Astable 555 Timer Circuit Acts as Proximity Sensor?

Thread Starter

Adam Clegg

Joined Jul 9, 2017
Hi guys first post and newbie to this stuff...

I build my first astable 555 timer circuit, using a TLC555CP chip. Unfortunately, it seems to have a problem. Sometimes, the output of the timer simply stops oscillating, with no apparent cause. Stranger yet, the problem often occurs at the precise time anyone moves their hand within a few inches of the circuit board (without actually touching anything!).

What am I doing wrong in assembling this circuit to cause such a problem?

I found this online elsewhere but I honestly don't know what is going on.

"the TLC555CP integrated circuit (“chip”) uses CMOS technology. Every year it seems I have at least one student who experiences this particular problem, usually as a result of hasty circuit assembly (not making all necessary connections to pins on the chip). This is a good question to brainstorm with your class on, exploring possible causes and methods of diagnosis."

I also did make sure to search this issue before posting!!!

Thanks guys.


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Joined Jun 19, 2012
CMOS inputs have super high impedance, if left floating, i.e. unconnected, the
voltage on this pin can be influenced by electrostatic fields nearby.

like this nifty little demo:



Joined Aug 1, 2013
The MO in CMOS stands for metal oxide. Basically, a CMOS transistor has a sheet of glass in it approx. 10 atoms thick. That is so thin that static electricity discharges too small to feel, like 1 kV or less, can puncture that layer and destroy the transistor. Many CMOS devices have anti-static protection designed in, but still have a very high input impedance - as in giga-ohms. A MOS transistor can detect someone walking past it 5 or 10 feet away, literally a disturbance in the force. Back in the 60's or early 70's, Popular Electronics had a "people detector" project based on a small metal triangle connected to a MOSFET gate.

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Joined Aug 12, 2014
You will be much better off if you follow the examples given in the datasheet than trying things you find on the web.
Best all-around piece of advice I've seen in a while!

There are so, so many terrible circuit ideas online, while most datasheets have example circuits that teach nearly everything you need.