It also analyzes transistor's characteristics such as voltage and gain. Frequency change with a varactor diode equipped tuned circuit is as simple as a voltage change.ĭirect SW Receiver for AM, AM-SSB & CW SignalsĮSR Meter / Capacitance / Inductance / Transistor Tester KitĮSR Meter kit is an amazing multimeter that measures ESR values, capacitance (100pF - 20,000uF), inductance, resistance (0.1 Ohm - 20 MOhm), tests many different types of transistors such as NPN, PNP, FETs, MOSFETs, Thyristors, SCRs, Triacs and many types of diodes. It allows more neighbor stations through and makes listening to either radio stations frustrating. A lower Q tuned circuit has a wider pass band. A tuned circuit with a higher Q has a narrow pass-band that makes it better able to pick out a station of many equally strong. Its disadvantages are a lower Q (quality), nonlinearity, lower voltage rating and a more limited capacitance range. Varactors are small and inexpensive, which makes their use advantageous in many applications. It provides an electrically controllable capacitance, which can be used in many different circuits. The varactor is also known as a variable capacitance or a varicap diode. This radio, as shown on Figure 1, uses a varactor diode instead of the usual mechanical rotary device. For those of you who would like to eliminate this mechanical component, here is a modern version of the classic detector set. Traditionally, in a crystal detector radio tuned circuits, a mechanical type variable capacitor is used. This is the reason why the CMOS version of the 555 timer performs much better than the standard bipolar, which has higher input bias current.Ĭrystal Detector Radio Receiver Set Varactor Varicap Capacitor Diode Tuned I had to invert the circuit and tie both high impedance analog pins, Threshold and Trigger to the radio signal input. If I used the standard approach and connected the input to the CV pin, the low impedance of the pin would prevent the circuit from receiving any radio signals. The 555 timer is configured as a pulse width modulator in a non-traditional configuration. So far this is exactly like an AM crystal radio. One end of the parallel configuration of L and C connects to an antenna (surprisingly long!) and the other end connects to a ground wire which is tied to the AC outlet ground (old books tell you to ground it to a water pipe). Here’s how the circuit works: The AM radio signal is tuned by inductor L, which is 300 turns of wire on a 1/2 inch diameter cardboard tube made out of a paper roll, along with the 100pF variable capacitor. You may be wondering how all this is accomplished with a 555. The tuning is accomplished with an inductor and a capacitor, and the LM555 acts as an AM demodulator and class-D power amplifier to drive the speaker. The only active device (silicon, germanium, or otherwise) is the LM555. ![]() 9V battery fits neatly inside a 30mm x 130mm long PVC tube.ĪM radio built around 555 timer chip. Insertion of earphones plug completes supply circuit and acts as an on/off switch. LM358 dual op-amp draws less than 1 ma so the battery drain is minimal. ![]() The two 10M resistors bias the detector diode and the op-amp input near mid-rail for better detector efficiency. Detector uses a biased 1N5711 (or similar) schottky diode with lowest forward-biassed voltage drop. Capacitance runs from 28 to 7pF which by formula gives a frequency range of 77 - 155MHz. Tuning capacitor is a 30pF Philips Beehive trimmer, with a short length of plastic tube glued - as a tuning shaft. The input tuned cct 'L' is a 2 turn loop, with 30mm diameter measured at 0.15uH on my LC Meter which intercepts RF directly as opposed to an LC cct fed with external aerial. Useful for listening to the pilot transmissions. This passive airband receiver is basically an amplified "crystal radio" designed to receive nearby aircraft transmissions on 121 - 133 MHz frequency.
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