Quirky 555 Timer Reset Function

How many have had legitimate problems with the 555 timer? Let me guess…it involved the reset line, did it not? We all know that pin 4 must be set high before oscillation begins, but what is its threshold? input current? and what happens when it is operated slightly out of spec? This may be the very first attempt in documentation of this obscure phenomenon.

555 pin 4 reset behavior schematic

555 pin 4 reset behavior Test Circuit 1


To determine reset threshold, a transistor integrator generates a low impedance, negative-going ramp voltage signal that integrates from +5V to –0.65V. To obtain the negative voltage, the power supply is split to provide –0.7V. While it is slowly changing at the rate of -1V /S (trace 1), pins 2 & 6 are monitored for oscillation (trace 2). A 3rd trace monitors the output (pin 3). All data is logged on the spreadsheet.

A total of (8) devices were tested, including (2) CMOS TLC555N devices.
When running, the 555 oscillates at approx. 100hZ. Vcc = 5V.




The threshold voltage is exceptionally low—lower than any true logic device. Specifications indicate a threshold range of 0.4 to 1.0V in bipolar devices, and 0.4 to 1.5V in CMOS device. Actual measurements indicate a range of 0.65 to 0.75V (bipolar) and 1.1V (CMOS).

What is most interesting is what happens when the voltage drops a little below –0.12V. At this voltage, pin 7 turns off and the voltage at pins 2 & 6 floats high while pin 3 demonstrates an abnormally high saturation voltage (0.8V)—normal is 0.1V. While this is an out-of-spec condition, it remains important information to know because it occurs very close to zero volts and may be subject to noise.

In some cases, this anomaly can actually occur above 0V. I recall from years ago some 555’s that exhibited this behavior at approx +20mV. The solution at the time was the addition of a series resistor in the reset line. Of course these devices were out of spec and therefore defective according to published specifications. Since then, I think manufacturers have been testing for this specific condition. I challenge readers to locate some of these strange 555’s and make comment—very simple to test for this condition by simply shorting pin 4 to common and see if pin 7 allows capacitor voltage to float up.

One of the CMOS devices actually started to oscillate again at -0.65V. I have had issues with this chip before, so I think that I can write it off as defective—may have taken a static discharge…

Not recommended as a voltage threshold function input

Due to the extremely low and variable threshold, the reset line must not be used as an input level threshold. I know it is used this way in numerous circuits, but I cannot recommend it—this may be an explanation why some circuits fail to operate.

Looks good on paper, but this is an out of spec condition

This is a typical interface to avoid—looks clean and simple and works with most logic interfaces, but does not work with the 555. What a simulator would show? A simulator cannot handle out of spec conditions.

Another way to preset /reset the 555

Open collector comparators etc. work well as an interface to the 555.

Input current

Reset current maxed out at 0V. Actual range was -13 to -33uA. The minus sign indicates that current comes out of pin 4 (sourced) as opposed to going into the pin (sinked). CMOS input current was understandably immeasurable. Actual specification for the bipolar version is 400uA typical. From this data you can determine what you need to drive it below the reset threshold voltage—Max series resistor value must not generally exceed 470Ω or so.


For the future

Quirky LM358 /LM324 /LM293 /LM339 common mode input range issues

Glossary of undocumented words and idioms (for our ESL friends)

write it off –idiom—consider defective—forget it—give up on that idea…

Recommended devices for the serious experimenter

DIP-8 Package
DigiKey LM555CNFS-ND, $0.43 each

DIP-8 Package, CMOS
DigiKey 296-1857-5-ND, $0.74 each


Join the conversation!

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  • russellsher

    Hi – thanks for that.

  • russellsher

    Thanks for the article – In the picture, Test-Circuit-3, you say: Any guesses why this will not function?
    Is this because the diode clamp D2 (0.7V +/-) will not guarantee a reset in all devices? Or is there another reason?

    • Jim Keith

      “to get the reset pin to work correctly, you need a resistor to bias the Reset pin to GND”

      You have it correct!

    • Charlie b

      yeah i know, comparitor direct to the cap is better than using the pin properly, i didnt thoROUGHly check the suggested alternatives… D:

    • Charlie b

      Judging by that 555 internals schematic, to get the reset pin to work correctly, you need a resistor to bias the Reset pin to GND. When you want the timer to run, pull the reset pin up to VCC with a PNP transistor (rather than use the suggested components on the capacitor). This is also comparitor compatible.


    • Jim Keith

      @russellsher, It is because when pin4 is pulled below about 200mV, the 555 misbehaves. This is an unstated condition for the 555. Looking at the internal schematic, it is obvious that the base to collector junction of Q25 forward biases and sucks the base drive out of Q14 so that it can no longer discharge the timing capacitor.

  • seelan

    thanks for info, I tried to use a diode in line to isolate pin 4 and it did not want to reset, needed to be completely shorted to ground

  • Dalik

    …i am sorry, my previous statement about control pin is not true..:)

  • Dalik

    Hi, excelent article, thanks!

    Its possible to use pin CONTROL as a some kind of reset.

    Pete: Voltages as -0,12V are actually negative! There is a diode which push 555 circuit 0,7 V above the level of the ramp. Its mentioned in the text…

    • Jim Keith

      I just reread the article and cannot find the specific errors that you are referring to. I can appreciate how confusing this may be to understand–standing on your head may help…

      What we are doing here is seeing what actually happens in the range of about +1 to -0.65V. In the past, I have seen 555s that exhibited strange behavior when the reset line was actually a little positive. However, I was not able to get any of my sample devices to exhibit this behavior in this bit of research–as a result, I ran the voltage a little negative to see really how sensitive or close to zero this phenomenon occurs.

      Concerning the possibility of destruction when inputs /outputs are forced negative, it is actually a function of available current–if the current is limited to a safe value, there will not be smoke. Generally in bipolar ICs, -0.6V will not hurt.

  • Pete

    Another excellent investigation, Jim!

    The only issue for me was that you tend to use the minus sign with voltages that are actually positive. For example, you mentioned voltages like -0.12V and -0.65V in a couple of places, when you meant an absolute value on the reset pin of +0.12V and +0.65V, respectively. We’d normally drop the sign when it’s positive.

    Since you’re measuring the voltages relative to ground, you need to be really careful not to use the minus sign as a placeholder or separator. As I’m sure you already know, voltages below ground will destroy most chips, not just the 555!

    I’m guessing that this is simply something you’ve picked up over time. But being precise about this will greatly help non-English readers and newcomers, who tend to take things quite literally!

    That said, I think you’re doing a fantastic job documenting the 555’s undocumented features! I can’t wait to look at some of your other articles. Well done, mate!


    Now this makes sense as I thought thet I was stupid and made silly mysteakes (misspelled on purpose)

  • yunush

    i hed comlete my sirvice in Gujrat.electy.board gujrat india and im allso electroniks hobbyest