555 Timer False Triggering Problem

I will explain some solutions for monostable mode but maybe it can be useful to astable mode too. First of all; if you are here, probably you encountered a problem like these;

1. Although there is not any trig signal on trigger pin, when you plug in the power supply of the 555 circuit, your output becomes active.
2. Sometimes suddenly output pin becomes active without any reason.
3. When another load is activated, 555 timer output becomes active.
4. You are giving only one trig pulse the input of the 555 but it's output remain active more than one times.

You are on lucky day today because your 555 timer will work soon. By the way I'm not english but I like this pronunciation, five five five timer. Yes your fif fif fiveff timer will be work. I hope.

Answer 1

This problem generally about power filter capacitors, other capacitor charging times, inductance effects etc. 555 timer is not a secret box, this is a logic electric circuit but minimal and compact version. So, first time transients are very effective on that circuit. There are many way to fix it but in my opinion the best way is using power-on reset circuit and clean power conditioning circuit. I will explain power problems in other answers.

Power-On Reset 555 Monostable

Power-on reset is a simple RC circuit on R pin of 555 timer. Thus, this circuit keeps 555 deactivated until the C6 capacitor is full. You can adjust this time with time constant of RC circuit. (Don't use values on the picture, they are only representative values.)

Answer 2

Please control your trig pin circuit. Probably you are using 555 with a sensor or a button or similar thigns. So, you should filter this signal. Because there is noise on everywhere. Such as circuit inductance, your cable length, your hand, other cicuits etc. You should use tiny ceramic filter capacitors, RC filter circuts. Consequently I mean EMI, EMC.

Another general reason is about your connected device. Some cheap sensors sometimes generates false output signal. You should remove any device from trigger input and observe behaviour of 555. If there is no any problem on that condition, false triggering caused by your sensor or conected device.

Answer 3 - 4

This solution also important for first and second problems also. Many digital circuit unexpected behaviours caused by poor EMI, EMC protection and according that poor power conditioning and sourcing.

You should supply 555 timer from clean power regulator. You should use LDO or classic linear regulators for supply the 555. If there is any noisy load on your project that supplied by same power supply, you should isolate it. (For example supply that another output of power supply) Fundamenetal solutions:

• Use power filtering capacitors. Capacitors should be parallel with different sizes and types.
• Capacitor inrush current limiting. Especially prevent for first plug false trigger problems.
• Supply 555 from an linear regulator.
• Supply digital circuit isolated from other loads.
• Use TVS and zener diodes on power stages and trigger and reset inputs.
• Use RC filter on trigger input and maybe for main power source input.
• Don't use long cables.

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