Good day everybody
After a long time searching for an old fashioned voltage tester screwdriver (and not finding one in any stores) I decided to build one as some have done in youtube videos. I made more than one and none work.

It is very simple from one end to the next:

Piece of electrical wire (instead of screw driver metal part) - 1M Ohm resistor - 5mm LED - electrical wire

I tried 1.5, 9.3, 1.1 M Ohms. I changed the direction of LED just in case.

Nothing worked. This is a very simple tester what I am doing wrong?
Thanks in advance
Elektrov

 

What voltage did you test it with?

You may have blown the LED due to high reverse voltage.
Put another LED or small diode (e.g. 1N4148) in inverse direction across the LED.

 

Is this a DC tester where a wire is used to complete the circuit back to the supply, or an AC tester where touching the back of the screwdriver in enough to light the indicator?

If you are making a DC indicator for low voltage work then your resistances in the megohms are far too high.

AC testers commonly use a neon indicator rather than a LED. The current requirement of a neon is in the order of microamps (tiny compared to an LED) which is compatible with your resistances in the megohms. If this is what you are making then a LED will not work as the current is too small.

 

MANY years ago a screwdriver voltage tester used a NEON light bulb (it uses about 70-V at a very low current to light).

 

Do you expect it to work by simply touching the screwdriver tip to a single wire? It is unlikely you could make that light an LED like that without an additional power source. The current would need to flow through your body, and you would feel it.

 

Hello
Thank you guys for your quick replies. It seems like I have become a victim of BS on youtube in this case. There are videos showing how they have made the same thing and test it and the blasted thing lights up like xmas tree. Mine does not.
To answer your questions:
1. I tested it with normal home electricity (120 V in Canada as far as I know).
2. You are supposed to press the metal part (wire in my case) into the socket and press your finger at the other end or touch the wire in my case to create a ground. If the wire is live the LED is supposed to light up.
3. I have owned and manage to break at least a dozen of these things and there is nothing more in them than a bulb (as somebody mentioned it is not LED but a bulb) and a resistor or diod. However the videos show using a LED and resistor. Some say 100k some say 1 M. I have tried with different things and none worked.
Of course I could just go and buy one of these modern touch-free things, but having a screwdriver when you use it is a plus and it does not need a battery and if it works why make it more complicated? BpbTPH has described it correctly (except that I never felt any current going through me). So still the question remains: why is it not working?
PS

for reference here is a link that is somewhat more credible there are some otherones when the builder is actually touching the live wire before passing through the resistor and claim it to work!

 

This is part of why I avoid the "cartoon channel" youtoob. Fakes and lies and malware.
Consider that an LED will require several milliamps to illuminate at all, and if that is flowing thru your hand you will feel it.

 

What voltage are you testing for? Reverse voltage breakdown.

This is part of why I avoid the "cartoon channel" youtoob. Fakes and lies and malware.
Consider that an LED will require several milliamps to illuminate at all, and if that is flowing thru your hand you will feel it.

I don't avoid it but you're right, ones needs knowledge to spot the cartoons, fakes and lies and malware. To blanket avoid it, is misguided IMO as there a a huge amount of very valuable videos there. Use discernment to separate the good, the bad and the ugly.

 

So still the question remains: why is it not working?

It isn't working for a few reasons:

1) The LED requires a lot more current through it than can pass through the high-value resistances and through your finger etc as leakage current.

2) If the resistances were low enough for the current to be sufficient to light a LED a) you would feel it and not be happy and b) the reverse voltage of the AC would blow the LED immediately as there is no reverse protection.

What you need is one of these neons or similar, which is what the old-style testers used.

 

It is also possible to use a liquid crystal for this application, because the LCD will change states with the application of very little power. I have seen such a device demonstrated a while back. Evidently the implementation is complex enough that it has not appeared on the cartoon channel yet.

 

The threshold for feeling an electrical shock is about 1mA. Some modern LEDs will light fairly well at that current. Presumably, some LEDs would light up at 1/2 mA enough to be visible. So I think you could actually make this work with two of those, back to back and a resistor somewhere in the range of 100K to 300K.

What you need is a very high efficiency, low current green LED (because green is the peak response of our eyes.)

 

Thank you all for your very useful replies.

 

Concur with @Audioguru again , traditionally a Neon lamp is used, the return path is through the body to ground, I have even got one to light from the top of a step ladder!
(Remember to hold the pocket clip, not the screwdriver body when applying).

 

With a hand as a resistor you do not need much external resistance.

 

The good old screwdriver / 'neon line tester' uses an NE-2 neon lamp as the indicator. The value of the series, current-limiting resistor is typically 1 MΩ for a 240 V tester.

Nandu.