DIY Doctor

How do I Make a Switch or Circuit That Powers on With one Switch and Powers off with a Second Switch?

Postby Silenus » Thu Aug 08, 2019 9:01 am

I want to know how to make a switch or circuit, or to buy one cheaply, that allows me to use one switch to power an alarm siren but another to turn it off again. I know I can make a flip flop circuit to achieve this with two NOR gates but I'm just wondering if anyone knows a more straightforward way?

For some background I'm putting an ignition kill switch in my van, I'm going to hide this switch but I had an idea that I could put in a second more obvious switch that a potential thief would find first and flick, when they flicked it they would instead activate an alarm siren - the problem with this is that if I just wired it in normally then they could also just turn the alarm off again. Ideally I would have a third switch hidden somewhere else that would turn the siren off.

Any ideas?
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Postby Mr White » Thu Aug 08, 2019 5:25 pm

This forum is about "electrics" not electronics, so I would have to suggest if you want something more complicated than a latching relay I can only suggest you try looking for an "electronics" forum.
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Postby ericmark » Mon Aug 12, 2019 8:22 pm

Many years ago I found the 74 series of chips had an EMC problem, it would work A1 on the bench, but put in in the car and it went daft.

I also remember problems with anti-theft devices, the method popular at the time detected volt drop as inertia lamp switched on, however there was a problem when the battery got old it would activate the alarm.

As a result there is now a maximum time an alarm can sound for, also problems where the car would start then alarm activate disabling the car some times in a dangerous place. One common one was switch on the electric petrol pump (days of the carb) so car would start then conk out, and thieves would run away.

As a result there have been sets of rules on how anti-theft can work, and I left the auto seen a long time ago.

So although the j-k flip flop would work don't know if permitted. Although I have a degree in electrical and electronic engineering it was a long time ago.

I would be considering a simple latching relay, one set of contacts work the ignition, the other latch the relay on, no volt release is the standard way to run an electric motor so with a power cut it will not auto restart.
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