So I have an incredibly simple circuit: a 3V battery pack, positive goes to a 330 ohm resistor, then to an LED, then to the collector of an NPN transistor, then out the emitter, then to the ground of the battery pack.
Obviously, this circuit should do nothing because there is nothing to activate the base and let the current through. However, I've found that, mysteriously, when I touch the base with my finger, the LED dimly flashes for... maybe 10-20 milliseconds or so, and then remains on EXTREMELY dim until I take away my finger.
Any explanation as to why this happens and/or how I can prevent it?
A transistor acts as a current amplifier: small current thru the base allows a larger current (maybe 200-300 times higher) to flow thru the collector.
Your body is a natural conductor, so it is possible that a few µA flow from your body to the base of your transistor, allowing a few hundreds of µA to flow thru the LED. That's enough to make some LEDs light a little.
The facts that the contact is not perfect and that your body is not a simple conductor such as a wire is, explains that the LED "flashes".
Try a 10K resistor from the base to ground to pull the base down.
Huh. I thought it might have something to do with static electricity, because the longer I wait after taking my finger away before touching it again, the brighter it flashes. I'm also just confused because the base isn't currently connected to anything, therefore the circuit shouldn't be complete, ...right?
Static electricity is "just" electrical charges building up in your body. When you come into contact with another object, these charges flow: that's what current is. If you touch a metal object, the flow is instant thus the current is short but high: that's why it hurts.
When you touch the base of your transistor, which has a relatively high impedance, the current is low thus your body takes time to discharge itself. Think of it as a capacitor in this situation.
Yeah, I originally thought it had something to do with the capacitor built into the battery pack I'm using, but I tried it connected to one without a capacitor and got the same result, so my next guess was static electricity charging up in me.
I mean: your body acts as a capacitor. Static electricity charging up = capacitor behavior.
Yes, I understand that, but what I don't understand is that a capacitor has an anode and a cathode. I'm only touching the base at one point, and no other part of me or the base is connected to anything, so how can current flow?
1 Like
I'm not an expert on that topic but I don't think this thing apply for static electricity. A static discharge is due to the difference of charges between 2 bodies. Nature prefers equilibrium so when they are able to, charges move to make this equilibrium. No closed circuit and return path for current needed here, that's not the same as with current in a circuit.
Static should discharge fairly quickly (depending on the amount of charge and the net input impedance into the transistor.)
Your body acts as an antenna, and it picks-up electro-magnetic energy from the power lines around you. If you've ever touched the input to an audio amplifier, you generally hear a hum or buzz (60Hz in the U.S., 50Hz in Europe). Guitar amplifiers are particularly sensitive to this, since they are high impedance. (The high impedance means you don't need much current, and the voltage doesn't drop when you touch the lead.)
You'll also measure AC voltage if you touch the leads of a multimeter. If I touch one lead of the meter with the other lead connected to the building/power-line ground, I'm getting about half a volt AC right now.
Interesting. I'll have to do some more experimenting/research to see how I can harness or prevent this quality. Thanks for giving me an idea of what's going on!
Squirt_5432:
Huh. I thought it might have something to do with static electricity, because the longer I wait after taking my finger away before touching it again, the brighter it flashes. I'm also just confused because the base isn't currently connected to anything, therefore the circuit shouldn't be complete, ...right?
You are actiing as an antenna picking up AC signals (mains, noise from the mains,
emanations from LCD screens, radio transmitters, whatever) - this you then couple
into the base which recitifies the AC so the DC component gets amplified into the collector circuit.
Squirt_5432:
harness or prevent this quality.
Harness this quality for a touch switch (Google "transistor touch switch")
Prevent this quality by not letting the base float (See reply #2)
Your not the first to spot this almost everyone does. Mainly when dealing with digital inputs on the Arduino that are a lot more sensitive than a single transistor.
You don't need two plates to get a capacitor, for instance the Earth has a capacitance
(relative to the rest of the universe).
Capacitance is simply charge/voltage, so you just something to measure the voltage
relative to (ie the rest of the universe if necessary). The earth's capacitance is about 700uF
If anyone still has a good old analog amplifier, try connecting an RCA patch cable to the amp, then touch just the inner conductor with your finger. You will likely hear a nice buzzing noise. Your body acts as an antenna picking up electrical interference. Fluorescent lights, AC powerline, your computer, an Arduino, etc.
An antenna does not have an anode and a cathode.