It seems the reset pin on the NE555 is connected to a PNP which could be left open instead of to upper rail.
What are the drawbacks of leaving the reset pin floating?
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2\$\begingroup\$ Do you have a definitive schematic of the internals of the device? Have you checked with the data sheet to establish that your assertion is viable? \$\endgroup\$– Andy akaCommented Dec 10, 2022 at 11:32
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\$\begingroup\$ The ST datasheet contains an schematic. But only contains information for "lowering" the RESET. st.com/resource/en/datasheet/ne555.pdf \$\endgroup\$– Steve SchneppCommented Dec 10, 2022 at 11:41
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\$\begingroup\$ And, i never connected the Reset pin. So it works for hobbyist purposes. But I was wondering what could go wrong. \$\endgroup\$– Steve SchneppCommented Dec 10, 2022 at 11:43
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\$\begingroup\$ Read the bottom of page 8 \$\endgroup\$– Andy akaCommented Dec 10, 2022 at 11:49
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3 Answers
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Can the NE555 reset pin remain floating?
From the data sheet you linked (at the bottom of page 8): -
From the Diodes Inc data sheet (page 7): -
And, just in case the recommendations for connecting the reset pin to Vcc are a recent thing, here is a picture of the 1973 Signetics data sheet (page 9): -
Maybe there is a 1972 version when the device first came out that someone can find?
answered Dec 10, 2022 at 11:53
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\$\begingroup\$ Now I am wondering why it might trigger a reset, but that is another question. As I kindof agree, the answer to this one is obvious in retrospect \$\endgroup\$ Commented Dec 10, 2022 at 12:19
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\$\begingroup\$ It's never a good idea to leave input pins floating @SteveSchnepp \$\endgroup\$– Andy akaCommented Dec 10, 2022 at 12:20
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\$\begingroup\$ I knew about it on CMOS, but didn't for TTL tech. \$\endgroup\$ Commented Dec 10, 2022 at 12:23
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1\$\begingroup\$ At least one of the example circuits in your link has the /RESET pin left open during operation. \$\endgroup\$ Commented Dec 10, 2022 at 15:22
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2\$\begingroup\$ They clearly haven't taken their own advice. \$\endgroup\$– Andy akaCommented Dec 10, 2022 at 15:26
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Drawback is it might not work at all, or it might work intermittently or unreliably.
Direct quote from TI NE555 data sheet:
To prevent false triggering, when RESET is not used, it should be connected to VCC.
The above quote is identical with what reads in Diodes Incorporated NE555 data sheet.
Direct quote from ST NE555 data sheet:
When Reset is not used, it should be tied high to avoid any possibility of unwanted triggering
Even if you have never had a problem leaving it open, it does not mean it is a good idea to leave it open as it might not always work.
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- It costs nothing to have RESET connected.
- If someone were to ever get your circuit and replaced the bipolar 555 with a CMOS 555, they’d be in trouble with an open input pin. They could get spurious triggering, excessive current consumption (potentially up to self-destruction at higher supply voltages and temperatures), and generally the circuit wouldn’t act reliably at all.
On the balance, I think there’s no reason not to connect it.
answered Dec 11, 2022 at 13:35
Kuba hasn't forgotten MonicaKuba hasn't forgotten Monica
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1\$\begingroup\$ The "no surprise" factor when you replace with a compatible part is indeed a very good reason. Wondering about the inners of a cmos ne555 now \$\endgroup\$ Commented Dec 11, 2022 at 16:48
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\$\begingroup\$ CMOS inputs generally can't be left floating. This applies to any CMOS logic device. \$\endgroup\$ Commented Dec 11, 2022 at 17:24