This is a DIY - vehicle anti-hijack alarm circuit.

COMMENTS SUGGESTIONS	Hijack Alarm No.3 - Testing Your Circuit	MORE VEHICLE ALARMS

Schematic Diagram	The Rest of Ron's Circuits	Write To Ron	More Free-to-Use Circuits	Circuit Exchange International

Hijack Alarm No.3 - Construction Guide

Introduction

The prototype of Hijack Alarm No.3 was built using only the Stripboard Layout as a guide. So - if you have faithfully reproduced that 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.

Setup

A resistor and an LED are all that's needed to demonstrate that the Buzzer output is working properly - and the same arrangement can de used to simulate the Siren. A couple of pairs of wires will serve as the Door and Reset switches - and for the Ignition - you can use any small single-pole switch.

Click Here For A Photograph Of The Prototype.

Connect an LED - in series with a 2k2 resistor - across the buzzer terminals. Do the same across the siren terminals - and connect an LED to the LED terminals. Connect a 2k2 resistor between the positive terminal and the Pole terminal. And connect two LEDs from the N/O and N/C terminals to the negative terminal. ******* I used five different colour LEDs in the diagram because I want to be able to distinguish between them. But you can use whatever colour LEDs you have available. ******* Next, connect the Ignition Switch between the positive terminal and the "Ignit" terminal. Move the switch to the "Off" position - and leave it there. ******* Connect two pairs of wires to the Door and Reset terminals. Strip the ends of the wires - but do not allow them to touch each other. Again, I made the pairs different colours so that I can distinguish between them. You don't have to. ******* Finally, connect the DC supply to the input terminals. Pay particular attention to the polarity of the supply. Note that the positive connection goes to the top terminal. ******* Turn On The Power ******* When you turn on the supply all the LEDs - except for the white one - should be off. Touch the ends of the Orange wires together - to simulate an open door. Nothing should happen. While the ignition is off - opening a door should not trigger the alarm. ******* Turn the Ignition Switch on. When you turn on the ignition - the alarm should trip every time - even if the doors are all closed. When it trips - the Yellow LED should flash briefly - meaning that the buzzer has given a short beep. And the red LED should light and remain lit - indicating that the alarm has tripped. In addition - Ry2 should energize - the white LED should turn off - and the blue LED should light. This means that YOUR cut-out device is disconnected - and the engine will start. ******* Touch the ends of the two Blue reset wires together momentarily. The red LED should turn off. This indicates that the alarm has been successfully reset. ******* Next - to simulate the opening of a door - touch the ends of the two Orange wires together. The yellow LED should flash briefly - and the red LED should light and remain lit. This means that the buzzer has given a short beep to warn you that the alarm has been triggered. And the red LED is providing a constant reminder that the siren is about to sound. ******* After three minutes or so - Ry1 should energize. And both the yellow and green LEDs should light and remain lit. This means that both the buzzer and the siren are sounding. ******* The blue LED should remain lit. And the white LED should remain off. This indicates that Ry2 is still energized - that YOUR cut-out device is still disconnected - and that the engine is still running. ******* When you're ready - turn off the ignition. Ry2 should de-energize. The blue LED should switch off and the white LED should light. This indicates that YOUR cut-off device has disabled the engine. ******* Turn the ignition on again. Ry2 should NOT re-energize. The blue LED should remain off and the white LED should remain lit. This indicates that the engine is still disabled - and it will not re-start. ******* There is no time limit on the siren or the engine cut-out. The circuit must be reset manually. Turn off the Ignition and - if you haven't already done so - separate the Orange wires. Then touch the Blue wires together briefly. ******* The alarm should reset. That is - the red LED should turn off. Ry1 should de-energize - and the yellow and green LEDs should turn off. This indicates that the buzzer and the siren are silent. ******* Now - when you turn on the Ignition Switch - Ry2 should energize. The white LED should turn off and the blue LED should light. This indicates that the engine is no longer disabled. ******* Of course - you have also tripped the alarm again - so you'll need to touch the Blue wires together briefly - in order to reset the circuit. ******* Do the test again. Start by touching the two Orange wires together. You should get a short flash from the yellow LED - and the red LED should light and stay lit. Let's say that this is an accidental activation. Make sure that the Orange wires are not touching - i.e. the doors are closed. Touch the two Blue reset wires together. The red LED should turn off. This means that the alarm has been successfully reset. ******* Do the same again - but this time twist the two Orange wires together and leave them that way. The yellow LED will flash and the red LED will light. Try touching the Blue wires together. The circuit should NOT reset - because the "Door" is still open. ******* Wait until Ry1 energizes and the green and yellow LEDs light - then turn off the ignition. Ry2 should de-energize - the blue LED should turn off - and the white LED should light. However, the siren and buzzer should continue to sound - even after the ignition has been turned off. This means that - once tripped - turning off the ignition will not prevent the alarm from functioning correctly. ******* Turn the ignition on again. It should have no effect on the outputs. Turning on the ignition will not energize Ry2. The red, yellow, green and white LEDs should continue to light. This indicates that the alarm has been tripped - that the buzzer and siren are both sounding - and that the engine has been disabled. ******* Untwist the Orange wires - and touch the ends of the Blue wires together briefly. The alarm should reset. That is - the red LED should turn off. Ry1 should de-energize - and the yellow and green LEDs should turn off. And - if the ignition is not already on - when you turn it on Ry2 should energize. The white LED should turn off and the blue LED should light. ******* Repeat the tests as often as you like - until you're satisfied that your circuit is working reliably. *******

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 circuit board should turn up the cause of the problem. If an 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 provided - 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 haven't used the specified transistors or SCR - check the pin configuration. Just because the components look the same - do not assume that they have the same pin configuration.

If you can't see anything obvious - 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 twelve 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. Look for unwanted solder bridges. Your felt-tip markings will tell you which ones should be there - and help you identify any that shouldn't be there. Again, if you backlight the board during the examination - it makes potential problem areas easier to spot.

If all else fails and you still haven't found the cause of the problem - work your way through the assembly instructions provided. 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, the transistors, the SCR and the electrolytic capacitors.