Timer for holding circuit

Thread Starter

FroceMaster

Joined Jan 28, 2012
632
Hi,
Need to make a curcuit for something, and to that i will need a timer,
When i activate by GND, the timer should activate an external relay,

Problem is that GND is activated allways, until user deactivate.

I want it to be activated when first time on GND and stay on for ex 15 min, then the relay deactivates again, and the user will have to disconnect from GND and connect to GND again to activate again.

Hope u understand, or feel free to ask.

Rgds
Sten
 
Last edited by a moderator:

ericgibbs

Joined Jan 29, 2010
10,846
hi FM,
When you say activated when first time on GND , do you mean when a control pin is connected to Gnd [ 0v] , the delay starts.?
Post a simple sketch showing how the timer will be wired in the project.
E
 

Reloadron

Joined Jan 15, 2015
5,646
Understanding Time Delay Relay Functions will give you a good overview timer functions and naming conventions. My guess based on your wording is that you want what is commonly called a "one shot" timer circuit.
Upon application of input voltage, the time delay relay is ready to accept a trigger. When the trigger is applied, the output is energized and the time delay (t) begins. During the time delay (t), the trigger is ignored. At the end of the time delay (t), the output is de-energized and the time delay relay is ready to accept another trigger.

You can build (roll your own) or buy an off the shelf turn key solution. In your case a user would push for example a momentary push button starting a timing cycle which would activate a relay for a period of 15 min at which time the relay would de energize and await another trigger signal to repeat the process. Considering 15 min On Time I would consider just buying a simple one shot timer designed for your application. You make no mention of switching voltage or current or trigger volyage?

Ron
 

SamR

Joined Mar 19, 2019
2,602
Square D manufactures a whole host of Octal Base timing relays in all of the various flavors. If you are looking for 120VAC control...
 

AnalogKid

Joined Aug 1, 2013
8,608
What voltages are available to run the circuit?

What is the relay coil voltage and current?

If someone presses the button during the timing cycle, like at 7 minutes into it, does the timer keep going for 8 minutes, or does it reset to zero and go on for 15 minutes from the time of the second button press? In other words, when the timer is running does it ignore the control switch?

What is your skill set for assembling a small circuit on perf board?

ak
 

Thread Starter

FroceMaster

Joined Jan 28, 2012
632
HAve drawn a little sketch,
The timer should be reset when "SW" is realesed. like after 7 min, and then SW is open, the timer is reset to 15 min, ready to start.

12 v dc, output of relay is around 20A.
Relay use max 75 mA.

5555 001.jpg
 

Reloadron

Joined Jan 15, 2015
5,646
You want a "one shot" now you can roll your own or just buy an over the counter turn key solution. Non-retriggerable one shot so after it is triggered any additional triggers will have no effect until the timing period times out. You want a 12 volt version and as SamR points out Square D markets them as well as NCC, Macromatic, Dayton and others as well as very inexpensive versions off the boat from China. Since you mention 15 min On Time just make sure whatever you choose will give you 15 min and meets any accuracy specification you may have.

One Shot Triggering.png

Note how subsequent triggers have no effect on the output. Most allow for using a positive or negative trigger. As to your relay handling 20 Amps DC I would just think about driving a 12 Volt automotive relay in a 30 Amp variety, they have a low coil current. That or a MOSFET.
The timer should be reset when "SW" is realesed. like after 7 min, and then SW is open, the timer is reset to 15 min, ready to start.
The trigger is just a fast momentary switch closure (push button for example) all additional triggers are ignored until the timing period times out.

Ron
 

djsfantasi

Joined Apr 11, 2010
6,848
Or an ATTiny25 with a tiny bit of code. You will need a programmer and Sparkfun sells one for ~$17. Or another microcontroller programmer to be a programmer.
 

AnalogKid

Joined Aug 1, 2013
8,608
15 minutes is a looooong time for a traditional one-shot, which is basically an R-C timer. Better to use something that counts a large number of clock cycles, like a uC running code.

OR, for the zero-programming crowd, something based on the CD4060, my go-to part for long timer stuff.

1 - CD4060
3 - R
2 - C
1 - MOSFET driver transistor for the relay
1 - diode to suppress the relay coil

I'll try to whip out a schematic later. My past posts have several examples.

ak
 

ElectricSpidey

Joined Dec 2, 2017
1,258
If you are using a 555, the trigger should be connected to ground with a capacitor and positive with a pullup resistor.

Then the trigger is also connected to ground with the switch (momentary), to restart the timing cycle again.

Then…

The timer starts on power up, then can be started again any time after the cycle ends.

If you need to restart the cycle from the beginning with the switch during any cycle, this is also possible, with some slight mods to the circuit. IE discharging the timing cap when the switch is pushed.

Cycle can be ended anytime with another momentary switch from reset pin to ground. (with pullup at pin).

Actually I use a SPDT momentary switch for this exact function on many of my projects. (push up...start, push down...end)
 
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Thread Starter

FroceMaster

Joined Jan 28, 2012
632
Have been thinking , and thinki will use a little MCU to make the timercurcuit.
Have just some questions.
First drawing here is how it's connected original.
Second is how i maybe will connect it. but have a small issue i think.
Will the "LED" in the switch "glow" due to 12v-5v = 7v over switch or ? when original switch is open
Have added the Diode,
When 15 minutes is up, the MCU disconnect the original Relay
888 001.jpg
 

Reloadron

Joined Jan 15, 2015
5,646
Problem is that GND is activated allways, until user deactivate.
Taken from your first post but that is not reflected in your drawings. You reflect 12 volts present via a pull up resistor with a switch making ground? So what is it?

If you don't want to use a one shot as suggested then have at it with a uC just make sure the uC of choice can do a 15 min count (delay).

Ron
 

Thread Starter

FroceMaster

Joined Jan 28, 2012
632
As i tried to explain the user activates "orignal switch" to GND as see on latest drawing.

The TOP of latest drawing shows the curcuit right now.
I want to make the modifikation.

The curcuit is for heated seat in car and the passenger forgets to turn it off , therefor a timer .

Have studie the seat switch but that one is connected to airbag box and by a resistor network, that i will not change. (Could have been easy to add an relay so when passenger gets out the seat heater cuts off)

No problem to make a 15 min timer function in uC . Just need to know if it gives trouble to connect as button part of latest drawing
 

Reloadron

Joined Jan 15, 2015
5,646
Then as drawn it should work fine. Have your code poll the uC input pin you choose. Pressing the push button will apply a logic low (ground) to your pin. If trigPin == Low then do something, delay and loop again. Something like that. I would likely use a 10K pull up or some uC chips have internal pull up which you can address in your code.

Ron
 
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