Delay circuit or IC with override - Page 1
Forum section: electronics / beginners

author: TGPCB
author_rank: Newbie
timestamp: July 18, 2025, 09:21:02 am
content: I have an application where an Arduino DUE will control an output pin that drives a relay (via a high side load switch). But if the Arduino does not boot up I need the output pin to turn on after a set amount of time (1-2s), ideally adjustable.I need an IC or circuit that will do the following:Turn the output on immediately when there is a signal on the input.If no input is present then turn on the output after a defined amount of time.Does such an IC existing, what term do I need to search for and do you know of any?I was thinking I could utilise a reset or watchdog IC or maybe even a push button IC like the TPS3808E-Q1, LTC6994-1, TPS380x, or similar and take advantage of the usual active low reset pin, but I having a hard time working out the logic of this.Any ideas? Thanks in advanced!

author: tooki
author_rank: Super Contributor
timestamp: July 18, 2025, 09:39:57 am
content: Just out of curiosity, why is the Arduino not booting a concern? Once your code is written and debugged, it should be very reliable.One approach I could see is a watchdog timer (or a missing pulse detector circuit) driven by a MCU GPIO that periodically resets it. The output of the watchdog and the output of another GPIO go to an appropriate logic gate to allow either one to turn on the relay.If this is for a safety-critical application, you might want to look into safety-rated controllers and relays (e.g. from Phoenix Contact). Just beware that they’re not cheap.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 18, 2025, 09:42:07 am
content: Easy enough to do with a few passives. Resistor/cap from the supply to turn on after delay, diode from MCU pin to override.Maybe a reset IC if you need a clean edge, controlling power to it via the RC network.

author: TGPCB
author_rank: Newbie
timestamp: July 18, 2025, 09:52:28 am
content: Quote from: tooki on July 18, 2025, 09:39:57 am1. Because its me writing the code! and 2. because the output pin has to be high if there is a failure of this module, 2. is the more critical point.Quote from: mikeselectricstuff on July 18, 2025, 09:42:07 amI was thinking about something like this or using a reset IC with a delay but struggling to get my head around it, what ever I do needs to be reliable in HW as the output must go high if there is an issue with the SW or other issue with the MCU.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 18, 2025, 09:56:27 am
content: If it's really important, have the hardware turn it on after the delay, and use a capacitively coupled charge pump to turn off, so it needs an actively changing signal from the MCU to turn off, that way if the pin is stuck high or low it will stay onThere are probably watchdog chips out there that will do this, but also doable with some passives.

author: TGPCB
author_rank: Newbie
timestamp: July 19, 2025, 12:47:10 am
content: I think I have an idea on how I can do this, but I have either over complicated it or it will not work at all...The TPS37A010122 is a dual voltage supervisor with separate programable sense and reset delay, channel 1 asserts reset low for over voltage, channel 2 asserts reset low for under voltage.https://www.ti.com/lit/ds/symlink/tps37-q1.pdfI have it configured as follows:P3V3_BAT is my permanent power source for the MCU backup domain.P3V3 is my switched source powered the MCU core and GPIOs.When P3V3 rises above ~3.3V for at least ~12.8ms FS_OUT will stay low for ~1.2s, after which FS_OUT will go high.When MCU_ACT rises above ~2.5V for at least ~128uS RSTn1 will pull down CTS2 which will discharge the capacitor and reset2 will be pulled up out of reset, therefore FS_OUT will go high.Timings are set arbitrarily at the moment, values just placed for now and can be tweaked.Will this work as I intend it to?My only concern is that the datasheet doesn't mention that you can pull down the CTS pins, so the behaviour is potentially unknown, however you can pull down the CTR pins albeit the internal resistor is 10x higher on the CTR pins.The MCU override feature isn't a critical feature, the most important part is the delayed backup, so I may put a 0R in between the RST1 and CTS2 pin, in case that doesn't work.