The one below is a past PCB work of mine, which was prepared to fit into a pocket flashlight. PCB drawing should be printed at 300DPI, here is a TIFF file already set. Also try to connect trimmer capacitor's moving part to the + side, which may help unwanted frequency shift while adjusting. Pay attention to the transistor's leads which should be connected right. Unfortunately I don't have any :(Įvery part should fit on the following PCB easily. An oscilloscope would make it easier, if you know how to use it in this case. Place radio far away from the circuit and re-adjust. If the reception on the radio lost in a few meters range, than it's probably caused by a wrong coil adjustment and you are in fact listening to a harmonic of the transmitter instead of the center frequency. Do not forget to fix inductance by pouring some glue onto it against external forces. You may get help of a experienced person on this point. Once it's OK, you may use trimmer capacitor to make further frequency adjustments. When you find the right point, adjust inductance's looseness and, if required, number of turns. Apply power to the circuit and put a iron rod into the inductance loops to chance it's value. Rest of the work is much dependent on your level of knowledge and experience on inductances: Have an FM radio near the circuit and set frequency where is no reception. Get an enameled copper wire of 0.5mm (AWG24) and round two loose loops having a diameter of 4-5mm. The critical part is the inductance L which should be handmade. Then note the value of the variable resistor and replace it with a fixed one. Not all of the condenser microphones are the same in electrical characteristics, so after operating the circuit, use a 10K variable resistance instead of the 5.6K, which supplies current to the internal amplifier of microphone, and adjust it to an optimum point where sound is best in amplitude and quality. Key to a well functioning, low consumption circuit is to use a high hFE / low Ceb (internal junction capacity) transistor. PN2222A, 2N2222A, BFxxx series, BC109B, C, and even well known BC238 runs perfect. Transistor is not a critical part of the circuit, but selecting a high frequency / low noise one contributes the sound quality and range of the transmitter. This way circuit especially valuable in amateur spy operations :) Transmitter even runs with standard NiCd rechargeable cells, for example a 750mAh AA size battery runs it about 500 hours (while it drags 1.4mA at 1.24V) which equals to 20 days. Main advantage of this circuit is that power supply is a 1.5Volts cell (any size) which makes it possible to fix PCB and the battery into very tight places. Simple FM Transmitter involves on a single transistor oscillator Frequency is determined by a parallel L-C resonance circuit and shifts very slow as battery drains out. circuit given below will serve as a durable and versatile FM transmitter till you break or crush it's PCB. Here is the last step of this evolution, the most stable, smallest, problemless, and energy saving champion of this race.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |