This project was found in a 1982 publication, using only two transistors. Signal injectors are instruments of great use in any repair shop or assembly of radios and amplifiers. The circuit given here takes two NPN transistors for general use and can be powered by 2 or 4 small batteries, that is, with 3 or 6 V. It is an astable multivibrator whose frequency is determined by the values of R2 and R3, as well as for the 4n7 capacitors. These capacitors can be changed to have lower or higher frequencies. The transistors used in the original design of the reader are AC 187, but more modern equivalents such as BC237, BC238 or BC547 can be used instead.
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