Category: Electronics Projects

Output frequency or tone of the project temperature controlled oscillator circuit varies with the temperature at which the input germanium diode is held. Reverse resistance of D1 varies from 500-Ω to 10 KΩ, when temperature varies between 200C and 800C. (At higher temperature germanium devices show very low resistance.) This variation of resistance changes the tone of the output sound of the oscillator which is built around the AF amplifier IC (IC1). Instead of germanium diode, base-emitter junction of an ordinary medium-power transistor such as AC128, AC188, 2N360 or 2N610…

The Project LED volume indicator circuit can be used in stereo amplifiers to indicate the balance level of volume. As the human ear is not much sensitive to differences in sound level, the LED volume indicator circuit indicates the difference in the sound level in any channel by means of LED. Input to the circuit is taken from the output terminals of each channel. The output signals are rectified by diodes D1 and D2, which are further filtered by capacitors C1 and C2. A DC voltage is developed across the…

What is Lie Detector Circuit? The instrument used to find whether a person is speaking the truth or not is called a lie detector or polygraph. It records the heartbeats, blood pressure, and body temperature, and tension level of the person undergoing the detector test. General questions along with other relevant questions are asked to the subject. When the person tries to speak lies, his body physiology changes. These changes are recorded and compared with the results that are obtained for normal questions. Such sophisticated instruments can however give wrong…

The telephone spy circuit published here is used to hear telephone conversations remotely or to amplify the telephone conversations and also transmits the speech signals in the MW range so that one can listen to a telephonic conversation on a radio set. The device picks up telephone conversations remotely if the telephone set is in the range of a few meters to some tens of meters, the device then amplifies the received signal and plays it through the speaker, and also transmits the signal as a Medium Wave signal. The…