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I have built the circuit from this answer.

I have R1 = 10k and C1 = 6.8nF which seems to be giving me about 7.5kHz output.

The timer works just fine but from a cold startup, the output is wildly unstable for 2-3 seconds. My scope shows it oscillating at a much higher frequency than intended until it settles on the expected one.

Is this simply a quirk of the 555? This circuit is being testing in isolation with a known good power supply, and decoupling caps (although the behavior is identical with or without them). I have seen this behavior occur regardless of the input voltage. I could see a very brief period of instability but for me to notice for multiple seconds when looking at the output seems indicative of a design issue on my end.

Other thoughts are that the C1 I'm using is a generic ceramic, perhaps it's exhibiting some thermal issues, or the RC circuit is taking a long time to build enough charge to saturate the BJT?

I can easily design around this and just be robust to an initially unstable timer with downstream components, but I am curious about what might cause this and how I might solve it.

brenzo
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    There are five schematics in the link you posted, and no indication if your circuit is identical to any of them. Please post a schematic of your circuit. Also, do you have any specific requirements such as a precise 50/50 output, Vcc range, etc. IOW, why not use the circuit in the datasheet? – AnalogKid Aug 03 '22 at 23:24
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    perhaps using a smaller capacitor on pin 5 will help. – Jasen Слава Україні Aug 04 '22 at 04:28

2 Answers2

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That's not to be expected at all- there is nothing in the circuit should have that kind of time constant.

Perhaps you have a bad solder joint or your timing capacitor is fault. Flaky electrical connections can behave strangely. Or something else that can't be imagined from this end.

In any case, I don't see much advantage to that circuit. If you need a 50% duty cycle it would be better to either use the standard datasheet astable configuration (at double frequency) with a flip-flop on the output or to use a CMOS version of 555 with feedback 'R1' from output to timing capacitor/trig/thresh and leave discharge open.

The transistor circuit will degrade the immunity to supply voltage changes (however the frequency will be lower than otherwise, not higher) because of diode drops vs. the purely ratiometric nature of the vanilla 555 connections.

Spehro Pefhany
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The 555 is a mixed analog and digital IC and has a min. operating voltage (e.g. 4.5V for the NE555).

So when powering it up, it can act weird as long as this minimal supply voltage requirement is not met.

Therefore, assuming that your supply voltage ramps up slowly (2-3 secs), I would not expect any other observations.

Stefan Wyss
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    I believe the 555 is considered a hybrid IC, as it has analog comparators and every circuit you build with them depends on analog levels. Only the output is digital. – Scott Seidman Aug 04 '22 at 22:31