Switching a relay momentarily when the switch input is removed but not applied.

Thread Starter

Cartech

Joined Jun 13, 2022
11
Hello, I'm trying to switch a 12VDC relay for approx 500mS when the trigger voltage is removed but not when it's applied. I have searched on here and there doesn't appear to be any solutions that fit.
If it helps, what I'm aiming to do is simulate a button push on the central locking unlock button when the ignition is turned off, but not turned on.

If the switch is held for more than a couple of seconds, the system locks out and stores a fault code for a fault with the switch so I'm aiming for 500mS to allow for system voltage variance changing the duration slightly.

I've investigated a couple of ready-made modules, unfortunately these seem to hold the output active until the trigger is removed then start the timer.

The relay has a coil resistance of ~200 Ohms and a rated operating voltage of 12VDC, these relays tend to drop out below 10 - 10.5VDC. As well as the switched 12VDC, I obviously have a permanent 12VDC and ground. The switch i'm trying to simulate pulls the output down to ground, when the switch is open, the voltage from the central locking module is a nominal 12VDC. I'd rather use a relay to isolate the central locking module from any other system, as it is currently.

Thanks in advance for any help or guidance you may be able to offer, I'm quite handy with a soldering iron but circuit design is something I have little experience of.
 

MaxHeadRoom

Joined Jul 18, 2013
25,469
The relay has a coil resistance of ~200 Ohms and a rated operating voltage of 12VDC, these relays tend to drop out below 10 - 10.5VDC.
That seems a high drop out value? A while a go I conducted an experiment on the hysteresis of different DC relays, including an automotive version and the drop out level was quite a bit lower than 10.5v?
 

Thread Starter

Cartech

Joined Jun 13, 2022
11
I work in Automotive tech, we find relays tend to start dropping out or cycling when the cranking voltage gets below 10.5VDC, there may well be transient volt-drop lower than that, but that is our 'minimum safe' voltage monitored by an average spec. DMM.

In isolation, it may well go down to 7.5VDC, but the electronics controlling the relays would have given up well before that.
 

sghioto

Joined Dec 31, 2017
3,315
what I'm aiming to do is simulate a button push on the central locking unlock button when the ignition is turned off, but not turned on.
Can you elaborate more on that statement?
I understand you want to operate a 12 volt relay for 500ms by pulling one side of the relay to ground, correct?
The trigger voltage is from the central locking unlock button? What is that exactly?
 

crutschow

Joined Mar 14, 2008
30,103
Below is the LTspice simulation of a two transistor circuit that does, I believe, what you want.
The relay coil current (yellow trace), and contact output (red trace) are activated at the fall of the input voltage (green trace).
The relay on-time is close to the R3*C1 time-constant value, and either of those values can be changed if you need to adjust the time.

1655228310161.png
 
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Thread Starter

Cartech

Joined Jun 13, 2022
11
Can you elaborate more on that statement?
I understand you want to operate a 12 volt relay for 500ms by pulling one side of the relay to ground, correct?
The trigger voltage is from the central locking unlock button? What is that exactly?
First part - not quite correct - it doesn't matter which side of the relay coil is switched, normally it's the +ve that's switched. The contacts are one side ground, the other is to the central locking module.

Second part, again, not quite correct - the trigger voltage is from the vehicle ignition circuit and is 12VDC, obviously this can be 'on' for just a few seconds to many hours.

Apologies if it's my description which is misleading.
 

Thread Starter

Cartech

Joined Jun 13, 2022
11
Below is the LTspice simulation of a two transistor circuit that does, I believe, what you want.
The relay (yellow trace) is energized at the fall of the input voltage (green trace).
The relay on-time is determined by the R3*C1 value, and either of those values can be changed if you need to adjust the time.

View attachment 269369
That looks ideal - thank you very much!
Can I just ask, what are the M1 & M2 pinout connections?

There isn't a hope in hell of me being able to come up with that - I'll put it together and let you know how it works out - thanks again!
 

eetech00

Joined Jun 8, 2013
3,210
Hello, I'm trying to switch a 12VDC relay for approx 500mS when the trigger voltage is removed but not when it's applied. I have searched on here and there doesn't appear to be any solutions that fit.
If it helps, what I'm aiming to do is simulate a button push on the central locking unlock button when the ignition is turned off, but not turned on.

If the switch is held for more than a couple of seconds, the system locks out and stores a fault code for a fault with the switch so I'm aiming for 500mS to allow for system voltage variance changing the duration slightly.

I've investigated a couple of ready-made modules, unfortunately these seem to hold the output active until the trigger is removed then start the timer.

The relay has a coil resistance of ~200 Ohms and a rated operating voltage of 12VDC, these relays tend to drop out below 10 - 10.5VDC. As well as the switched 12VDC, I obviously have a permanent 12VDC and ground. The switch i'm trying to simulate pulls the output down to ground, when the switch is open, the voltage from the central locking module is a nominal 12VDC. I'd rather use a relay to isolate the central locking module from any other system, as it is currently.

Thanks in advance for any help or guidance you may be able to offer, I'm quite handy with a soldering iron but circuit design is something I have little experience of.
If you want to use passive components, here's an alternate circuit.
The circuit below shows how you can delay the drop out of a relay with cap and resistor.

1655219850660.png
 

crutschow

Joined Mar 14, 2008
30,103
Clarify please.
According to the TS's post (partially copied in my post # 13), the switch pulls the output to ground when the switch is "open", which I assume means when the switch is not pressed (NC contact).

So, in your circuit, that would have the relay be on whenever the button is not pressed.
 
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Thread Starter

Cartech

Joined Jun 13, 2022
11
What a relief.
There must be hope in hell after all. :D
I meant the circuit - what threw me was the 'M' designation and (I presume changed since I was at school) a different symbol for a Transistor. But yes, I see why you read it the way you did! ;)
There's a little electronics shop (proper electronics, not Chineseium pre-made stuff) in a town near me, the parts will come to less than £5, so with an hour or so of putting it together, fitting into an enclosure and connecting to the vehicle, It should be all sorted by the end of the week!
 

eetech00

Joined Jun 8, 2013
3,210
That circuit will have the relay on whenever the switch is in the "open" state (connected to ground).
I understood the TS to mean the control circuit output is normally pulled up (high) and goes low (to ground) when activated.
My schematic shows the switch open, so the relay will be initially off.

Anyway, my focus was on showing the TS that semiconductors are not necessarily needed to provide a relay drop out delay.
 

crutschow

Joined Mar 14, 2008
30,103
Cartech, please clarify --

Do you want the relay to activate when the signal from the button goes high or when the signal goes low?
 

Thread Starter

Cartech

Joined Jun 13, 2022
11
...my focus was on showing the TS that semiconductors are not necessarily needed to provide a relay drop out delay.
Thanks, however what I was after was a short on-off cycle (not a delay) once the trigger was removed, not applied. The trigger could be on for a period of seconds to hours, my requirement was fulfilled by the circuit provided by Crutschow.
 
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