Delay off relay switch

Thread Starter

Natsosoyami

Joined Jan 9, 2020
3
Hello friends. I need a diagram for a 12v adjustable (0s-2min) delay off relay switch that the input will serve as the triggering device. As soon as the circuit recieves power, the output is switched on and also the turn off timing is activated. The circuit will be activated only once when it recieves the input.
 

crutschow

Joined Mar 14, 2008
24,055
Here's the LTspice simulation of an example 555 circuit with a delay off.
The maximum delay with the shown component values is about 165 seconds (middle blue trace) and the minimum is about 2 seconds (yellow bottom trace), by adjusting pot U3.

1578590198307.png
 

MisterBill2

Joined Jan 23, 2018
4,840
A N.O. push button, a FET, a capacitor, and a relay. The pushbutto connects the capacitor to a charging voltage momentarily, and the voltage on the FET gate keeps it in conduction, closing the relay in the drain circuit, until the capacitor discharges to the level that lets the FET switch off, releasing the relay. This is by no means an original design, it has been around for at least 20 years. Cheap and easy if your time delay does not have to be easily set or very precise.
 

crutschow

Joined Mar 14, 2008
24,055
Here's the LTspice simulation of MB2's suggested circuit:
For this circuit, V(out) is low when the relay is ON.
The minimum delay time is about 200ms and the maximum about 150s for the chosen components and the nominal MOSFET Vgs threshold voltage.

It will work fine but the delay time depends upon the MOSFET threshold voltage as well as the capacitor and resistor values, so will not be as stable as the 555 circuit, and will have more initial variation for a given R and C value, depending upon the particular MOSFET you use.

1578600852095.png
 

MisterBill2

Joined Jan 23, 2018
4,840
Here's the LTspice simulation of MB2's suggested circuit:
For this circuit, V(out) is low when the relay is ON.
The minimum delay time is about 200ms and the maximum about 150s for the chosen components and the nominal MOSFET Vgs threshold voltage.

It will work fine but the delay time depends upon the MOSFET threshold voltage as well as the capacitor and resistor values, so will not be as stable as the 555 circuit, and will have more initial variation for a given R and C value, depending upon the particular MOSFET you use.

View attachment 196547
The circuit is close to what I suggested except the bottom of the cap C1 goes to common and I had not included the resistors, which would mean that the cap could be a much lower value. And I never stated that it would be stable or easy to adjust.
BUT thanks very much for producing the drawing.
 

ci139

Joined Jul 11, 2016
1,086
12v adjustable (0s
0s is impossible because of the device delays - try again
as in fact ↑↑↑ this sets requirements for device initialization time - there is no point to suggest anything if we don't know that
As soon as the circuit recieves power, the output is switched on and also the turn off timing is activated.
Question 3 : ? does that mean that power ON triggers relay (at the rising edge of power) . . . should the further presses inhibited during "count-down"

Edit :________
( you will not like my version )
Random - Op-Amp - OSC - 0205bx.gif
 
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Thread Starter

Natsosoyami

Joined Jan 9, 2020
3
Thanks for the reply guys. I just need something like this easy circuit, but the problem is that it has a push button and the timer starts when the button is released not when pressed. This circuitwill be connected to a digital timer. After it recieves the input from the digital timer, the circuit will start timing and will run. The digital timer will set as the triggering device and the circuit will just run once when triggered by the digital timer. 12v input and output
 

Attachments

Tonyr1084

Joined Sep 24, 2015
4,028
Here is an older article on 555 One Shot circuits (actually only one circuit - but - - - ). While you CAN use a MOSFET with a resistor and cap to hold a voltage long after the supply is gone - it sounds to me like you want the timing to begin the moment you push a button and not at the moment it is let go. The problem with the FET circuit is that it can be re-energized at any time of any press of a button. That means - suppose you set for 2 minutes. You walk in the door and push the button. Two minutes later the FET powers off. BUT if you walk in the door and push the button, then a minute and a half later someone else pushes the button - you reset the two minute delay to the last time the button was pushed. With a One Shot - the moment you push the button you start the two minute countdown. No matter how many more times or how long the button is pressed and held, the two minute delay will start at the beginning of the signal and not allow any sort of retriggering. It's a little more complicated, but my honest opinion is a One Shot will work the way I believe you want it to.
 

MisterBill2

Joined Jan 23, 2018
4,840
0s is impossible because of the device delays - try again
as in fact ↑↑↑ this sets requirements for device initialization time - there is no point to suggest anything if we don't know that

Question 3 : ? does that mean that power ON triggers relay (at the rising edge of power) . . . should the further presses inhibited during "count-down"

Edit :________
( you will not like my version )
View attachment 196599
Your version is a lot more complex than the one that I proposed. A resistor could always be added in parallel with the capacitor to speed the turn off, and to make the delay more repeatable. And like I stated, tha cap would be a much lower capacitance.
 

ci139

Joined Jul 11, 2016
1,086
actually i was attempting to get reasonable init times for 555 - just completed the functionality later . . . there are a lot of things that can be changed -- either coz they will be not reliable in real circuit (the TLC555 substitute model is simplified and some of it's functionality is adapted from NE555 d/s) or then if the linear/precision/(predictable) charging current is not required . . .
resistor could always be added in parallel with the capacitor
a good hint !
 
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MisterBill2

Joined Jan 23, 2018
4,840
actually i was attempting to get reasonable init times for 555 - just completed the functionality later . . . there are a lot of things that can be changed -- either coz they will be not reliable in real circuit (the TLC555 substitute model is simplified and some of it's functionality is adapted from NE555 d/s) or then if the linear/precision/(predictable) charging current is not required . . .

a good hint !
Why use a quite complicated scheme to generate a delay when there is the much simpler method that is much more compact and also smaller and cheaper?
 
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