![]() The victim is then cross-questioned and, according to theory, the victim's skin resistance will then change, causing the bridge to go out of balance if he/she lies or shows any sign of emotional upset (embarrassment, etc.) when being questioned.įIGURE 4. In use, the victim makes firm contact with the probes and, once he/she has attained a relaxed state (in which the skin resistance reaches a stable value), RV1 is adjusted to set a null on the meter. The lie detector of Figure 4 is an experimenter's circuit, in which the victim is connected (via a pair of metal probes) into a Wheatstone bridge, formed by R1-RV1-Q1 and R3-R4 the 1 mA center-zero meter is used as a bridge-balance detector. By connecting the probe to suitable points in a radio, the tracer can thus be used to trouble-shoot a faulty radio, etc. The resulting audio signals are then amplified and heard in the earpiece. ![]() Any weak audio signals fed to the probe are directly amplified and heard in the earpiece, and any amplitude-modulated RF signals fed to the probe are demodulated by the non-linear action of Q1. When SW1 is switched to TRACE position 2, the Figure 3 circuit is configured as a cascaded pair of common-emitter amplifiers, with the probe input feeding to Q1 base, and Q2 output feeding into an earpiece or head-set. By choosing a suitable injection point, the injector can be used to trouble-shoot a defective radio. This waveform is rich in harmonics, so if it is injected into any AF or RF stage of an AM radio, it produces an audible output via the radio's loudspeaker, unless one of the radio's stages is faulty. When SW1 is in INJECT position 1, Q1 and Q2 are configured as a 1 kHz astable, and feed a good square wave into the probe terminal via R1-C1. Morse code practice oscillator.įigure 3 shows an astable multivibrator used as the basis of a "signal injector-tracer" item of test gear. The circuit can be used as a Morse code practice oscillator by using a Morse key as S1 the tone frequency can be changed by altering the C1 and/or C2 values.įIGURE 2. It can be used to generate a non-symmetrical 800 Hz waveform that produces a monotone audio signal in the loudspeaker when S1 is closed ( Figure 2). The astable multivibrator circuit has many practical uses. Thus, if RV1 is adjusted so that the signal output is amplified to this peak level, the noise peaks will not be able to greatly exceed the signal output, and intelligibility is greatly improved.įIGURE 1. Q1 amplifies both waveforms equally, but D1 and D2 automatically limit the peak-to-peak output swing of Q1 to about 1.2 V. Here, the signal-plus-noise waveform is fed to amplifier Q1 via RV1. This problem can often be overcome by using the noise limiter circuit in Figure 1. Unwanted electronic "noise" can be a great nuisance when listening to very weak broadcast signals, for example, peaks of background noise often completely swamp the broadcast signal, making it unintelligible. This final episode rounds off the "Transistor Cookbook" subject by presenting a miscellaneous collection of practical and useful transistor circuits and gadgets. The opening piece of this eight-part series described basic transistor principles and configurations ( Part 1) subsequent articles went on to describe a wide variety of practical transistor circuits ranging from common-collector amplifiers ( Part 2), common-emitter and common-base amplifiers ( Part 3), and small-signal audio amplifiers ( Part 4), to various practical oscillator ( Part 5), multivibrator waveform generator ( Part 6), and audio power amplifier ( Part 7) circuits.
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