Simple Dual Audio-Signal Tracer Circuit Diagram

The circuit described here is of a dual audio-signal tracer. An audio-signal tracer is a simple device used to troubleshoot for audio signals in radio and other electronic circuitry.

The device is a battery-powered stereo amplifier with two loudspeakers, packaged into a small handheld unit with test probes. An optional diode detector can be incorporated for the detection of amplitude-modulated signals. A test signal (audio) is injected into the tracer at CON1/CON2 from various points of the circuit under test. So long as the signal is heard, the circuitry up to that point is (at least minimally) functional. However, if the signal disappears, a fault can be assumed to be present in the stage of the circuit just preceding it.

Fig. 1: Circuit diagram of the dual audio-signal tracer

The simple external diode detector is not only sensitive to amplitude modulation but even circuits that are normally used for other modulation schemes (such as FM radios) can be tested in some cases.

Circuit and working
Fig. 1 shows the circuit diagram of the dual audio-signal tracer. It is built around LM358 (IC1) and TDA2822M (IC2). LM358 has two independent, high-gain, internal-frequency-compensated operational amplifiers, which are designed specifically to operate from a single power supply over a wide range (3V to 32V) of voltages.


TDA2822M is a low-power stereo amplifier. Its outputs can be directly coupled with the speakers through decoupling capacitors. The tracer is sensitive enough to capture signals from most microphones.

The relatively-high input resistance of 1-mega-ohm (VR1 and VR2) makes it possible to capture audible signals from many tone-control circuits and filters. Input signals are applied to connectors CON1 and CON2 and amplified by the two amplifiers in IC1.

Both amplifiers (channels) are completely independent. Resistor R3 and diodes D2 and D3 protect the input of the first amplifier. Similarly, resistor R4 and diodes D4 and D5 protect the input of the second amplifier. Resistors R1 and R2, diode D1 and capacitor C1 provide the necessary bias DC voltage for the operational amplifiers in IC1.

Potentiometers VR1 and VR2 adjust the volume for the first and second channel, respectively. Op-amps of IC1 are configured as non-inverting amplifiers with gain fixed at ten. Voltage gains can have any values starting from one, depending on the need. However, it is not a good idea to have voltage gains higher than 30 because then the circuit becomes too sensitive to electromagnetic fields and may produce a lot of noise in the loudspeakers.

IC2 works as a dual-channel output amplifier. Each amplifier works with internally-fixed gain of around 100. The amplifier can drive loudspeakers and headphones with resistance equal to or higher than 8-ohm.

Fig. 2: An actual-size, PCB layout of the dual audio-signal tracer

Fig. 3: Component layout of the PCB

Construction and testing
An actual-size, single-side PCB of the dual audio-signal tracer is shown in Fig. 2 and its component layout in Fig. 3. After assembling the circuit on a PCB, enclose it in a suitable plastic box.

The circuit works well off 6V battery; the preferred power supply is 6V and above. The circuit is appropriate for the signal tracer, as general-purpose dual audio amplifier and as low-power stereo amplifier. The output power can reach 1W over 4-ohm with 9V power supply, which is more than enough for listening to the produced sound in a small room.

Sourced By: EFY Author

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Simple Motion Detector Using NE555 Timer

This circuit is based on a passive infrared (PIR) sensor, which automatically switches on a device when someone comes close to it. It can be used for detection of theft or an unauthorised person entering a restricted area or building. It can also turn on lights when someone approaches the area where it is installed. Applications of this circuit include security systems, corridor lights and bathroom lights, among others.

 Fig. 1: Circuit diagram of the motion detector

Circuit and working

The circuit diagram of the motion detector using NE555 timer is shown in Fig. 1. It is built around 230V AC primary to 9V, 300mA secondary transformer X1, bridge rectifier DB107 (BR1), 6V voltage regulator 7806 (IC1), timer NE555 (IC2) and a few other components.

The 230V AC mains is stepped down to 9V, 300mA through step-down transformer X1. Secondary output of X1 goes to bridge rectifier BR1. Capacitor C1 filters the ripples and the rectified output is given to regulator 7806 (IC1). IC1 provides 6V regulated DC output to operate the circuit. LED1 is used as a power-on indicator.


IC2 is configured in monostable mode. Time period of IC2 is based on resistor R4 and capacitor C3, which is around ten minutes in this case. By changing resistor R4 and capacitor C3 you can change the time period of IC2.

Output pin 3 of IC2 is connected to the base of relay driver transistor T2. Contacts of relay RL1 are connected to the load, which could be a CFL or a bulb connected across CON2.

The PIR sensor is a pyroelectric device developed for detection of human body infrared radiations. It has a single output that goes high when a valid motion is detected. That is, the load is switched on whenever the PIR module senses a body in motion nearby.

Under normal conditions, transistor T1 is cut off and collector output is high around 6V. When motion is sensed, output pin (OUT) of the sensor becomes high, making transistor T1 to conduct for a few seconds, and voltage at its collector goes low momentarily. This low signal triggers IC2. Output pin 3 of IC2 goes high for around ten minutes, energising relay RL1 through transistor T2, turning on the load for ten minutes.

In brief, when someone comes in front of the PIR module, its output triggers IC2 to turn on the load. Thereafter it is disabled automatically.

Fig. 2: Actual-size PCB of the motion detector

Fig. 3: Component layout of the PCB

Construction and testing
An actual-size, single-side PCB for the motion detector using NE555 timer is shown in Fig. 2 and its component layout in Fig. 3. Enclose the PCB in a box and install it at a suitable location. Connect the PIR module across CON1.

Connect transformer X1 to 230V AC mains. Verify various test point voltages as given in the table to ensure proper working before using the circuit.

Sourced By: Efy  Author: Kumar Abhisekh

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