Touch Activated Astable Multivibrator

249

02:43:20

The pushbutton isn’t a normal push button, but rather two wires next to each other that if you touch the tip of your finger to it creates a connection between them via your fingertip and if your finger has a resistance under 35kOhms it will turn on the circuit. If your finger is more resistive, the circuit will make a dimmer light and possibly run at a different speed depending on how sensitive astable multivibrators are to supply voltage.

published 7 years ago

hurz

7 years ago

You pump 8mA into a finger. 5kOhm in and 5kOhm out skin resistance would need a voltage of 80V

eekee

7 years ago

It still works with the resistor in series with the switch increased to 10kΩ to simulate the skin resistance. The right-side LED sometimes flickers a little. 20kΩ (skin + 10kΩ anti-short resistor) is too much unless the forward beta of the NPN is increased.

jason9

7 years ago

I specifically listed that any finger resistance below 35kOhm would still work. The 1kOhm resistor is just for current limiting in case the “finger” wasn’t actually a finger and was made of a conductor.

hurz

7 years ago

Right, i saw HBM human body models which handel the skin resistance very variable, it can be from 100 to 100000 Ohm. To calculate with something they took 5kOhm for the hand/foot skin.

Issacsutt

7 years ago

Good job, you should see if you can make it latch with as little components as you have.... a little challenge that you could build yourself! -if successful.

jason9

7 years ago

I tried making this circuit some weeks ago but I could only get it to work with the pushbutton instead of a touch sensor like described in the description. It used every transistor I had (two PNPs and one NPN).

Issacsutt

7 years ago

Try adding a high value resistor across the touch switch to make it more sensitive (only because of how little components you have to work with), (maybe between 100K-1M Ohm)... High enough that it won't fully saturate the transistor by itself, but low enough that when you touch it with your finger, your finger adds resistance in parallel with it, thus creating a 2nd path for current to flow, and overall reducing the total resistance.

Issacsutt

7 years ago

But take away the 1k Ohm, or swap it for a 100 Ohm if you want to keep one there for extra protection.

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