Screwdriver with microcontroller

It is indeed a Sziklai pair. Quite unusual.

Big Clive

Clive! I had a light bulb go off (no pun intended) and I had to pause to find it. I saw the topology of the transistors once before on eevblog: It's a Sziklai pair. Because it incorporates a PNP transistor, the on voltage is lower than a typical Darlington: around 0.6v. It makes sense (again no pun intended) for the application where sensitivity is a key factor. I didn't load all the comments before posting (someone already covered it) but you got me excited because I actually remembered seeing it. I love those screwdrivers. If they keep improving we will all have Doctor Who sonic screwdrivers at some point. :)

Alex

Wild! I have a "volt stick" sort of thing or two but my confidence is more 100% if I actually probe something. Maybe that's just me.

Dave Frederick

"Live wire" => 火线 The Google translation is "line of fire". The first logogram even looks like fire, which is pretty normal considering the writing system is so old. This also explains how the only difference between Mandarin and Cantonese is verbal. The writing is identical, so I'm told. I've said it before but I actually think this dual visual / verbal system is probably more powerful and intuitive. Not that I'm about to learn it ....

Mike Page

The classic neon screwdrivers are still sold.

Big Clive

All voltsticks measure current through a circuit comprising various capacitances. They convert this current to an equivalent voltage-to-earth. The capacitance from the user's hand to the voltstick is around 20pF. The capacitance from the user to Earth is around 100pF. The capacitance from the tip of the non-contact stick to the conductor is a bit under 1pF, varies with position, and picks up electric fields from every nearby conductor. The contact voltstick gets round this by connecting directly, and using an internal capacitor of around 2pF, whose value is reliable. When calculating measuring circuit current, series capacitances don't add. You need to add their reciprocals ("Elastance"). From this it's clear the internal capacitor dominates and thus provides a reliable voltage threshold, as well as allowing the user to decide which wire or terminal they're testing. However, because it doesn't load the conductor, phantom / ghost voltages remain an issue and two pole testers remain the gold standard.

Mike Page

Interesting. How do they work ? What I like about the neon is it's reliabilty, based on it's simplicity. It has a resistor that's designed not to fail short-circuit (vanishingly rare, anyway) and checks the actual path of fault current. It can also be quickly and easily checked for functionality. I'm somewhat wary of replacing that with a bunch of electronics and an unknown earth reference.

adrian

I see what it's doing. It's an LED screwdriver with additional "earthy" contininuity, probably using an unrelated frequency, maybe even DC (0 Hz).

Mike Page

There is a third category of tester which makes contact with the conductor being measured, but NOT with the user. It's far more decisive than a voltstick while retaining the safety.

Mike Page

The Chinese tend to be very descriptive about things. And in pain terms the live wire is indeed full of fire.

Big Clive

That's the first time I've come across the name Sziklai pair. Interesting that it does show the whole transistor as equivalent to an NPN device with the collector (PNP emitter) switching the load.

Big Clive

No matching divider resistors and the PWM control of the LEDs.

Big Clive

The two transistors connected like that are called a Sziklai pair. The advantage over a darlington is that it will turn on at a lower voltage. I'm not sure how much difference it makes to this design though.

Hmm looking on Aliexpress at these and love the description of live as "fire line"... might pass as I've still got an old RS one in my toolbox.

Rik Kershaw-Moore

I think the RS toolkit I have has one of the old school neon ones in it.

Chris Crowther

has PWM

Bill Kerr

I still own my 1970s neon screwdriver. I used it yesterday to test something.

Nuts 'n' Proud

I'd trust a neon screwdriver over a voltstick any day

adrian

Could that IC be a dual op-amp or comparator?

There is another,.. in the bad old days, an identical type clear plastic screwdriver, inside, a neon, a resistor, the pocket clip was the ground, you could jam it into a wall socket and find the hot side and you could use it on the high tension lead for a spark plug. Ahh the good old days when possible electrocutions were just part of the job.

For people that enjoy jamming screwdrivers into sockets.

Curtis Hoffmann