Hi
So, I'm sort-of-ok with using NPN BJTs as switches, so I decided to try out a Darlington pair to make a "human contact-ometer". When I hold an electrode in each hand, I want it to detect the tiny current running through me (I measure my resistance as 1.2MR between hands) and light up an LED.
So I set up a Darlington like this:
If I just touch the flylead, the LED switches on. I didn't expect this because I wanted it to switch on the LED only when I touch both leads (the second lead tied to 5V, not illustrated). I figured the Darlington was too sensitive so I added a resistor between the emitter of Q1 and the base of Q2. (I tried 1kR and 470kR)
I also tried adding a resistor between the flylead and base, thinking the Darlington was picking up noise or something. (I was just winging it at this point)
Then I figured this Darlington was just too crazy sensitive, so I replaced it with a single transistor like this:
This worked a bit better, especially when I put a 1kR resistor in series with flylead2. It still made the LED glow slightly when touching only flylead2.
Is the P2N2222 - or transistors in general - that sensitive? The P2N2222 has an HFE of around 35. Assuming I'm an ohmic resistor, I should only pass 4.2 microampere. In the darlington, as far as I know, shouldn't that give me x35x35 collector-emitter current? (ie., 5.1mA) Then as far as my dodgy maths is concerned, the darlington should give just enough current for the LED when I attach myself to both leads, not only one?
What am I experiencing here? I hope I haven't shown my ignorance too much in this post
So, I'm sort-of-ok with using NPN BJTs as switches, so I decided to try out a Darlington pair to make a "human contact-ometer". When I hold an electrode in each hand, I want it to detect the tiny current running through me (I measure my resistance as 1.2MR between hands) and light up an LED.
So I set up a Darlington like this:
If I just touch the flylead, the LED switches on. I didn't expect this because I wanted it to switch on the LED only when I touch both leads (the second lead tied to 5V, not illustrated). I figured the Darlington was too sensitive so I added a resistor between the emitter of Q1 and the base of Q2. (I tried 1kR and 470kR)
I also tried adding a resistor between the flylead and base, thinking the Darlington was picking up noise or something. (I was just winging it at this point)
Then I figured this Darlington was just too crazy sensitive, so I replaced it with a single transistor like this:
This worked a bit better, especially when I put a 1kR resistor in series with flylead2. It still made the LED glow slightly when touching only flylead2.
Is the P2N2222 - or transistors in general - that sensitive? The P2N2222 has an HFE of around 35. Assuming I'm an ohmic resistor, I should only pass 4.2 microampere. In the darlington, as far as I know, shouldn't that give me x35x35 collector-emitter current? (ie., 5.1mA) Then as far as my dodgy maths is concerned, the darlington should give just enough current for the LED when I attach myself to both leads, not only one?
What am I experiencing here? I hope I haven't shown my ignorance too much in this post