Table of Contents
![Touch based bistable 555 timer circuit with LEDs output learning electronics lesson 0065](https://electronzap.com/wp-content/uploads/2021/09/Touch-based-bistable-555-timer-circuit-with-LEDs-output-learning-electronics-lesson-0065.png)
The human body picks up and generates electromagnetic activity. Therefore it can transfer voltage and a very small amount of current to a sensitive circuit.
The trigger, threshold, and reset pins of a 555 timer are very sensitive to any voltage.
Those 3 input pins have high impedance. That means that they don’t let current through them, other than a small amount of leakage current. It’s such a small amount of current, that it’s almost always OK to just think of it as being no current.
An input that doesn’t let current flow (high impedance) is useful because it doesn’t alter the voltage of a relatively weak signal. It just responds to that voltage.
Video:
- Trigger pin (2) with 1/3 or less of the supply voltage (Vcc) across it sets the output high (as close to the positive supply (Vcc) as it can get). The one million ohm pull up resistor connecting pin 2 to the 5V supply holds it at 5V against very weak signals. Touching it causes a voltage oscillation that is sometimes below 1/3 supply voltage. Signals from your body are strong enough to overcome a voltage through a 1MΩ resistor.
- Threshold pin (6) sets the output low (pretty good connection to ground/0V) if it gets more than 2/3 of the supply voltage (Vcc) applied to it. The one million ohm resistor connecting it to ground (pull down) holds it at 0V against very weak stray signals, but is low enough resistance to allow the oscillating signals from your body to raise the voltage enough.
Good pages to check out next:
To support this site, check out the following links:
Become a Patron! I currently share my new diagrams with patron supporters and will also share them with anyone that joins my YouTube channel through the following link.
Become a member of my YouTube channel!
- Check out my YouTube videos! https://www.youtube.com/c/Electronzap/videos
- Products I used in my videos or otherwise think look like a good buy. As an Amazon associate, I earn from qualifying purchases. My Amazon affiliate page showing products I think look good
- Information on this site is not guaranteed to be accurate. Always consult the manufacturer info/datasheet of parts you use. Research the proper safety precautions for everything you do.
- Electronzap is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com.