Four small battery-powered transistor alarm circuits - you can build yourself - using cheap off-the-shelf components.

Transistor Alarms - Test


Transistor Alarms Schematics The Rest of Ron's Circuits Write To Ron More Free-to-Use Circuits Circuit Exchange International


The prototypes of all four Alarms were built using only the Stripboard Layouts as a guide. So - if you have faithfully reproduced the layout - you will have a working circuit.

Once you're satisfied that your layout is correct - and you have made a careful and thorough check of the underside of the board - it's time to power-up the circuit and test its operation. This is always an anxious moment. If you construct a lot of circuits - you might consider building the Current Limiting Power Supply - or alternatively - you could add the Simple Current Limiter to your existing PSU. Both will let you set an upper limit on the amount of current supplied to your circuit - and so protect it from any serious damage.


All four alarms are tested in the same way. You can simulate the trigger-switches using short lengths of flexible multi-stranded wire - such as alarm cable. An LED and resistor is all that's needed to demonstrate that the Siren/Buzzer Output is working properly.

Details of How to Prepare The 
Transistor Alarm - For Testing

If You Find a Problem

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. If you are using transistors other than those specified - check the pin configuration. Just because they look like the specified transistors - do not assume that they have the same pin configuration.

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. It only takes the smallest strand of copper to cause a problem. If you backlight the board during the examination - it makes them easier to spot.

Carefully examine the full length of each track. Look for unwanted solder bridges. If you backlight the board during the examination - it makes potential problem areas easier to spot. In the case of Transistor Alarm {C} - check that you have made and correctly placed the one solder bridge next the relay coil terminal.

If all else fails and you still haven't found the cause of the problem - work your way through the assembly instructions on the 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. Pay particular attention to the orientation of the diodes and the electrolytic capacitor. Take your time and examine each individual component carefully.

Transistor Alarms Schematics The Rest of Ron's Circuits Write To Ron More Free-to-Use Circuits Circuit Exchange International

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