Delay timer using mosfet - Page 1
Forum section: electronics / beginners

author: paulhm81
author_rank: Regular Contributor
timestamp: May 31, 2018, 05:12:17 pm
content: HelloI'm trying to figure out if there is a way to use the schematic in the picture to cut power but not gradually. I would like to avoid using a relay because they are big and my load will be small (0.2amps) and I would also like to avoid microcontrolers or timers. In my car I have an ionizer and my cigarette lighter is always powered. I thought about changing it to work on contact but the ionizer needs some time to do it's job, so I was thinking on delaying the power off for 2 hours but It's not ok to power off slowly decreasing the voltage. Any ideas?Thank you!

author: Wimberleytech
author_rank: Super Contributor
timestamp: May 31, 2018, 05:31:08 pm
content: Quote from: paulhm81 on May 31, 2018, 05:12:17 pmHow many components are allowed for your minimalist solution? How much area is allowed? The circuit you have will not work according to your needs, which I think you already know.

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 31, 2018, 05:34:31 pm
content: If you don't want to use a microcontroller, then:The way to go is to use a low frequency oscillator, followed by a divider string.A 2.27Hz oscillator followed by a 14 stage divider would do the 2 hour trick. This can be done with a single CD4060 IC.

author: paulhm81
author_rank: Regular Contributor
timestamp: May 31, 2018, 05:41:12 pm
content: I'm only concerned about the size and I'm not that into microcontrolers. I was thinking there is a solution that I'm not aware about like using an additional transistor or a zenner or something. The simplest way I can think of is using a 555.

author: Zero999
author_rank: Super Contributor
timestamp: May 31, 2018, 05:50:00 pm
content: I recently posted a circuit in another thread which does this.https://www.eevblog.com/forum/beginners/a-one-time-timer/msg1574389/#msg1574389However, in practise a simple RC timer is unsuitable for even a 5 minute delay, let alone 2 hours. The only sane way is with an oscillator and counter, such as the CD4060 mentioned above or a proper time IC such as the MC1/CD4536B or MC1/CD4541B.https://www.onsemi.com/pub/Collateral/MC14536B-D.PDFhttps://www.onsemi.com/pub/Collateral/MC14541B-D.PDF

author: paulhm81
author_rank: Regular Contributor
timestamp: May 31, 2018, 05:52:07 pm
content: Quote from: schmitt trigger on May 31, 2018, 05:34:31 pmThank you for the idea. I used it once to turn off my soldering station if I forget it on. I thought I can do it without ic's.I'm going to use a cd4060 thenA question about hero999's schematic even if it's not suited for such a long delay: The trigger could be a switch to V1?

author: Zero999
author_rank: Super Contributor
timestamp: June 01, 2018, 12:17:45 pm
content: Quote from: paulhm81 on May 31, 2018, 05:52:07 pmOf course it's possible without ICs. The old school way to do this would be with a cam timer.https://en.wikipedia.org/wiki/Cam_timerQuoteYes, that will work. If the switch will never be activated for longer than the time delay, then the diode, resistor and capacitor are not needed.

author: paulhm81
author_rank: Regular Contributor
timestamp: June 01, 2018, 06:33:21 pm
content: Quote from: Hero999 on June 01, 2018, 12:17:45 pm

author: Zero999
author_rank: Super Contributor
timestamp: June 03, 2018, 06:50:11 pm
content: Quote from: paulhm81 on June 01, 2018, 06:33:21 pmYou may laugh but before the advent of ICs, this was the only sane way to implement a time delay of 2 hours. A semiconductor design, would require a large number of discrete components, even with an approximate time delay. An astable could be used, with a period of 7.5 minutes, requiring 2 transistors, followed by 15 stage a frequency divider, to divide the frequency by 16 to give a period of 120 minutes, requiring another 30 transistors: two per stage. A total of 32 transistors, not counting the discrete components or any additional transistor logic for starting/stopping/resetting the timer.

author: Yansi
author_rank: Super Contributor
timestamp: June 03, 2018, 06:55:38 pm
content: Cam timers are stillwidelyused in cheap washing machines!

author: Zero999
author_rank: Super Contributor
timestamp: June 03, 2018, 08:21:17 pm
content: Quote from: Yansi on June 03, 2018, 06:55:38 pmAre you sure? I thought microcontrollers have been cheaper for a long time.There might still be cam timers in some washing machine models, but I wouldn't expect them to be the cheapest. They do have advantages such as: continuing from the same point, after the power has been interrupted, although this would be possible with an MCU with flash memory/battery backup and the user being about to manually adjust the programme, moving it on or backwards, as they see fit.

author: Yansi
author_rank: Super Contributor
timestamp: June 03, 2018, 08:37:41 pm
content: I think you are about correct, however the warehouses may be still full of these old mechanical thingies from the past, so you will still be able to find them in shops hanging around here and there occasionally.

author: Wimberleytech
author_rank: Super Contributor
timestamp: June 03, 2018, 09:01:31 pm
content: Quote from: Hero999 on June 03, 2018, 08:21:17 pmhttps://www.bestbuy.com/site/amana-3-5-cu-ft-8-cycle-top-loading-washer-white/5369600.p?skuId=5369600Looks like a mechanical timer to me. Here is the breakdown at BB

author: Zero999
author_rank: Super Contributor
timestamp: June 03, 2018, 10:10:04 pm
content: Quote from: Wimberleytech on June 03, 2018, 09:01:31 pmIs it the cheapest in its range? I'd be surprised if it is. Microcontroller based controllers, are so much cheaper to make!Sorry, the specifications and price don't mean much to me, as they're in US units.

author: Wimberleytech
author_rank: Super Contributor
timestamp: June 04, 2018, 01:06:12 pm
content: Quote from: Hero999 on June 03, 2018, 10:10:04 pmYes, the cheaper ones are electromechanical. If you want digital, you pay more.

author: Zero999
author_rank: Super Contributor
timestamp: June 04, 2018, 01:12:45 pm
content: Quote from: Wimberleytech on June 04, 2018, 01:06:12 pmThat's surprising. I suppose there may be other factors I haven't considerer: the timer is only a small part of the machine and software development?Is this just in the US? I haven't looked at washing machines before. Most of them in the UK are font-loaders and have digital controls.

author: Wimberleytech
author_rank: Super Contributor
timestamp: June 04, 2018, 01:32:05 pm
content: Quote from: Hero999 on June 04, 2018, 01:12:45 pm"Just in the US?" Good question...don't know. As to SW development...as I am sure you know...that cost is in the development bucket and is not directly attributed to per-unit costs.I, like you, feel like the digital solution would be cheaper. Perhaps it is just marketing.