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.
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.
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
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.
For the future
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