Which wire goes where?  Where do I connect the battery positive terminal?
Don' have a multi-meter to find out?
Build this simple ELECTRIC CONTINUITY TESTER and it will be a great help!

Step 1:

This project and instructions are basically for the ‘neophyte’ or beginner in the things electrical. If you have basic electrical knowledge then you may find these instructions a bit simplistic and pedantic.
Basic theory info:
Any electrical device (preferably using battery based power) can be used.

For example devices making a sound, such as bells, buzzers or radios, can be used.
Also any device that lights up can be used: for example – a flashlight or a key chain light.  I have used a regular flash light many times in my early days of electronic testing adventures.

The method for making an electrical continuity tester is quite simple.
  a)     Break the connection at the power source (the battery).
  b)     Connect probe wires at the break point.
  c)     Use these probe wires to test electrical continuity of any wire or circuitry.

For example, if you touch the two probe wires together, the light will light up if your test device is a flash light, or there will be sound if your test device is a bell, or buzzer or radio.

Similarly, when you touch the probes to wires or points in your circuit that you want to test then if there is continuity you will hear a sound or see a light as explained above.  If there is no electrical continuity then nothing will happen.

In this instructable a simple 1 AA cell battery powered flashlight purchased at a "Dollar" Store for about $1.25 will be modified to be used as an electrical continuity tester. 
If you do not have a multi-meter to measure whether there is current flowing on a certain electrical path then this simple tester will do the job.  For example, you need to know which wire is to be connected to the positive terminal of a battery and which wire connects to the negative or ground.  By connecting this tester to various points you will be able to determine the correct connections.
With a multi-meter you would determine the proper connection when the meter is set to read resistance and the readout shows zero ohms resistance.  This tells you that you have a direct path from point A to point B.  
With this simple falshlight tester the light will come on when point A to B is a direct path.

Step 1: For this project - purchase a simple inexpensive flashlight.

CAUTION: Do not use this device for testing continuity on ‘live’ AC voltage circuits or on ‘live’ high-voltage DC circuits.  Also do not use around high-voltage electrolytic capacitors in circuits, because even if the circuit is not live, the capacitors may still hold a big enough charge to ‘zap’ you and they could be damaged.


Step 2:

Here you can see the basic parts of a dis-assembled flashlight.
Note that the key parts of our interest are the connection (switching - ON/OFF) components.
In this particular flashlight there is no physical switch, but the metal strip extending from the spring base that touches the negative end of the battery is one end of the "switch" and the metal edge of the lamp itself completes the "switch" connection when the metal strip is forced to touch the side of the lamp.  The tip of the lamp touches the positive terminal of the battery.
In a flashlight with an actual switch you would simply connect one wire to one end of the switch and another wire to the other end of the switch ( and remove the switch or leave it in the off position ).
The next few steps should make this much clearer.

Step 3:

The yellow dots show where I drilled holes in the body of the flashlight to make the necessary wire connections.
One wire connection lead directly to the negative end of the battery and the other to the lamp and the positive end of the battery.
When you touch the two wires together the lamp comes on (because the wires are in essence the two end points of a physical switch).  You will see how this very simple feature is actually the electrical continuity test mechanism.

Step 4:

This step shows the electric continuity tester in circuit diagram form.
Notice that the switch mechanism on a flashlight, radio or buzzer would be installed across the two points that we have labelled PROBES.
As originally indicated, you can use a buzzer, or a radio and its sound to do the electrical continuity test or, as in our case a lamp or a modified flashlight can be used.

Note: For the advanced enthusiast:
This particular test device is powered by a 1.5 volt AA battery.  1.5 volts will limit effective testing over great distances of several tens of feet or more because of the increasing electrical resistance introduced by the increasing lengths of wire.  Although, in most cases, this sort of testing will not be done. 
However, if you are looking to find positive and negative wire connections that are already installed throughout a house for speaker systems, alarms or door bells, then this testing can be accomplished from one end of the circuit using a diode, but in such cases you will need a tester with more than a 1.5 volt battery since a diode usually drops from .4 to 1 volt in the circuit.  In such cases there are ways to increase the effective voltage of the tester. Contact me for a further discussion of this possibility.

Step 5:

The finished modified flashlight ready to be used as a DC electric continuity tester.
I used an old audio cable so that the RCA plugs could function as solid probes for touching test points or ends of wires.
Alligator clips become quite handy in many cases for attaching to the ends of wires and the probe points.
The red probe is attached to the wire coming from the lamp.  This gives a handy reference to the positive and negative terminals of the battery inside the lamp and can be used to identify positive and negative connection in more complex situations where you might use a diode to identify the current flow in a wiring setup.

Step 6:

Here is the operating device.  The first step would be to test the electrical continuity of the device itself.
By touching the two probes together or by connecting them with a jumper wire of some sort (which is the same thing electrically speaking), we can test whether the device is working.  The light coming ON indicates that the battery is providing power and that the lamp is not burned out. Therefore, this shows electrical continuity throughout the tester circuit.  All other continuity tests are just the same thing happening in different layouts or configurations of wiring.  
The picture of the multi-meter showing a reading of 0.00 ohms is the equivalent of this "flashlight tester" having its light come on. 

Step 7:

This step shows the device being used to find out which wire is connected to the negative part of the cigarette lighter socket.
Notice that in the picture the gray colored RCA plug (PROBE) is inside the socket touching the inner metal ring. 
Next the red probe is touched against each of the two wires coming from the socket.  (In this case I used a yellow jumper wire to connect the probe to the socket wires).
(When the tester's light comes on that is an indication that electrical continuity exists between those two points.  In other words, The wire the red probe is touching is directly connected to the metal ring inside the socket.  You can be sure that you have now found the negative connection wire of this socket..
By elimination, you also know that the other wire is connected to the plate at the back end of the socket and it is designated as the positive wire of the cigarette lighter socket.  I used this test to make sure that this very socket was properly connected to the battery pack in my instructable "EMERGENCY POWER PACK THE EASY WAY". However, in that case I used the multimeter, but now I have shown you that I get the same result using this electric continuity tester.

In the above test example where the gray probe is touching the inside metal ring of the socket:  If the red probe causes the light to come one when you touch each one of the two wires individually then the test tells you that there is a malfunction in the form of a short circuit in either the cable/wire connection or in the socket itself. 
Thus this tester can also be used to find SHORT CIRCUITS (which is a state of electric continuity where it is not desired).

Step 8:

This photo is just a reminder of the project that used the cigarette lighter socket for which electric continuity was tested in the previous step in this instructable.
The following note is extra information for those who are curious about how cigarette lighter plugs and sockets are wired.
Note: sometimes the negative wire is identified by some sort of marking such as a white stripe.  In this case, the socket wires actually had one wire casing with a slightly 'grooved' surface.  However, to be absolutely certain that this was the negative and not the positive wire, I tested the continuity.
The following is general information on cables that plug into a car's cigarette lighter socket:
1. They are usually designed to accept power from a 6 volt  or 9 volt car battery.  Thus the plugs are often designed with built-in circuitry to handle a range of voltages from 6 to 12 volts.  However, there can be plugs specifically designed  for one particular voltage only, so take care to read the spec sheet or manufacturer packaging information in every case.
2.  The tip of the plug is usually designated as the positive terminal and it makes contact with the positive plate at the back of the female cigarette lighter socket.   
3.  The two springy contacts on the outside of the plug contact the inside metal ring of the female socket and are generally connected to the negative electrical terminal.

In  2 and 3 above, in rare cases it could be that the wiring is opposite, but I have never encountered this.  Therefore, electric continuity testing can be useful in many situations.
Great idea!
Thanks man.
nice idea, also useful for the flashlight that can be always used as it is!
Thank you mario <br>walt

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