Simple Signal injector circuits using transistors

One of my friends is an electronic technician amateur in repairing of television or radio receivers. He said he is made a signal injector circuit to detect the signal in various sectors.

For example: in the sector amplifier, when the input signal of this circuit, if the amplifier is good, we can hear the sound from the speakers. Or we can check signal the following points by the oscilloscope more easily. Or we will apply to check the vertical yoke driver of the TV, too.

We have two circuits for you try to make them below.

1. Simple Signal injector circuits

When see in simple signal injector circuit as shown figure this below, he used two transistors join together to be astable multivibrator which have signal output frequency of 1,000Hz in square wave form.

this circuit used only the power supply of 1v to 5V so he has only battery aaa 1.5V, doing level amplitude voltage peak output lower than 1vp be safe for the circuit is tested by this tool.

The creation of this circuit is simply only composted all parts in the perforated board.

The schematic of this projects is easy!

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________________= 2 pcs.
C1,C2_________0.015uF 50V___ceramic Capacitors________________________________= 2 pcs.
C3____________10uF 25V_____Electrolytic Capacitors_____________________________= 2 pcs.
R1,R4________3.3K__1/4W Resistors tolerance: 5%_______________________________= 2 pcs.
R2,R3________47K__1/4W Resistors tolerance: 5%________________________________= 2 pcs.
And others.

2. 1KHz Signal injector for electronic technicians

A signal injector is important tools for the electronics technicians it small and cheap. This project is better the simple circuit because it has 1KHz and many frequency harmonic.

The normal signal injector has a square wave output about 1KHz. Which the square wave has lot of harmonic and up to many MHz so can be test a radio frequency section. While the fundamental frequency scale 1Khz, was used to test the frequency sound circuit as well.

The 1 Khz Frequency oscillator is generated to ON and OFF switch as flashing on range about 0.2 Hz. In Figure 1 show the completely the circuit diagram.

1KHz  Signal injector for electronic technicians

The 1KHz Signal injector circuit for electronic technicians

The oscillator circuit include of N1 and N2 are connected as the astable multivibrator circuit. This signal is used to drive a LED D3 through T1 to indicate the circuit power on.

The output of 1KHz oscillator is connected to buffer by both transistors T2 and T3, And the final signal is adjusted a level by P1. The maximum output voltage will be about the supply voltage (5.6 volts).

The Diodes D1 and D2 use to protects T2 and T3 from the external transient signal, The capacitor C6 is used to separate this circuit out of the tested DC signal.

If we will take this project to test the high voltage circuit (example the television), should use the voltage of C6 more than 1,000 volts.

the PCB layout and components layout
The copper PCB layout and the components layout

D1 and D2 must tolerate the good voltage and current as well. For the circuit of the power supply from 4 x 1.4 volts mercury battery or others. This circuit use current at 7mA only so long times. While working has LED1 show power on.

Parts will you need
IC1______CD4011___Quad 2-Input NAND Buffered B Series Gate ____________________= 1 pcs.
T1______BC558,BC557,BC559,2N3906____45V 100mA NPN Transistor___________________= 1 pcs.
T2,T3______BC548,BC547,BC549,2N3904____45V 100mA NPN Transistor________________= 2 pcs.
C2,C3_________0.47uF 50V___ceramic Capacitors________________________________= 2 pcs.
C4,C5_________100pF 50V___ceramic Capacitors________________________________= 2 pcs.
C6____________0.1uF 50V___ceramic Capacitors________________________________= 1 pcs.
C1____________100uF 25V_____Electrolytic Capacitors_____________________________= 1pcs.
R1,R2________10M__1/4W Resistors tolerance: 5%_______________________________= 2 pcs.
R3___________100K__1/4W Resistors tolerance: 5%_____________________________= 2 pcs.
R4___________470 ohms___1/4W Resistors tolerance: 5%__________________________= 2 pcs.
R5,R6________10M__1/4W Resistors tolerance: 5%_______________________________= 2 pcs.
R7__________27K__1/4W Resistors tolerance: 5%________________________________= 1 pcs.
P1__________1K____50K___Trimpot_______________________________________________= 1 pcs.
D1,D2________1N4148____75V 150mA Diodes_______________________________________= 2 pcs.

And others.


Friends. Instrumentation just is not enough. We see other circuits. You should make a better use.
1. Concentration test circuit

2. Simple transistor tester circuits

3. Morse Buzzer Alarm or Continuity Tester by IC 4011

4. Logic probe tester circuit


  1. RICK HODGSON June 18, 2012
  2. MR.OHM June 18, 2012
  3. Mr,OHM June 19, 2012
  4. Mr,OHM June 21, 2012
  5. bhavesh September 14, 2014
  6. bhavesh September 14, 2014
  7. chayapon September 17, 2014

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