@▷ Signal Injector and Tracer Circuit | Diagram for Schematic

Signal Injector and Tracer Circuit

Circuit Project Description

The circuit was designed to produce a simple test circuit for monitoring, tracing and locating any fault or defect in a radio or analog equipment and other electronic circuitry.

TerminologySingle Pole Double Throw (SPDT) – an electromagnetic switch that has common, normally open and normally closed connections; the common gets connected to the normally closed connection when the relay coil is de-energized while the common gets connected to the normally open connection when the relay coil is energizedBC109C – an epitaxial NPN silicon planar transistor having the collector electrically connected to the TO 18 metal case Receiver – an electronic device that converts incoming signals, in the form of microwave, electrical or radio, to a form that is perceivable by humans such as pictures, sound, light, navigational positions, digital data, measurement, and othersCircuit Explanation

To illustrate the operation of this circuit, it starts by injecting a test signal into the device that is to be tested. The signal can be followed around the various components of the radio receiver using the signal tracer. The circuit is believed to be working for as long as the signal is still being heard. The presence of a fault in the circuit is signified by the disappearance of the signal.

In this circuit, the signal tracer is a low noise, high gain amplifier, with a small speaker for audible monitoring while the signal injector is an audio oscillator that acts as the signal source in the absence of an input signal being acquired. To switch between the trace and inject mode, a single pole double throw (SPDT) switch is used. From the diagram, the switch is in contact with the earpiece circuit while connecting to the collector of the transistor, which means the state of the circuit is in trace mode. Both transistors are supplying high gain and both conducts as emitter follower. The 1 nF capacitor at the probe end provides blocking of DC because both the tracer and injector operate at AC source or a 9V or 12V battery source. Since the capacitor is in series with the signal, the energy is transferred between the by means of capacitive coupling of the two stages.

When the SPDT switch is turned to the other side, both transistors will be conducting as a generator of astable square wave. With this, there will be sufficient harmonics from audio up to hundred kilohertz, which can also be useful in investigating AM radio receivers. Since the circuit is a high gain type, proper shielding and grounding is necessary. It is advised to enclose the circuit in a metal casing or aluminum foil lined plastic case.

In general, a signal tracer is a simple device that is mostly powered by a battery which consists of a loudspeaker and an amplifier. The device is typically packed into a small, hand-held test probe and the detector is sensitive to amplitude modulation but can also be applied for testing circuits that are using other types of modulation such as FM radios. Signal tracers that utilize LED for displaying digital levels are considered sophisticated types.


This device will provide benefit and great troubleshooting for audio circuits from the input to the speaker. It has been designed to detect or inject into an audio circuit a specific signal during the repair and for easy fault or problem detection and tracing of equipments. The signal tracer may also be utilized as a simple monitor. They can be used for various purposes and applications such as monitoring signals in electronic devices, testing high frequency modulated signals in antenna, RF amplifier and IF section of a receiver, in audio systems, in electronic data processing, and repairing receivers and amplifiers.

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