I have a question.......+) with pics

I’m sorry that I posted a duplicate article because I didn’t know the rules properly.
Not only is it my first time posting here, but I am a Korean, so it was hard to check all the notices written in English. Actually, I’m still writing with the help of a translator. I want you to understand a little bit about my behavior. I’m very sorry again.
After learning the rules, delete the additional writing and attach the photo to the existing writing.
I don’t know why the picture doesn’t come up directly on the original, but it’s just attached.

I am a 19-year-old student in South Korea and I would appreciate it if you could understand even if my English expression is awkward. :’(
I don’t have a good place to ask. Leave a question here.

I was curious about the amplification effect of transistors, so I bought a breadboard from the Arduino site. When a finger touched, the microscopic current in the body was amplified by the transistor, and the LED was turned on. Despite trying many times, the experiment continued to fail. I’d like to attach a photo of the electronic circuit I designed, but I can’t. :’( :’(

  • I’ve used three different resistors, but do I have to install the smaller resistors in order for the amplification to occur?
  • The book says we need to design circuits from forward to reverse.
    How do you judge that when you design an electronic circuit?
  • Is there any other requirement for the transistor to amplify?
    I want to know why my electronic circuit failed.

I’d appreciate it if you could answer me. :blush:

We can't answer you. It is not because your English is bad. Your English is OK, although some of the things you said are ambiguous or confusing. We can't answer because of all the information you did not include in your question. Read what you posted and ask yourself how anyone, even the word's greatest genius, could give an answer using the tiny amount of information you gave.

To post a photo, you first need to have a photo. You may need to limit it to less than 2 Megabytes in size, but it needs to be no less than 2 megapixels resolution, preferably at least 3 or 4 in .jpg or .png form.

Then you need to click "preview" on your post. You will now see an option to attach your photo before you click "post". If you do that, one of us will embed it in the discussion page to make it easily seen. :grinning:

Despite trying many times, the experiment continued to fail.

Many things could cause this not to work.
The transistor type what is its part number?
The environment, are you close to a mains power source? It is normally voltage picked up by your body that causes it to switch on.

The resistor in unimportant as long as there is nothing connected to the other end.

You could have misidentified the pinout of the transistor and so have wired it up wrong.

Finally you skin could be very dry, try putting it on you tongue first before touching the base of the transistor.

Finally you skin could be very dry, try putting it on you tongue first before touching the base of the transistor.

...but only once and wash your hands first. there are some bad substances that get on electronics during manufacture and storage.

OPs pics.

Can you attach a circuit diagram please?
What is the transistor part number?
This is really an Arduino subject forum, but we can possibly help your simple problem.
Do you have a DMM?
Tom... :slight_smile:

You got the circuit wrong. I don’t know what it is supposed to be, that is what a schematic would tell us. But what ever it is it is not what you have built.

It is against the forum rules to post the same question again, especially when you got a lot of answers the first time you asked. You are just wasting peoples time asking again.

Cross-posting is against the rules of the forum. The reason is that duplicate posts can waste the time of the people trying to help. Someone might spend 15 minutes (or more) writing a detailed answer on this topic, without knowing that someone else already did the same in the other topic.

Repeated cross-posting will result in a timeout from the forum.

In the future, please take some time to pick the forum board that best suits the topic of your question and then only post once to that forum board. This is basic forum etiquette, as explained in the sticky "How to use this forum - please read." post you will find at the top of every forum board. It contains a lot of other useful information. Please read it.

Thanks in advance for your cooperation.

So now the thread has been moved (merged), let's have another go at those pictures:




In the middle picture why is the bottom 1K resistor labelled as 1uF ohms?

There appears to be two transistors here, a point not mentioned before.

We still need a schematic of what you are trying to make.

As you have been told before the value of the resistor your hand touches does not matter.

I don't think you understand how this works.

You need to take a blank sheet of printer paper and draw a proper schematic , take a photo of it, and
attach it to your post.

You should have no problem figuring it out using online schematics and datacheets



Did you bother to measure your body voltage by holding the black meter lead in your left and the red lead in your right hand ?

I got 0.080 Vdc at 0.000020 A (35mV@20uA).

The transistor needs 0.7Vdc to TURN ON.
Let's see...

Your Input voltage: 0.080V
Required input voltage : 0.7+Vdc

0.080-0.7=-0.62 Vdc

which leaves you 0.62 Vdc SHORT

Your input voltage is 0.62 Vdc BELOW the MINIMUM required voltage !

(nevermind that the current is probably insufficient).

You need an op amp input amplifier with a gain of at least 50 to 100 to increase the low input voltage.

I have not tried that but if you try it you will need to touch the op amp input GND with your left hand
while you touch the op amp input with your right hand.

FYI , obviously it's possible because my bedside lamp is touch activated.

FYI , obviously it's possible because my bedside lamp is touch activated.

Mind you, the "touch" voltage on your bedside lamp is automatically 110 V AC while this circuit (unless you are holding Vcc as well) relies on the AC radiation from the house wiring exceeding the 7 V necessary to overcome the reverse breakdown of the first transistor. :grinning:

" 7 V necessary to overcome the reverse breakdown of the first transistor."

7volts ?
Are you sure about that ?
I could have swarn it was 0.7V but Icould be wrong...

Poor guy.

He has already apologised for his double posting in the first post but still gets hammered for it by multiple members.

Here is a young bloke that is trying to learn electronics. His English is not perfect but he is trying and using a translator to help which is a lot more than a lot of people that post on these forums. Remember that a lot of you were probably in the same boat at some point.

Now regarding his problem, I have made many projects along these lines before and they worked. Sometimes they worked and sometimes they didn't.

As raschemmel mentioned he measured the voltage between one hand and the other and measured 0.08V. This voltage is dependent of the resistivity of the body; moisture of the skin, salt content and many other factors. This was measured by holding one probe in one hand and the other probe in the other hand. I got similar readings. I then tried it putting one probe on the household earth and holding the other probe in a hand. I got about 250mV.

raschemmel also mentions that the transistor requires 0.7V to switch on, incorrect. This voltage is the saturation voltage. This also varies between transistors, some will saturate at 0.5V and others at 0.8V. A lot of transistors will start conducting from as low as 0.2V.

So @ the OP, keep trying. Play around with resistor values and different transistors. If you are still having trouble try replacing the first transistor with a Germanium transistor, they have a much lower Vbe to switch on. Good luck.

Whether it's less than 0.7V is academic because
whatever it is it's far beyond what can be achieved
without addition circuitry to amplify the voltage and
provide an extremely high input impedance (>1 M ohm) to prevent loading the feeble input signal.
Most op amps are at least 10 M ohm input impedance which should be sufficient but a gain of
50 to 100 is also necessary. Why don't you
design the circuit ?

Challenge accepted.

We haven't seen the OPs circuit yet, the pictures seem to show two BJTs, hopefully in Darlington Pair configuration.
The OP hasn't come back in 12 days, so we may never know!!

Tom... :slight_smile:

Don’t bother the OP has gone away. He has deleted all his posts but the initial one, probably because the forum software does not allow you to delete the first post in a thread.

He was getting stick for refusing to answer any questions in a meaningful way. He never ever posted a schematic despite being asked many times.

This might help.



Really ?
Did you test it ?

For starters it doesn’t meet the criteria of working with one hand touching GND and the other touching the ‘input’.
It might even be fair to say that’s ‘cheating’.
Obviouslyif you run ENOUGH current at a sufficient voltage theough the human body eventually you
will turn on a transistor. That’s really not what the
OP described. He described a circuit that turned
on when he touched it. We gave him a handicapby allowing him to ground one side of his body , but
providing a power supply is one thing , ( to power the circuit), injecting voltage into his body is
another all together. Why not use 12V or 24V ?
Both are less that the 50V maximum allowed by
the Fire Marshal for circuits exposed to humans without any protection. No, I’m sorry, that’s
not going to work because you’re using the
subject’s body as the base resistor.
Get it to work with a passive input .
I’m not saying it can’t be done . Just that
to do it that way is cheating. If you are
restricted to a passive input but allowed to use
one hand as the GND , that narrows it down a lot and you will wind up the opposite of what you
proposed. I know Grumpy Mike knows whete I’m
going with this. The solution is fairly simple but it’s not that simple. (component wise).
Hint: resistors are passive components and the body a resistor you can measure with a DMM.
Just design a that has a passive (meaning no voltage on it) high impedance input but is missing one passive component , a human body (preferably alive)…