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555 Fast Reset Circuit

555 Fast Reset Timer

A common problem with on-delay timers is the timing capacitor reset. Typical timers (555 included) rely upon both complete power loss and finite elapsed time to assure that the timing capacitor is fully discharged. If this important detail is ignored, the timeout period is reduced and inconsistent. This 555 timer circuit resets almost instantly when Vcc drops below +10V. This circuit avoids the use of the 555 pin 4 reset feature that often causes more problems than it solves – this anomaly was previously documented in this article: Quirky 555 Timer Reset Function.

Background

The initial application for this circuit was a reset timer in which the output was hard-wired across a reset pushbutton. The terminology is a little confusing because this is a reset timer that resets digital electronics, and the timer itself must reset instantly upon power loss by dumping the charge in the timing capacitor.

When digital components (including some Bluetooth stuff) are interconnected in automotive applications, initialization is sometimes an annoying problem that is complicated if both pieces of equipment do not have the same reset criteria (e.g. one component resets upon complete power loss, while another component resets when Vcc drops only briefly or slightly). In this application, a 50sec reset duration was found to be effective (I cannot provide more information because I was helping someone of a different language in a distant country).

More advanced software features provide a “warm” reset feature that automatically restores lost communications – in such a case, a reset timer is not required.

Schematic

555 Fast Reset Circuit

Circuit Function

The 10V threshold is basically the drop of zener D1 and the base to emitter junction voltage of Q1. If a lower voltage is desired, a 9.1V, 8.2V or other zener may be used instead. When the voltage dips, the zener immediately stops conducting, Q1 turns off and the bias current through R3 (approx 1mA) is then diverted to the base of Q2. Q2 has an hFE of 60 @ 50mA, so its collector current will be a minimum of approx 60mA. The 47µf timing capacitor is discharged at this current and is fully discharged in less than 10mS. Note that it is discharged well before the 12V supply fully collapses. Note also that when Q2 turns on, the reset output function begins immediately and is completed 50sec after Vcc is restored. Of course, the reset output period may be adjusted per your requirements simply by adjusting the capacitor value.

I used a TLC555, but any 555 should function OK. In this circuit, the 555 timer is applied as a Schmitt trigger driver. This circuit detail is further described in the previous article: 555 (TLC555) Relay Driver Circuit

Oscillographs

The first oscillograph shows what happens without the fast reset circuitry (Q1 & Q2). Should the power be reapplied before the capacitor discharges to zero, the reset time period is indeterminate.

The second oscillograph shows the timing capacitor discharging in about 5mS – FAST!

The third oscillograph illustrates normal operation where the reset output is complete at the end of the timeout period. To simplify the taking of data, a shorter (14 sec) timeout period is shown (C1 = 10µf ). Aluminum electrolytics generally have a poor capacitance tolerance (-10%, +50% is typical). Also, the effect of the 10M scope probe tends to reduce the charge rate of the timing capacitor thus adding about a 10% timing error in this case. Otherwise, a 10µf capacitor would yield an 11sec timeout period in this circuit.

Photos

Fast Reset 555 Protoboard

For the future

  • 555 Fast Reset On Delay Timer (two versions)
  • 555 Off Delay Timer (very rare animal)

Undocumented words and idioms for our ESL friends

warm reset – idiomatic phrase – in electronics it indicates a software reset function (partial or complete) that occurs automatically or in process without the requirement of pushing a button or recycling the applied power. This is one detail that makes software friendly.

friendly software – idiomatic phrase when applied to software – friendly software is software that is easy and intuitive to learn or use.

animal – idiom when applied to an inanimate object rather than a living creature

3 Comments

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  • rgbassettgmail-com

    Hi, I am wishing to use this circuit as a 20 secont blanking after a start beam has been triggered for a race. The signal from the beam goes to a race timer then it blanks out for 20 seconds before resetting to allow for the next compeditor to start the race.
    I am new to this and built the circuit with a 22uf for C1. Can anyone help where I place the output wires to the timer and likewise where I connect the Negative 12 charge on the circuit?

    • Jim Keith

      The “negative charge” or negative power rail connects to the ground or common symbol. The output connects between the positive rail (positive power supply) and the Q3 transistor collector terminal. Or you may eliminate Q3 and take the output from U1-3 and common.

  • Tariq996

    I’ve tried to make this circuit & it worked well. But, when I experimented on the circuit further i found that there is no need of some components.
    R1,2,3 Q1, Q2 D1 & C2.
    I’ve experimented this circuit with 100uF capacitor as C1 & 560K as R4 which gives output of about59-65 seconds.
    & 1 more amazing thing is the circuit also works very well @ 9V (battery source).

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