Lack of space prevented us from describing the power supply circuit for the Speed Alarm last month, so we’ll give a brief rundown on how this works before moving on to the construction. You will need to refer back to Fig.1 on page 19 of the November issue to see the circuit details.

As shown, a +12V rail is derived from the vehicle’s battery via the ignition switch. A 10W 1W resistor and 47mF capacitor decouple the supply, while 16V zener diode ZD1 protects the circuit from transient voltage spikes above 16V.

The decoupled ignition supply voltage is fed to regulator REG1, which provides a +5V rail. This rail is then used to power all the circuitry with the exception of IC2 which is powered directly from the decoupled +12V rail. A second 47mF capacitor plus several 0.1mF capacitors are used to decouple the regula-tor’s output.

OK, so much for the circuitry. Of course, most of the clever stuff takes place inside the PIC16F84 micro-controller under software control. For a broad overview of how this software works, take a look at the accompanying panel.

Construction

Fortunately, you don’t have to understand how the software works to build this project. Instead, you just buy the ready-programmed PIC chip and "plug it in".

As mentioned earlier, all the parts (except for the piezo alarm and the speed sensor) are installed on two small PC boards. These are coded 05310991 and 05310992 and are stacked together using pin headers and IC sockets.

Before installing any of the parts, check both boards care-fully for etching defects. In particular, note that a hole is required just below DISPLAY3 on the display board (05310991). This must be large enough to accept a small screwdriver so that you can later adjust VR1 on the processor board.

Fig.4 shows the assembly details. We suggest that you assemble the processor board (05310992) first. Begin by installing the seven wire links, then install the resistors, diodes D3 & D4 and zener diode ZD1. Make sure that all the diodes are oriented correctly and note that the seven 150W resistors are all mounted end on.

Fig.4: install the parts on the two PC boards as shown here. Note particularly the orientation of the three pushbutton switches and switch S4. The terminals of S4 must be oriented as shown.

Next solder in a socket for IC1 (but don’t install the IC just yet), then solder IC2 in place. Take care to ensure that pin 1 of IC2 is nearest to the lefthand side of the board.

REG1 can now be installed with its metal tab flat against the board and with its leads bent at rightangles so that they pass through their respective mounting holes. Note that the hole in its metal tab must accurately align with the board mounting hole – see Fig.4.

Next, install the capacitors in the locations shown. Watch the orientation of the electrolytic types and note the following points:

(1) the two 47mF capacitors below REG1 have their leads bent at right-angles, so that the capacitors lie horizontally over REG1’s leads (see photo);

(2) the 47mF capacitor to the left of D3 & D4 has its leads bent at right-angles, so that it lies across these two diodes.