Weird Science

Miniature Radio Transmitter: How It Works and How to Build One

Polish ver­sion is here

Lit­tle Spy

Sim­ple wire­less micro­pho­nes have long fasci­na­ted begin­ner elec­tro­nics enthu­sia­sts and radio ama­teurs. Many are easy to build, and suc­cess­fully get­ting them to work is highly rewar­ding. The minia­ture radio micro­phone pre­sen­ted here is a varia­tion of a design found on Czech elec­tro­nics web­si­tes.

The micro­phone's maxi­mum range does not exceed 30 meters (~98 ft), which sho­uld be suf­fi­cient for ini­tial expe­ri­ments with radio tran­s­mit­ters. It tran­s­mits on the 88 to 108 MHz band, making it pos­si­ble to rece­ive the signal using an ordi­nary FM radio rece­i­ver.

War­ning: Cur­rent regu­la­tions gover­ning the con­struc­tion of radio tran­s­mit­ting devi­ces, even those with low power, are very strict. Be sure to fami­lia­rize your­self with the rele­vant laws before con­duc­ting any expe­ri­ments in this field!

Con­struc­tion

You will need the fol­lo­wing com­po­nents:

Don’t be inti­mi­da­ted by this shop­ping list—all the com­po­nents nee­ded to build one tran­s­mit­ter are ine­xpen­sive and easy to find.

Assem­ble the com­po­nents accor­ding to the fol­lo­wing sche­ma­tic:

Ilustracja

The coils sho­uld be wound with the num­ber of turns spe­ci­fied in the sche­ma­tic, with a dia­me­ter of appro­xi­ma­tely 0.5 cm (~0.2 in). They can be conve­nien­tly wound on a 5mm (~0.2 in) drill bit. Align the coils so that their axes are col­li­near, with a spa­cing of 0.5 to 1 cm (~0.2 to ~0.4 in) between them.

The cir­cuit con­si­sts of two main func­tio­nal blocks. The first block, using the BC546 tran­s­i­stor, is a sim­ple low-fre­qu­ency ampli­fier that boo­sts the audio signal from the elec­tret micro­phone. The ampli­fied signal is then pas­sed thro­ugh a 100nF capa­ci­tor and a 100kΩ resi­stor to the second bloc­k—the actual radio tran­s­mit­ter. The tran­s­mit­ter is based on the BF199 tran­s­i­stor, con­fi­gu­red as a capa­ci­ti­vely cou­pled reso­nant cir­cuit known as a  Col­pitts oscil­la­tor. The oscil­la­tor's ope­ra­ting fre­qu­ency depends on the capa­ci­tance and induc­tance of the reso­nant cir­cuit. With the given com­po­nent values, it ope­ra­tes at appro­xi­ma­tely 100 MHz. A short length of wire can serve as the antenna.

To tune the cir­cuit, start by powe­ring on an FM radio rece­i­ver and set­ting it to a fre­qu­ency between the sta­tions bro­ad­ca­sting on the FM band. Then, power the tran­s­mit­ter. Adjust the coil in the tran­s­i­stor’s col­lec­tor cir­cuit by com­pres­sing or stret­ching it to fine-tune the tran­s­mit­ter to the desi­red fre­qu­ency. You sho­uld hear a distinct feed­back whi­stle as the tran­s­mit­ter’s signal is detec­ted. Be sure to move your hand away from the tran­s­mit­ter between adju­st­ments, as pro­xi­mity can affect its fre­qu­ency. Altho­ugh tuning may seem chal­len­ging due to the oscil­la­tor’s sen­si­ti­vity to small coil adju­st­ments, a lit­tle patience will yield good results. After tuning, the micro­phone is highly sen­si­ti­ve­—ca­pa­ble of func­tio­ning as a liste­ning device. Howe­ver, aside from ethi­cal con­cerns, using it in such a man­ner is stron­gly disco­u­ra­ged due to poten­tial legal con­se­qu­en­ces.

Fur­ther rea­dings:

Marek Ples

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