Free-Power AM Radio

Posted on Feb 5, 2014

Here`s something that might intrigue you Crystal Radio builders out there: A one-transistor (Silicon, yet!) AM broadcast receiver that derives its power from ambient AC hum and `spherics [static crashes]! Gotta give credit when I can: The germ of the schematic was borrowed from Darryl Boyd`s `Stay Tuned` website which features over 170 articles an

Free-Power AM Radio
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d plans on crystal radio construction. The URL for the site is and my No-Power - or, more accurately, Free-Power - circuit was derived from #2 of the 3 circuits found in the site`s project #153, "How to Build Free-Power Radios. " That website has three jpeg pages which are copies of the original article, by Terry L. Lyon, from an old Popular Electronics magazine issue. The 2nd page is the page containing the 3 schematics of single-transistor, free-power receivers. My version has some modifications of the original circuit, which I will describe below. Terry Lyons` original circuit was designed around a common-emitter amplifier, with emitter grounded; mine has a 100K ohm potentiometer inserted between the emitter and ground. The wiper of the pot connects to a tap on the antenna coil, 5 turns up from ground, with the pot determining how much of the emitter current is permitted to flow through the coil winding in a Hartley Oscillator feedback configuration, so that a fraction of the incoming RF signal is fed back into itself after having been boosted by the transistor. With enough battery voltage present, advancing the pot to full clockwise would connect the transistor`s emitter directly to the coil tap, causing the circuit gain to increase through positive feedback (regeneration) until the circuit "plops" into oscillation at the tuned frequency. In our "free-power" receiver, there isn`t enough voltage or current to achieve...

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