If - in the course of the test - you find that something is not working properly, then a careful inspection of the relevant area of the circuit board should turn up the cause of the problem. If the LED is not lighting - check that it's connected the right way round. Where you've cut the board to size - look for small loose strands of copper left behind by the saw. Check the board for short-circuits caused by component leads touching each other. It can also happen that the stripboard itself is faulty. I have seen cases where the copper tracks have not been completely severed from one another during manufacture.
If you've built your circuit using the specified components - and you've followed the step-by-step construction guide described on the Support Page
- then the chances are that any bug will be caused by something minor - a component connected the wrong way round - a missing or unwanted solder bridge - an incomplete cut in the track etc.
If you can't see anything obvious then adopt a systematic approach to faultfinding. Begin by double-checking that all of the cuts in the tracks have been made, that they are all - In The Right Place
- and that they sever the track completely. Use a magnifying glass - and backlight the board. It only takes the smallest strand of copper to cause a problem.
When you're satisfied that the tracks have been severed in all the right places, check that you have made - and correctly placed - all eight solder bridges. Mark each bridge with a felt-tip pen - or something similar - so that it can be easily identified later.
Next, carefully examine the full length of each track. If you backlight the board during the examination - it makes potential problem areas easier to spot. Look for unwanted solder bridges. Your felt-tip markings will tell you which ones should be there - and help you to identify any that shouldn't be there.
If all else fails and you still haven't found the cause of the problem - work your way through the assembly instructions on the Support Page
. Check each individual component and link - to make sure that it's present and correctly positioned.
Print out the drawings and mark off the components as you go. Take your time and examine each individual component carefully. Pay particular attention to the orientation of the diodes, transistors and electrolytic capacitors. Make sure that Q1 is a PNP transistor - and that the other three transistors are NPN.
If you'd prefer a more traditional approach to faultfinding - here are a few readings you should find helpful. The black figures are the voltages in standby mode - with the N/O switch open. And the red voltages are roughly what you should get - while the N/O switch is closed - and the siren is sounding.
Since the siren is switching on and off fairly quickly - it's unlikely that your meter will be able to react fast enough to produce stable readings - particularly at pins 2, 3, 6 & 7 of the IC. But temporarily increasing C4 to 100uF - will slow down the oscillations - and give the meter time to catch-up.