200 Minute Timer Circuit Design Help – 8A 120VAC Load.

[JJ_023]

I am asking the communities help in designing a circuit with the following parameters.

Timer needs to be fine-tuned once and then left as is.

The timer is going to be roughly 200 minutes.

The circuit is going to switch an 8 A load at 120VAC 60 Hz.  (In this particular case the load is primarily a heating element.) with a momentary LED switch.  The switch initiates the timer as well as abruptly ending the timer sequence should that be necessary.

I have built plenty of these types of circuits in the past. They all pretty much start out the same.
I take a 120V AC to 12V DC power supply and use that for the momentary switch as well as for the timer circuit.  The momentary switch is a lighted LED switch.  The timer circuit is usually some sort of microcontroller.  I have even used off-the-shelf products that are compatible with home assistant to get the ability to control the device from anywhere as well as giving Wi-Fi accessibility to the device.

The timer circuit then drives the SSR (solid-state relay) to switch the load on and off.  The SSR is typically rated to 100A AC and is controlled by 3-32V DC at 20mA (typically).   

The load also incorporates a fast blow fuse in its own dedicated fuse holder wired on the hot side.

I am primarily looking for advice on how to implement the timer part of the circuit, maybe other variants.  However, any overall ideas are also greatly appreciated in terms of improving what I am currently doing.

Based on my previous posts on here on the 555 it was suggested to get a CMOS variant.  I now have those in stock primarily the LMC555 CMOS Timer from Texas Instruments.  https://www.ti.com/lit/ds/symlink/lmc555.pdf?ts=1715555308343

Is it worth creating a timer circuit out of this chip (Monostable mode)?  The resistor values are in the 1G ohm range with a 100uf capacitor, or is it better to just stick with a microcontroller? 

Is there a better solution than those two for a timer to accomplish what I am looking to do?

As always thank you to everyone in advance that participates

[themadhippy]

binary counter and a few and gates?

[PCB.Wiz]

Timer parts like HEF4541, HEF4521, and even 4060 x 2 avoid very large values and make testing easy.

You could also look at
https://www.eevblog.com/forum/projects/how-to-shorten-the-signal-to-my-doorbell/msg5467111/#msg5467111
Where a timer-relay was the best (simplest,quickest,least wiring) solution.

[xvr]

The timer circuit is usually some sort of microcontroller.

Is there a better solution than those two for a timer to accomplish what I am looking to do?

Why not microcontroller? You can use some tiny MCU with 8 pins, it will be even simpler than 555 timer and post dividers.

[Gyro]

It depends on the OP's skill set, either method is valid. The technique they intend to use to determine and adjust the required time may also be relevant.

Edit: Scrub that, he has used microcontrollers before. Also, trying to use a monostable with such high resistance and capacitance values will be far too sensitive to board leakages, contamination, atmospheric humidity even. I'd consider that a non-starter.

[JJ_023]

binary counter and a few and gates?

I am familiar with both of those components and the proposed solution although I have never done it.  However I am unaware of how to implement it.   I will have to look into it.   

What would be the advantage of this over a microcontroller or a cmos 555?

[Gyro]

Sorry, I was just posting an edit to my above post when I saw yours. As I explained, there's no way you'll be able to do it in a single CMOS 555. You need to start with a faster clock and divide it down to stand any chance over 200 minutes.

[JJ_023]

Timer parts like HEF4541, HEF4521, and even 4060 x 2 avoid very large values and make testing easy.

You could also look at
https://www.eevblog.com/forum/projects/how-to-shorten-the-signal-to-my-doorbell/msg5467111/#msg5467111
Where a timer-relay was the best (simplest,quickest,least wiring) solution.

  I have read the data sheet for the HEF4541 it would definitely work and it is similar to the first responders post just an all-in-one solution.

I read that post  they had some interesting suggestions.   Thank you for sharing all that information.

[xvr]

What would be the advantage of this over a microcontroller?

There is impossible to beat MC over everything. 1 IC without any external circuitry (except blocking capacitor) - there is impossible to do any smaller/better.

[JJ_023]

A microcontroller is what I currently use as I stated in the opening post.  An Arduino would work as well.   I was just looking to expand my knowledge and see if anything could be improved upon or be made aware of something I was not.  Thank you.

[JJ_023]

Edit:  Also, trying to use a monostable with such high resistance and capacitance values will be far too sensitive to board leakages, contamination, atmospheric humidity even. I'd consider that a non-starter.

That was my concern with the monostable approach.   Thanks for pointing out and confirming this.  What do you think would be a good solution for a long timer besides microcontroller (that's what I am currently using)?   Definition of long timer being three hours.

[JJ_023]

Sorry, I was just posting an edit to my above post when I saw yours. As I explained, there's no way you'll be able to do it in a single CMOS 555. You need to start with a faster clock and divide it down to stand any chance over 200 minutes.

Thank you for elaborating on this.  I had reservations about the time length that's why I wrote this post.   I wanted to see if there is another solution besides a micro controller.   Don't get me wrong I have no issues with using a microcontroller.  I've done dozens and dozens of projects with them.   I just want to see if there is another solution to expand my knowledge and maybe broaden my exposure to things that are out there that I had no clue exist.

[JJ_023]

What would be the advantage of this over a microcontroller?

There is impossible to beat MC over everything. 1 IC without any external circuitry (except blocking capacitor) - there is impossible to do any smaller/better.

That was my thought process as well.  Also for people that might be reading this in the future that are not comfortable with microprocessors I would recommend looking at some of the home automation modules which interface with home assistant. 


You don't really need any programming skills to be able to get them to function in this intended manner.

[Gyro]

Sorry, I was just posting an edit to my above post when I saw yours. As I explained, there's no way you'll be able to do it in a single CMOS 555. You need to start with a faster clock and divide it down to stand any chance over 200 minutes.

Thank you for elaborating on this.  I had reservations about the time length that's why I wrote this post.   I wanted to see if there is another solution besides a micro controller.   Don't get me wrong I have no issues with using a microcontroller.  I've done dozens and dozens of projects with them.   I just want to see if there is another solution to expand my knowledge and maybe broaden my exposure to things that are out there that I had no clue exist.

As PCB.Wiz points out, using a 74HC4060 (or two) even a CD4060, will give you a 14 stage binary divider (1/16384), a simple cmos gate oscillator and a clear input, all in one chip. Your RC time constants then become a lot more sensible. I've used them before for very basic battery charger timers. The datasheet should give you all you need.

[themadhippy]

However I am unaware of how to implement

clock a binary counter once a second (the above mentioned cd4060, a 32.7khz crystal and a dived by 2 circuit for example)  and combine the counter outputs with AND gates so when  the count  reaches  10111011100000  it stops

[mariush]

Easier than a microcontroller? Probably a small motor and a bunch of reduction gears that would cause a wheel to make a half rotation or full rotation in 200 minutes.

basically a simplified version of this



Or... thinking cassette tapes ... have a cassette tape run at slow speed so that the tape ends at exactly 200 minutes... you can detect tape end with an optical sensor (tape becomes transparent at the end)

[Gyro]

...
Or... thinking cassette tapes ... have a cassette tape run at slow speed so that the tape ends at exactly 200 minutes... you can detect tape end with an optical sensor (tape becomes transparent at the end)

I sense a competition coming on. 

[JJ_023]

Sorry, I was just posting an edit to my above post when I saw yours. As I explained, there's no way you'll be able to do it in a single CMOS 555. You need to start with a faster clock and divide it down to stand any chance over 200 minutes.

Thank you for elaborating on this.  I had reservations about the time length that's why I wrote this post.   I wanted to see if there is another solution besides a micro controller.   Don't get me wrong I have no issues with using a microcontroller.  I've done dozens and dozens of projects with them.   I just want to see if there is another solution to expand my knowledge and maybe broaden my exposure to things that are out there that I had no clue exist.

As PCB.Wiz points out, using a 74HC4060 (or two) even a CD4060, will give you a 14 stage binary divider (1/16384), a simple cmos gate oscillator and a clear input, all in one chip. Your RC time constants then become a lot more sensible. I've used them before for very basic battery charger timers. The datasheet should give you all you need.

Thank you for that.  I looked at the data sheet for the Texas Instruments.   Maybe I will order one and play around with it.

[JJ_023]

However I am unaware of how to implement

clock a binary counter once a second (the above mentioned cd4060, a 32.7khz crystal and a dived by 2 circuit for example)  and combine the counter outputs with AND gates so when  the count  reaches  10111011100000  it stops

I get the gist of this but do not fully understand how to do this.   I'll try to look into it.   Thank you.

[JJ_023]

Easier than a microcontroller? Probably a small motor and a bunch of reduction gears that would cause a wheel to make a half rotation or full rotation in 200 minutes.

basically a simplified version of this



Or... thinking cassette tapes ... have a cassette tape run at slow speed so that the tape ends at exactly 200 minutes... you can detect tape end with an optical sensor (tape becomes transparent at the end)

That is a great post apocalyptic design.     Thank you for the ingenuity.

[Zero999]

A microcontroller is what I currently use as I stated in the opening post.  An Arduino would work as well.   I was just looking to expand my knowledge and see if anything could be improved upon or be made aware of something I was not.  Thank you.

You can use the smallest, simplest, 6-pin  microcontroller you can find and program it in assembly, rather than C.

[mag_therm]

I am  working on a separate "hardware" watchdog timer to stop ham sdr radio transmitter in event of cpu hang.
The timer I selected is "DC 12V FRM01 Timer" one of the generic modules available on Amazon, $17 for (2).

It has 18 modes, 0.1 s to 270 h  and  a 14 page programming manual that can be downloaded separately.
 The c/o contact is rated 125V AC 10 Amp, but recommended rating is 1/2 for resistive  or 1/3 for inductive/capacitive.
The AC contacts look well sized, separate from circuit and surrounded by ground plane
The modes are programmed with onboard buttons and led numeric display. There is a terminal for 1 20 ms pull up/pulse contact.

This timer comes in versions for 5V, 12V  and 24VDC supply, and there are a number of similar ones to this part number.

[ledtester]

Also, trying to use a monostable with such high resistance and capacitance values will be far too sensitive to board leakages, contamination, atmospheric humidity even. I'd consider that a non-starter.

Might check out the "C005" timer COB module which can produce delays from "5 seconds to over 40 days."

https://westsideelectronics.com/delay-timer-ic/

[schmitt trigger]

...
Or... thinking cassette tapes ... have a cassette tape run at slow speed so that the tape ends at exactly 200 minutes... you can detect tape end with an optical sensor (tape becomes transparent at the end)

I sense a competition coming on. 

How about a narrow throated funnel, filled with fine sand, which drops on a plate below which is attached to a strain gage? 

Now seriously , the CD4541 or other similar long range counter is actually your best bet, as previously suggested.

[PCB.Wiz]

Timer needs to be fine-tuned once and then left as is.
The timer is going to be roughly 200 minutes.

Questions here are how much fine-tune and for what values of 'roughly' ?

Digital dividers are very simple, and you can get very accurate binary multiples, from any frequency. (mains, or 32.768kHz)
With a programmable divider, like a 40103, you can get finer digital control

RC oscillators are easy to adjust, with a trimpot or fixed resistors,  and should be stable to 1%~ 0.1% from setpoint.

A small MCU would need to have some ADC ability, to allow for the adjust side.

If it is done using Logic, or a MCU, some test pin that is meter-able quickly, can let you know what the setting is, without waiting 200 minutes 

Some of those logic parts have a fast-test pin so a MCU design could do similar.