Need help making a relay latch with a reset

Thread Starter

Qmavam

Joined Nov 6, 2016
16
I thought this would be simple and maybe the problem is I need a DPDT relay, But I don't have one available.
I have a relay that closes for 30 seconds then opens. It is a SPDT relay. I can retrigger the relay to run another 30 seconds.
I want to add a second latching relay with a reset.
When the 30 second runs out and the relay is set to the N.O. contacts, I want a buzzer to sound, I also want to be able to reset the latching relay to shut the Buzzer off and I want it to stay off until the next time the relay runs through a 30 second cycle and drops out to the N.O. contacts.

Long story short, I have a timer, I push start and it closes the relay N.O. contacts for 30 seconds, then the relay opens and the N.C. contacts are closed. When the N.C. contacts close, I want them to latch a relay with a reset button. Reset is to shut the buzzer off, while work is performed before the next 30 second cycle is triggered.
Thanks, Mikek
 

DickCappels

Joined Aug 21, 2008
10,246
(Some text removed for clarity)
Long story short, I have a timer, I push start and it closes the relay N.O. contacts for 30 seconds, then the relay opens and the N.C. contacts are closed. When the N.C. contacts close, I want them to latch a relay with a reset button. Reset is to shut the buzzer off, while work is performed before the next 30 second cycle is triggered.
Thanks, Mikek
If we just think in terms of when the normally open contacts are open and closed it should make the description a little simpler.

Push start
Relay closes and stays latched for 30 seconds
During the 30 seconds a buzzer sounds
A RESET button can stop the buzzer at any time.

Does that adequately describe what you want the circuit to do?
Can the circuit incorporate things like a timer ship (NE555) and/or transistors?
 

crutschow

Joined Mar 14, 2008
34,841
Here's the LTspice simulation of an example circuit using an added DPDT relay that acts as a latch:
When the 30s timer times out (green trace) it sends a pulse from the capacitor C1 to turn relay U2 on, which is then latched through switch S1 and powers the buzzer (yellow trace).
Opening the PB contacts (red trace) unlatches relay U2 and stops the buzzer.

The capacitor value may need to be increased to obtain a sufficiently long pulse to latch U2, depending upon the relay coil characteristics.

Note that the PB reset switch needs to be a normally closed type.

1684737581523.png
 
Last edited:

Ya’akov

Joined Jan 27, 2019
9,264
Does the buzzer time itself? If so then all you really need is to connect the power for the buzzer and for the secondary relay to the secondary relay's NO contact through a NC pushbutton.

When the primary relay activates, the power for the secondary relay is provided from the NC contact of the primary but then the NO contact of the secondary powers both the buzzer and its (the secondary's) own coil.

When the NC pushbutton is pressed it interrupts the power to the really which drops, and the buzzer is also de-energized.

I am not in a position to draw a schematic so I hope this narrative is clear enough. Assuming I understand your constraints I believe this arrangement does what you want.
 

Thread Starter

Qmavam

Joined Nov 6, 2016
16
Hi all,
Thanks for all the responses, I received a circuit from another group and got it working late last night.
I did find a DPDT relay in my pile of relays.
The circuit below is what I have now, it works fine, and does exactly what I wanted.
BTW, I'm using this cute little $5 countdown timer, adjust 1 to 99 seconds and 1 to 99 minutes, with a time display.
https://www.mpja.com/Retriggerable-Time-Delay-Relay-Delay-on-Break-12VDC/productinfo/37663+RL/
Thanks, Mikek
Oil water latched relay with reset for buzzer.jpg
 

crutschow

Joined Mar 14, 2008
34,841
The circuit below is what I have now,
Good circuit.
Doesn't require the capacitor that mine does.

But you should add a diode across the relay coil to avoid arcing and pitting of the push-button contacts from the coil inductive spike when turned off.
 

Thread Starter

Qmavam

Joined Nov 6, 2016
16
Hi all,
Thanks for all the responses, I received a circuit from another group and got it working late last night.
I did find a DPDT relay in my pile of relays.
The circuit below is what I have now, it works fine, and does exactly what I wanted.
BTW, I'm using this cute little $5 countdown timer, adjust 1 to 99 seconds and 1 to 99 minutes, with a time display.
https://www.mpja.com/Retriggerable-Time-Delay-Relay-Delay-on-Break-12VDC/productinfo/37663+RL/
Thanks, Mikek
View attachment 294702
I have decided to incorporate a relay (an SSR) to switch an AC load, for the relay that I said I may or may not use.
However, I see that after the 30 sec countdown the first relay is latched causing the second relay to stay on after the countdown has ended, I want it off after the countdown has finished.
Is there a fix for that?
Thanks, Mikek
 

crutschow

Joined Mar 14, 2008
34,841
I have decided to incorporate a relay (an SSR) to switch an AC load, for the relay that I said I may or may not use.
However, I see that after the 30 sec countdown the first relay is latched causing the second relay to stay on after the countdown has ended, I want it off after the countdown has finished.
Is there a fix for that?
Sorry, what you want now is confusing, since it apparently is different from your original requirement, as the latching relay was for the buzzer.
Can you show a diagram of what you now want exactly?
 

Thread Starter

Qmavam

Joined Nov 6, 2016
16
The schematic is in post #7. The relay on the far right is the one I'm now concerned about. It is still engaged when the 30 second countdown ends, and won't release until the reset is pushed. I want it to release when the 30 second countdown ends.
I probably need to add another SPDT relay. Or maybe a transistor.
Thanks, Mikek
 

crutschow

Joined Mar 14, 2008
34,841
You just need to add a diode (D2 below) to isolate the latch voltage from the output relay U3 (blue trace).

1684784053103.png
 
Last edited:

Thread Starter

Qmavam

Joined Nov 6, 2016
16
You just need to add a diode (D2 below) to isolate the latch voltage from the output relay U3 (blue trace).
Super, I haven't tested it but I see it will work. I'm busy cutting holes in the enclosure I'm putting it all in. I'm going to add an AC cord input receptacle and an outlet. The timer can be used alone with the wallwart, if you just want the timer, or if you want to control the load, you can plug in a power cord and and plug your load into the receptacle.
Thanks for the help!
Mikek
Oil water latched relay with reset for buzzer with diode to make 3rd relay operate properly.jpg
 
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