Circuit knowledge

Small Single Chip FM Transmitter Circuit Schematic Diagram

The theoretical circuit is given in Fig. 1 and consists of a Colpitt’s oscillator using 8MHz crystals. Six channels are shown in the schematics—three, in fact, `T were used for the prototype but there is no reason why many crystals cannot be included by using a suitable multiway switch and increasing the number of islands on the board; using the smaller HC25 series crystals would permit more channels to be fitted in the space allotted. The trimmers in series with each crystal allow easy netting to the assigned frequency. The f.m. is applied to the oscillator by a reactance stage, fed by two audio pre-amps. Deviation is con- trolled by a 10kQ potentiometer and the maximum attained on the prototype was 8kHz. Notice the inclusion of decoupling in the audio stages to prevent r.f. pickup so often a cause of poor audio quality in home·constructed equipment. The printed board layout is shown in Fig. 2.

Making PCBs

 Cut a piece of single-sided copper board to the size shown and with some {ine abrasive paper, clean the copper surface to remove any oxide or tarnish.

Using a soft, lead pencil, draw out the islands on the board, and then draw around these and the inter- connections of the earth plane edge. The small islands and fine connections are then filled in by means of an etch-resist pen or ine paint brush, using quick drying paint, such as car touch-up paint, thinned down if necessary. The larger areas are then put in care- fully and when the board is dry, each island and connection examined to make sure no copper bridges exist between them. One should also ensure adequate clearances.

Place the board in a suitable plastic or earthen- ware container and pour on just sufficient ferric chloride solution as is necessary to cover it. The solution can be purchased ready-mixed from most radio component stores, or can be made up by a chemist. It is however a corrosive, albeit a mild one, so handle carefully and wash off any of the solution that comes into contact with the skin immediately.

 Initially, leave the board submerged for about twenty minutes, agitating occasionally. You will see the chemical action taking place quite clearly and when all the unwanted copper has been eroded, take out the p.c.b., wash in clean water and then dry. Using a wet abrasive pad such as a Brillo pad- the paint is now removed and a final wash and dry will leave the copper gleaming. After a final check of the work, drill the mounting holes for fixing to the metal chassis.

Each board in the transmitter is etched in this way and provided the simple instructions are followed you should easily be able to provide good examples.

Mounting Components

There is no hard-and-fast rule about fixing the components to the board, but the Author favours soldering the resistors first, followed by the capacitors, the coils and finally the transistors. Keep lead lengths  short typically 6-12mm for transistors and solder neatly, holding the iron in place just long enough for the solder to flow to the joint. An iron of 15W rating with a bit size of 3mm or so is to be preferred for work of this nature.

Testing the long range transmitter circuit

Connect a 15 volt supply to the board having first established that the polarity is correct and check the voltages shown: a 15 per cent error is quite acceptable, due to component tolerances. With a 6009 microphone and a pair of earphones across C11 to the earth line, check for clean audio and the operation of the deviation control. The oscillator can be tested by connecting a suitable 8MHz crystal in position (i.e. 8-08335MHz for S20-145-5MHz) and listening for the 8MHz signal on a tunable h.f. receiver, coupled loosely to the vicinity of the oscillator stage.

After this feed an audio signal at the input and try to receive this over any standard 2 mtere band receiver unit placed at about 10 meters distance.

With little trial and error you would be able to receive a crystal reception, done! Now you can take the unit to some other far away location and confirm the same.