555 ic toggle on/off WIERD behaviour. Please share your inputs.

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ZOGAM

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Hello friends,
The attached diagram is a 555 IC which output HIGH i.e. LED is ON when toggle switch is pressed, and when pressed again, the output changes to LOW i.e LED is OFF.

As per the attached diagram, I have setup the circuit on breadboard with 0.01 mF ceramic capacitor added between pin 5 and GND. Hence, everything works well fine as supposed to be.

However, I noticed some weird behaviour/strange things as discussed below:

1. With the LED ON or OFF, when either the positive or GND of the power supply is touched through TWEEZERS, the LED turns on and off correspondingly. So, I add a 100 nF ceramic capacitor between pin 2 and GND. Yes, this issue is solved PARTIALLY.

2. Again, in this case, with LED ON, when either positive or GND of the power supply is tapped again, the LED turns OFF immediately. Is not it supposed to stay ON? Because the toggle switch is still maintained at open position. The LED is supposed to be OFF if and only when the switch is closed. So, in this particular case, how to fix this issue of LED turns OFF when either positive or ground side is tapped???? However, with LED OFF, the LED remains OFF even though the said wires are tapped individually.

3. And last one, I noticed is that, with LED ON, I hold the pin 2 and once again, tapped the aforementioned wires. Surprisingly, in this case, the LED CONTINUES to remain in ON state. But when I remove my hand from pin 2 and do retapping on the said wires, the LED turns OFF. This is something weird behaviour. Still I got the same issues when I hold the positive side of the 1 mF capacitor or both with pin 2.

So, please thoroughly read the information and help me out to fix these issues
 

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betwixt said:
Where is the capacitor across VCC and GND required to keep the supply low impedance?

Brian.
Click to expand...
With adding or without adding the capacitor, the issue still persist. Hence, the diagram.

Please carefully watch here the demonstration of the issues.
 

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C
betwixt said:
Wiring lengths like that are bound to cause problems, especially so on a prototyping board like that. Keep the wiring length to a minimum and keep the capacitors as close to the IC as possible.

Brian.
Click to expand...
Can you please setup the same circuit on breadboard from your end to see the same issues? This way everything will be vividly explain to you.
 

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Mechanical induced electrical problems usually contact issues. Breadboards
like you are using (me included) over time can develop poor contact engagement,
and get dirty also causing contact issues.

Touching hi impedance nodes in a circuit can cause problems. In this case the
trigger pin. Especially labs with fluorescent lighting causing whole body induced line
potentials. Basically body becomes capacitively coupled and when one touches a
hi Z node induces unwanted signal.

The breadboard you are using, on some the horizontal pin rows at top and bottom,
used as power busses. Many are split, eg. so if you use one for ground only half of the
row is at ground unless in the middle you jumper the two busses together.


Regards, Dana.
 

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I have no doubts that it's possible to get parasitic touch sensitivity by building the circuits as large breadboard, possibly with ungrounded wall-plug transformer powering it.

I'd rather ask the opposite question. Can you make it operate reliable, what's an appropriate circuit layout?
 

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Note most solderless breadboards are marked if horizontal busses are split or contiguous, but
not all. You can flip board upside down and/or just use ohmmeter to see if leftmost to right most
pin hole on board are a contiguous buss or not.



Regards, Dana.
 
Last edited:

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Regarding breadboard, I have not issues at all. The breadboard is perfectly working fine.
 

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Why are you concerned that the circuit changes state when randomly touching various nodes?
Is that an odd requirement for the finished design?

A pick: mF is millifarad but I assume you mean uF (microfarad).
 

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ZOGAM said:
Regarding breadboard, I have not issues at all. The breadboard is perfectly working fine
Click to expand...
Just my two-cents (or two Euros, or whatever). I agree that that “layout” is a mess. I’m not the least bit surprised you’re seeing these problems.
 

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