Hi

I am wiring a set of warning strobes in a vehicle, and want to include a warning sounder to remind the driver that the strobes are on. However, the sounder needs to have the ability to be silenced until the strobes are switched off and then turned back on again, so that if the vehicle is being driven - as opposed to stationary - there is not a continual alarm, but there is no danger of forgetting to turn the alarm back on.

In other words, the sequence of events may be:

1. Switch on the strobes; the alarm also sounds.
2. The vehicle needs to be driven for a while so the alarm gets silenced.
3. At the end of the journey, the strobes get switched off.
4 .The next time the strobes are switch on, power should automatically be returned to the alarm.

It is a 12V system.

My gut feeling is that I should be looking at some type of latching relay, but I'm not sure.

I have looked at various options but, because it is an automatic restoration of power, it would normally be an unsafe function and I can't find anything that would suit (maybe I'm using the wrong search criteria?).

It needs to be very simple because it's over 30 years since I did my City and Guilds 224 and my brain is considerably less agile, so any help (and hand-holding) would be much appreciated!

Thanks,
Ian

 

Yes, a relay configured as a latch will work for that.

Below is the LTspice simulation of such a circuit.
It uses a standard SPDT,12V relay (U1), and a NO momentary push-button (S1).

When the strobe power is applied (from the strobe switch, not shown) at 0.5s (red trace), the alarm power (blue trace) is also applied through the NC contacts of the relay (which is de-energized at that point).
When the push-button is momentarily pressed at 1.5s (yellow trace), the relay latches on through the NO contacts and the alarm power is turned off.
When the strobe power is removed at 2.5s, the relay is unlatched, and so the alarm is again powered with the strobe power at 3.5s.

The diode prevents relay contact erosion from the relay coil inductance.

RAlarm is just a dummy load for simulation purposes.

 

Thank you very much, crutschow for your very un-curmudgeonly reply. That makes perfect sense and I will give it a go.

Stay safe!

Ian

 

Another approach:

The +12vdc goes to the strobe power input. The SCR is triggered on power up sounding the Sonalert.. The switch shorts out the SCR, turning it off until the next power up.

 

Thanks KMoffett. I will have a look at that one, too!

Cheers

Ian

 

Below is the simulation of KMoffett's circuit.
The blue trace is the current through the alarm.