|OddMix CIRCUITS - CRYSTAL RADIO|
Crystal Detector Radio Receiver Amplified Battery-less One Transistor Set
The radio described here is unique in that it allows quality, higher volume radio reception even in areas without any outside power. This germanium, point contact diode and point contact germanium transistor detector radio produces an amplified audio signal, which is louder than the usual audio output produced by single diode sets. It does that, however, at the expense of some extra work and components.
Figure 1 shows the schematic diagram for the basic crystal detector radio. This set uses a 200 uH coil on a 1/4-inch coil form with a tunable ferrite screw in it. Ferrite has higher magnetizing losses than air, but an air core coil would use a lot longer wire with a higher resistance, the ferrit coil will increase the overall receiver efficiency. The antenna coil could be made using the universal winding technique. C1 is a good quality, air dielectric rotary capacitor.
Heavier solid wires can also be used if the preferred Litz wire is not available. To better match the detector impedance to the tuned circuit, the D1 germanium point contact diode, marked crystal, is connected to the center tap on the coil arrangement via a switch. Use long wire, high antenna, tune in a station and set the switch to a position that produces a stronger sound. So far the radio is not unusual, but it is used as a base to develop the amplified version.
Parts list for one transistor crystal detector radio: L1 - Primary, Antenna coil - 200 uH C1 - Rotary Capacitor 500 pF C2 - Filter Capacitor - 1 nF, Disc, 6-10 VDC C3 - Electrolytic Capacitor - 47 uF 10 VDC D1 - Diode - Germanium, point contact Q1 - Transistor - Germanium, point contact
Figure 2 shows the schematic diagram of the transistor version of the amplified semi-reflex crystal detector radio. The low capacitance, germanium point contact transistor is connected to the D1 crystal diode via C3, a 47 micro-Farad electrolytic capacitor. Other small signal, germanium transistor may be used, experimentation required to find the best. A solid dielectric or a low leakage tantalum capacitor would be ideal in this circuit. A strong rectified audio signal would develop an operating voltage for transistor Q1, and the audio signal will appear stronger across the 2 Kilo-Ohms magnetic headphone set. The D1 crystal detector can be connected as shown in the diagram, or if it produces a stronger audio, it can be connected to the antenna terminal like it shown on Figure. 1.