Neon test screwdrivers + LED modification (no batteries)

[00:00:00]So, today let's explore some neon screwdrivers. How do they work? What's inside? And let's also build an LED equivalent of it, which hopefully would be much brighter because these tend to be always quite dim. These simple devices are used to indicate minus voltage using a small neon lamp. But this one is not very bright and you can't even see anything under the studio lights. The tip goes to the AC mains voltage and the return path is this touch pad, which gives it a resistive and capacitive coupling through the human body to the earth. But of course the current is so low that you don't get a shock from it. This typically contains just a high resistance resistor and a neon lamp, a small glass bulb filled with neon gas, and the touch pad. And the dim glow of this lamp indicates the presence of AC voltage on the tip. And this circuit is coupled through the human body, which is basically another resistance, but also a capacitance if you're not touching anything grounded directly.

[00:00:53]This resistor is typically about 1 or 2 megaohms, and this lamp requires about 50 to 90 volts to light up. Now you have to significantly reduce the lighting for the camera to actually see it. Let's plug it in and you can see the neon lamp up in it. If I touch the ground pin, it actually gets way brighter.

[00:01:12]And then even a closer look at it with me not grounded and grounded.

[00:01:18]Not grounded.

[00:01:19]Grounded. The slightly different type of it. Again, another grounded. Grounded.

[00:01:26]And then even older one. Not grounded and grounded.

[00:01:31]Without the grounded connection and with it. This one has a differently shaped neon lamp, but the principle is the same.

[00:01:41]The human body capacitance seems to be the main limiting factor for the current, the main bottleneck. That's why it gets much brighter when I touch the ground. It basically bypasses this capacitance, which appears to be the highest impedance for the AC circuit, and only leaves this resistance and the resistance of human body as a limiting factor. The lamp itself does not limit the current. It just drops a certain voltage, but it would pass almost any current if it didn't have any impedance in series. So, if you use just the lamp without the resistor in series in it, you could get quite a shock from it. And this resistor for safety has to have enough resistance. It shouldn't go much below 1 megaohm because otherwise the current would be higher and you could already feel it. This one is the newest one, probably 2020-ish.

[00:02:25]Not different from the modern ones. And there's a neon lamp in it and here a resistor in a series with it. Can I open it? This one isn't easy to open. They have a much more worn 2000-ish one which unscrews easily. So, let's take a look inside of this one.

[00:02:42]Here's this touch pad going to a spring.

[00:02:45]And then it contains this neon lamp with a built-in series resistor in this glass tube or actually a plastic tube.

[00:02:55]The lamp is glass, but the outer tube is of course plastic.

[00:02:59]And this end is connected to it.

[00:03:02]And then it just connects to this tip.

[00:03:04]This one is quite worn. Closer look at this. It's scary how tiny this resistor is. It's the only thing between the mains voltage and you. And based on the marking, it's 820 kiloohms. Now, let's explore the much older one. It says 500 volts AC and this unscrews.

[00:03:26]And it reveals again a spring.

[00:03:29]A neon lamp, a completely different shape now. You can see two electrodes in it.

[00:03:37]And and there's a resistor and another spring and that's it.

[00:03:43]And this resistor says 2.4 megaohms in the older marking 2M4 and they basically cut its pins and sandpapered it. So, instead of penis, it just has two conductive pads on it. And the old one actually feels significantly safer because they used three times the resistance and also a way, way bigger resistor. Looking at this tiny resistor makes you kind of afraid that it will arc over and give you a shock. The screwdriver says 120 to 250 volts on it.

[00:04:14]The newest one actually says 200 to 250 volts category three. This is much more difficult to open.

[00:04:25]And a spring. Here's the neon lamp and let's take it completely apart. Removing this, there is a resistor in it and we can pull it out heat shrink.

[00:04:36]It's spot welded onto the lamp and either lead of the lamp is actually going here to make a contact with the spring. And the neon lamp is just two electrodes in a tiny glass bulb filled with neon. And the resistor based on its marking actually says 1 megaohm. At least it's a bit bigger than the other one. So, this one seems the dodgiest.

[00:04:56]It's back together for some measurements but because it has the lowest resistance resistor and also smallest, we will use this one for the modification for an LED. The bigger safer resistor. And the oldest one actually uses the highest resistance and the biggest resistor.

[00:05:10]They even have the original housing and the manual. It says it's for 100 to 500 volts AC.

[00:05:17]And either side of it and it says 1983 here. Nice. Let's try to measure what current is going through the neon lamp and the human when it's measuring. About 5 microamps when I'm not grounded and over 100 microamps when I'm grounded.

[00:05:34]About 110. And I stress this is microamps not milliamps. That was the dodgiest one. And the new one again about 5 microamps not grounded and about 100 microamps grounded. And the oldest one 4.5 microamps not grounded. And about 50 microamps grounded.

[00:05:55]The resistor inside of it doesn't make much difference when I'm not grounded because the main limiting factor is the capacitance of the human body. But when I'm grounded, the current was lower because this one contains the highest resistance in it. And two of them can be stacked, so it would probably be able to indicate half of the voltage. 115 volts.

[00:06:13]The current when you're not grounded is just about 5 microamps. It's a super low current. This of course explains why it's so dim. You can't really get more current for it because the capacitance of a human body is just not going to change. It's typically about 100 to 150 picofarads. So you just have to work with super low currents. If you wanted brighter, you can of course use an LED, some amplifying transistor too. Or a Darlington transistor. Basically two transistors in one package. And a battery to power the LED. Of course the battery can discharge. It can a leak.

[00:06:45]You have to keep replacing it. So let's try to make a passive LED version of it.

[00:06:49]With no batteries where the LED is powered just from the capacitive AC current. Of course it has to be a super bright, super sensitive LED. But the problem is LEDs drop only about two or three volts. They might be about 30 times more efficient than the neon lamp.

[00:07:03]But they also use about 1/30th of the available voltage only. So it kind of cancels out and they are still dim. With an LED you could basically do this.

[00:07:12]Connect it instead of the neon lamp using a bridge rectifier.

[00:07:15]Because unlike the neon lamp, the LED needs DC. But the problem is the LED only uses two or three volts. That's about 1% of the available voltage. And 99% is wasted in this resistor and the human body resistance and capacitance.

[00:07:29]So even a very efficient LED is still going to be kind of dim. So is there any passive solution to this without any batteries? Well, when I was disassembling this RGB white remote controlled LED lamp, we found the special LEDs inside of it. This thing is basically powered from rectified mains and uses some LEDs driven by a linear regulator from the rectified mains, which is 325 V.

[00:07:53]And it still only uses eight white LEDs in a series and five colored LEDs in a series. So, all these LEDs, especially the colored ones, have to be high-voltage LEDs. I actually discovered each of them contains about 16 to 18 LEDs in a series in one tiny SMD package. These colored LEDs about 50 V at a high current or about 40 V at a low current. They're super efficient and they would use way more of the available voltage and could actually be way brighter than the neon lamp. Of course, the green ones are the visually brightest. They don't have to be necessarily the most efficient, but a white LED in a white ambient daylight wouldn't be as noticeable. Also, these are more in a series, so they contain less chips in series in one package, about 10 or 12. These are about 16 or 18. So, we decided to try the green ones. These are super tiny, but each of them actually contains about 16 to 18 blue chips inside and a green phosphor to convert the light to green.

[00:08:49]They're super efficient and the light of about 18 LEDs in a series is going to be concentrated in one tiny spot. I'm testing these in a breadboard, they're actually quite noticeable even under studio lights at just 2.7 microamps. And at such a super low current, they actually appear more cyan than green. In comparison to a cool white, the green is actually brighter and the blue one does almost nothing in comparison to the green. The red would be usable, but it's also noticeably dimmer in comparison to warm white, which doesn't even look warm white at such a low current. Now, let's give this LED a tiny bridge rectifier.

[00:09:25]We're actually working with current so low that we should check the leakage current of the bridge rectifier. But, the leakage current turned out to be too low for this multimeter's microamp meter, so I'm using the voltmeter as a nano amp meter. The internal resistance is 10 megaohms, so 0.038 V means 3.8 nano amps. So, the leakage of the bridge rectifier is about three orders of magnitude lower than the current we're going to be working with.

[00:09:51]The leakage current goes up steeply with temperature, but this is going to mostly work at the room temperature anyway. And even if the temperature goes up by a couple tens of degrees Celsius, the leakage current is still low enough.

[00:10:02]The leakage current actually goes up just by breathing at it or by hitting it with my finger. And I'm measuring the leakage current of the bridge rectifier at 300 V.

[00:10:11]The actual voltage on it is going to be about 40 V, so the leakage current is actually going to be even lower. Let's take a piece of a blank circuit board, the single-sided, and let's cut a thin stripe which fits into this instead of this tube with the neon lamp.

[00:10:28]It shouldn't be wider, but it can be a bit longer because the spring can be compressed more or shortened.

[00:10:33]Sandpapering the sides of it to be a bit slanted.

[00:10:37]So, we can insert it this way because this is going to be the empty side and this one is going to have the components on it. Let's come up with some component layout.

[00:10:48]What else did you expect?

[00:10:53]That's the board.

[00:10:56]And of course, soldering the components on it.

[00:11:02]The LED and the bridge rectifier are on the board, and notice the big safety gaps under the resistors.

[00:11:10]So, that's it. We have the LED in it, the bridge rectifier, two resistors in a series for redundancy, for more safety, loops of wire as end contacts, and that's basically it. Now, of course, the rosin is cleaned from it using ethanol and a toothbrush, and it looks much better. I've put two 680 kiloohm resistors in it, an B1F bridge rectifier, and the multi-chip LED. And now, testing time. Let's actually make a comparison before and after.

[00:11:37]This is before. This under studio lights, you don't really see much, do you?

[00:11:50]And after in the same lighting conditions.

[00:11:56]And you're going to see the massive difference.

[00:12:08]And in a much dimmer light, you can actually see the neon lamp.

[00:12:12]This is probably the best angle for it.

[00:12:14]And the new lamp, well, that's quite a massive difference, isn't it? It actually slightly glows even when I don't touch the pad here.

[00:12:26]That's way, way, way brighter. And the close-up of it, the LED.

[00:12:36]And then back to the neon lamp. That's really quite a difference. Let's try to alternate between the modified and the other one.

[00:12:44]And it's quite a difference.

[00:12:48]If I try both, just the LED one lights up. The LED has a lower voltage drop than the neon lamp.

[00:12:59]And it grounded.

[00:13:06]This is super bright.

[00:13:10]Both.

[00:13:13]And a comparison in a picture. Before and after in the same lighting conditions. I added some heat shrinks on it. Way too many layers and extending way too far here, but now it's basically a category 100,000.

[00:13:26]It replaces this broken sleeve and I put some super glue under it so it doesn't slide off and that's it. It's all made possible by these LED lamps with the super special LEDs in them. Of course, there is some space for modifications.

[00:13:38]You could use two anti-parallel LEDs, red and a green one for example, to indicate the polarity, but these are mostly for AC voltages so it doesn't make much sense. And with just capacitive coupling, you can't test the DC anyway. And you could also put two or three LEDs in a series in it, each facing a different direction to make it all directional like the original neon lamp. Of course, this one in this version has to be treated like the old one which has a window and you have to look at it from just one direction. And these resistors could actually be a higher resistance. It could be for example 1 megaohm or even 1.5 megaohms each. It wouldn't really make it any dimmer in the capacitive mode. Of course, these resistors have to be rated for enough voltage to be safe. And everything has to be well separated on the board. And also, these test screwdrivers might not be reliable. You should never fully trust them. It's always better to verify it using something else and double-triple check it before you conclude there is no voltage and you can safely touch it. And I was complaining about this one having a too small resistor for safety, but actually, I found more of these which might be even legit, but these will get a separate video. That's it. And if you like my videos, please consider subscribing, supporting my channel on a Patreon, or using the things button. And big thanks to all of you who already support me.