this circuit is 1V to 5V. We recommend a AAA 1.5V battery; it makes the level amplitude voltage peak at the output lower than 1.2Vp-p. It is quite suitable for experimenting with general circuits such as amplifiers, radio receivers, and so on.

Creating this circuit

My daughter assembled this circuit on a breadboard and used a scope to examine its signal. There is a relatively square waveform signal with an amplitude of approximately 1.3 Vp-p at a frequency of approximately 856 Hz, close to 1 kHz.

If you really want to use it, you might build the PCB below. It was easy to create this project.

PCB layout of simple signal injector using transistor

How to Builds.

The Copper PCB layout and The components layout
You can see the PCB layout and the components layout in above cause you builds easily this projects.

Parts will you need
Q1,Q2: BC548, BC547, BC549, 2N3904, 45V 100mA NPN Transistor
C1,C2: 0.015uF 50V, Ceramic Capacitor
C3: 10uF 25V, Electrolytic Capacitors
R1,R4: 3.3K, 0.25W Resistors tolerance: 5%
R2,R3: 47K, 0.25W Resistors tolerance: 5%
And others.

Recommended: If you are a beginner, read Electronic components list with images.


Although this circuit is simple, it may make your work easier. It may also help in learning more about other electronic circuits. In addition, we may try to create circuits related to other measuring devices.


I always try to make Electronics Learning Easy.

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8 thoughts on “Simple Signal injector circuits using transistors”

  1. i have an audio component that was not working. brand National(AFD) Rx C51. i need circuit diagram all of this component. please help me.


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