Parts list: Resistors R1 10k 1/4W metal film R2,5 5k6 " R3,4 22k " Semiconductors TR1 BC547 TR2 BC557 D1 1N4148 Miscellaneous 9 pin D connector 3.5mm mono socket PCB Transmitter connector - as required
Construction
Construction is simple. The component layout is shown above. Fit the resistors and diode first, then the transistors, making sure they protrude no more than 7mm above the PCB. Double check the transistors are the right way round. One is NPN the other PNP. Install the 3.5mm socket snug with the PCB. Bend the earth (middle) pin on this socket down to minimise the depth of the solder joint as the case is tight here.
Next fit the 9pin D socket. This is the one area that requires a little extra attention. First solder wires into pins 1,4,5 and trim to 10mm length. Leave pins 2,3 as these are not used. Bend the wires sharply away from the middle, at right angles, such that the wire does not protrude any further than the end of the socket pins. Then solder wires into pins 6 to 9, trim to 13mm length and bend at right angles 2.5mm from the end of the pins. Insert the wires into the PCB. Push the PCB right up the pins 1 to 5 and make sure that the end of the PCB butts right up the insulator around the pins with no more than 0.5mm gap. This will enable the board will fit snugly into the cover.
The cover needs minor trimming to accept the circuit board. One half of the cover has tabs which extend into the other half. With this half remove the internal web near the cable exit and the two little tabs next to these. A cut with a sharp knife at either end of the web and then twisting with a pair of pliers works fine. The two little 2mm tabs simple twist off. On the other case half just remove the internal web. Leave the two little tabs. The completed PCB assembly should fit like a glove. Give it a good test for fit in each half cover and test electrically before closing the covers. Once closed I do not know how to open them again!
