My school has a practical lesson where the students wire up a mains voltage plug. The current set-up is to use a 200mm length of 3 core cable and a plug. (for safety, the plug has a rivet attached through the earth pin to prevent it being fitted into the mains sockets on the pupil's benches. The teacher has to inspect every plug to check it is wired up correctly but I would like to create a simple set of circuits so that if the wire is fitted into the correct location in the plug, it will light an LED. Ihave a working prototype using three separate PP3 batteries, which ensures LEDs do not light up when the wire is fitted to the incorrect terminal. I would like to reduce the number of batteries to one, so it packages better within the plug socket box and also reduces the overall cost of the set-up (I am building 10 sets). Please can you let me know if there is a way to run the set-up with just one battery, without allowing the LED to light up erroneously? For info, for my planned set-up with the LEDs, I have modified the earth pin socket so the plug can be fitted with the rivet attached.

 

what do you mean by mains voltage? i hope this is used to test not powered extension cords.

 

Guys - please read what's written, its not homework - its a teacher or a support tech asking - and it specifically states no mains voltage - its to teach wiring a UK plug properly.

Gary, welcome to AAC. Its an interesting and non-trivial problem. PP3's being expensive, I'd look to run this on 2 AA or AAA cells. But it can't be done, AFAIK, with just wiring, it needs an 'active' solution, for which I have a couple of options. Are you adverse to a little soldering exercise?

 

Certainly it can be done with only one battery, BUT it gets a bit complicated. The trick is to have either a 3 positin switch or three buttons, to apply the battery to one lead at a time: Mains, Neutral, and "ground." On the test outlet there are three LEDs, identified for the three connections: Mains, Neutral, and Ground.Simple but different feom before in that only one line at a time gets tested.

 

Certainly it can be done with only one battery, BUT it gets a bit complicated. The tricj is to have either a 3 positin switch or three buttons, to apply the battery to one lead at a time: Mains, Neutral, and "ground." On the test outlet there are three LEDs, identified for the three connections: Mains, Neutral, and Ground.Simple but different feom before in that only one line at a time gets tested.

Replace the 3-position switch with a tiny 8-pin mcu, a PIC or an ATtiny85 and it will look like it's simultaneous... under 50p a unit and you could run it forever on a couple of AA cells.

 

If the goal is to reduce the complexity, adding a processor module and it's I/O and power switch will INCREASE the complexity AND require programming. And most processors are rather sensitive to polarity reversals and shorted outputs. ALSO, it would need to be mounted someplace and physically protected. aside from that, some connection to the other end of that 20CM length of cord would also be required.

The best check for an open circuit connection is to check that there is no continuity between pins. The single battery test can apply a positive voltage to the "Line" conductor of the cord thru an LED, and tie the battery negative thru a button switch to the "neutral" conductor. The test outlet can also have an LED connected between the "line" and the "neutral" sides. Then a correct connection will light both LEDs at the same time. For checking the third "Ground" connection, a wire to the test outlet will be required, tied to the battery positivethru another LED, with the cord "ground" conductor tied to the battery negative. So inserting the plug in the outlet will immadiately light the "ground " check LED, while the "power" circuit test will require operating the button.

 

The teacher has to inspect every plug to check it is wired up correctly but I would like to create a simple set of circuits so that if the wire is fitted into the correct location in the plug, it will light an LED.

It's a nice idea, but :-
Correct wiring of the plug also requires (a) the right amount of insulation to be stripped from each of the three wires, (b) the three wires to be of the correct individual lengths to avoid mechanical stress and to ensure the earth wire is the last to be pulled out if the cable is tugged hard, (c) no stray strands which could cause a short circuit, (d) the cable sheath to be correctly clamped inside the plug by the strain relief member, (e) terminal screws tightened.
So the teacher would still have to inspect inside the plug.

 

If the goal is to reduce the complexity, adding a processor module and it's I/O and power switch will INCREASE the complexity AND require programming. And most processors are rather sensitive to polarity reversals and shorted outputs. ALSO, it would need to be mounted someplace and physically protected. aside from that, some connection to the other end of that 20CM length of cord would also be required.

1. There is no I/O or power switch,
2. it would be significantly smaller than a PP3 battery, would be mounted in the same space as a PP3, inside the socket box
3. would be no more complex - has exactly the same # of connections - and would use the same 3 connectors to the students 'cable' as there must be in the 3 battery version and in your proposal
4. requires no expensive and hard-to-mount push buttons which would take up real-estate on the socket box
5. Would be cheaper to run, just 2 AA cells rather than a PP3,
6. here is no possibility of a polarity reversal or shorted outputs, given the hard-wired nature of the device.
7. As to programming, well yes I'll give you that, but I'd be happy to do that for free, or for less than the price of a decent PP3 Gary could buy a programmer off AliExpress!

 

I don't think this is quite right, but maybe someone can analyse and correct? If the battery is connected between the Live and Neutral pins and the cable has green and red LEDs connected as shown, If Live and Neutral wires are correctly connected then the Green LED will light, but if they are connected the wrong way around, the Red LED will light. I'm fairly confident of that!

But I'm not happy about the Earth connection. The idea is that if the live and neutral are correctly connected the transistor will turn on, turning the blue LED on, so long as it is connected to the plug earth pin. Problem is that, if the neutral wire is connected to the earth connection on the plug it still turns on the green LED. I know it's wrong, but maybe someone can correct?

 

Nice try Jerry, not sure what the fix is, but its an interesting approach.

 

I think the practical simplicity of three batteries is going to be hard to beat.

I expect these test units would have a very small usage cycle - lights on occasionally compared to unused or stored - so how about using high-efficiency red leds (a few milliamps to run) and 3volt CR2032 cells with approx 1k resistors. Those batteries are very small and would last a long time.

 

I think the practical simplicity of three batteries is going to be hard to beat.

I expect these test units would have a very small usage cycle - lights on occasionally compared to unused or stored - so how about using high-efficiency red leds (a few milliamps to run) and 3volt CR2032 cells with approx 1k resistors. Those batteries are very small and would last a long time.

My experience is that the cost of "3volt CR2032" cells is a whole lot more than the cost of the AA cells, and while they probably do last quite a while, they would not be cost effective in this application. In other parts of the world that may not be the case.

 

Guys - please read what's written, its not homework - its a teacher or a support tech asking - and it specifically states no mains voltage - its to teach wiring a UK plug properly.

Gary, welcome to AAC. Its an interesting and non-trivial problem. PP3's being expensive, I'd look to run this on 2 AA or AAA cells. But it can't be done, AFAIK, with just wiring, it needs an 'active' solution, for which I have a couple of options. Are you adverse to a little soldering exercise?

Hello Irving.
Many thanks for your reply. I am very happy to add some electronics to the circuits. I can solder discrete components like transistors and diodes but don't have the technology to successfully solder ICs .

 

what do you mean by mains voltage? i hope this is used to test not powered extension cords.

Hi Panic Mode,
Thank you for your comment.
Yes, please be assured that 9V is the highest voltage that will be applied to these circuits.

 

Certainly it can be done with only one battery, BUT it gets a bit complicated. The tricj is to have either a 3 positin switch or three buttons, to apply the battery to one lead at a time: Mains, Neutral, and "ground." On the test outlet there are three LEDs, identified for the three connections: Mains, Neutral, and Ground.Simple but different feom before in that only one line at a time gets tested.

Hello Mister Bill,
Thank you for your idea of the three-way switch. If there is not a solution using discrete electronic components, I will probably go with this idea.

 

If the goal is to reduce the complexity, adding a processor module and it's I/O and power switch will INCREASE the complexity AND require programming. And most processors are rather sensitive to polarity reversals and shorted outputs. ALSO, it would need to be mounted someplace and physically protected. aside from that, some connection to the other end of that 20CM length of cord would also be required.

The best check for an open circuit connection is to check that there is no continuity between pins. The single battery test can apply a positive voltage to the "Line" conductor of the cord thru an LED, and tie the battery negative thru a button switch to the "neutral" conductor. The test outlet can also have an LED connected between the "line" and the "neutral" sides. Then a correct connection will light both LEDs at the same time. For checking the third "Ground" connection, a wire to the test outlet will be required, tied to the battery positivethru another LED, with the cord "ground" conductor tied to the battery negative. So inserting the plug in the outlet will immadiately light the "ground " check LED, while the "power" circuit test will require operating the button.

Hello Mister Bill 2
Thank you for your suggestion. Would it be possible to sketch your idea so I fully understand how to build the circuit?

 

It's a nice idea, but :-
Correct wiring of the plug also requires (a) the right amount of insulation to be stripped from each of the three wires, (b) the three wires to be of the correct individual lengths to avoid mechanical stress and to ensure the earth wire is the last to be pulled out if the cable is tugged hard, (c) no stray strands which could cause a short circuit, (d) the cable sheath to be correctly clamped inside the plug by the strain relief member, (e) terminal screws tightened.
So the teacher would still have to inspect inside the plug.

Hello Alex_t
The students only have about 45 minutes for the practical. Over the ten years I have been preparing this practical, I've learned that pre-cutting the three core leads to the correct length for the plug and tinning the stripped wire has enabled all the students to complete the practical on time. I have also removed the clamp features as these were being too easily lost. The focus of the practical is to make sure the wires are fastened into the correct fixtures of the plug. I'm just glad that all mains electrical equipment seems to come with moulded-in plugs, so although the students have an appreciation of how a plug is wired, there is only a small possibility that they will ever need to do it in practice!

 

I don't think this is quite right, but maybe someone can analyse and correct? If the battery is connected between the Live and Neutral pins and the cable has green and red LEDs connected as shown, If Live and Neutral wires are correctly connected then the Green LED will light, but if they are connected the wrong way around, the Red LED will light. I'm fairly confident of that!
View attachment 366342
But I'm not happy about the Earth connection. The idea is that if the live and neutral are correctly connected the transistor will turn on, turning the blue LED on, so long as it is connected to the plug earth pin. Problem is that, if the neutral wire is connected to the earth connection on the plug it still turns on the green LED. I know it's wrong, but maybe someone can correct?

Hello Jerry Hat Trick,
I will have a go and build your circuit, but I don't think it will achieve exactly what I am looking for. Many thanks for sending me this idea.

 

I think the practical simplicity of three batteries is going to be hard to beat.

I expect these test units would have a very small usage cycle - lights on occasionally compared to unused or stored - so how about using high-efficiency red leds (a few milliamps to run) and 3volt CR2032 cells with approx 1k resistors. Those batteries are very small and would last a long time.

Hello Boostbuck.
Many thanks for your suggestion. I like the idea of using smaller batteries and have some holders specifically designed for CR3032 batteries. I will keep this design in mind if I'm not successful, using the other designs that have been posted.

 

Hello Alex_t
The students only have about 45 minutes for the practical. Over the ten years I have been preparing this practical, I've learned that pre-cutting the three core leads to the correct length for the plug and tinning the stripped wire has enabled all the students to complete the practical on time. I have also removed the clamp features as these were being too easily lost. The focus of the practical is to make sure the wires are fastened into the correct fixtures of the plug. I'm just glad that all mains electrical equipment seems to come with moulded-in plugs, so although the students have an appreciation of how a plug is wired, there is only a small possibility that they will ever need to do it in practice!

The teacher will still be looking at each plug to see it is wired correctly. I am hoping that the LEDs will allow the student to have quick feedback on their wiring and if they don't get the correct lights showing, give them time to try again before the class finishes. I am also hoping that the LEDs give the practical a bit of a Wow factor to make the practical a bit more interesting for the students.