Help with reseting an alarm circuit

[scottyboy]

Hi, Im new on here and wondering if anyone can help me with a control circuit. Im using a 24v power suppy. Im basically going through a flow switch( gas flowing across it ) on normally open contacts, so when it sees flow the switch closes , triggering a relay to give me a light and a buzzer. My problem is where to put the reset button and buzzer in the circuit so the lamp is maintained on but the fault buzzer can be cancelled. I can get my head round the lamp and how it comes on and off through the relay. Any help with the circuit diagram will be greatly appreciated. Thanks in advance

 

[TBennettcc]

What about having an on/off switch for the buzzer that takes it out of the circuit?

 

[MagicMatt]

I think you'd need to drive it with some sort of latched switch - power transistor driving a second relay maybe, and an RS flip-flop on the input, so that when you "trigger" the reset, it switches off the second relay, but the loss of power switches resets the flip-flop ready for the next alarm event.

My concern with having a simple switch, is that it could be left "off" after an alarm condition, then not sound the next time.

 

[scottyboy]

I agree with magic matt, the switch would be no good as someone could forget to turn it back on. I dont have a flip flop to hand, is there no way this could be done with relays only?
cheers for your help.

 

[TBennettcc]

You could make a flip-flop with bipolar-junction transistors?

http://en.wikipedia.org/wiki/Flip-flop_(electronics)#Implementation

 

[MagicMatt]

I make flip-flops out of Quad 2-input NAND chips... they only cost about 20p and you can get 2 flip-flops from one chip.. ;-)

The link TBennettcc posted has an example.

 

[scottyboy]

Well thanks but I havent got anything like that to hand and a bit difficult to get my hands on. I have seen this done before utilising only relays, buzzer, light and reset button (non latching ) and the pressure switch but cant remember how its done.

 

[MagicMatt]

Ok, a little head scratching and I've come up with this.

Personally I still prefer the logic route, as relays are expensive. Hope it solves your problem though!

Difficult to get things like that though?
Maplins, RS, Farnell (CPC)... standard stock. Personally I use RS as they've never let me down on next-day deliveries and have always found their customer service vey helpful.

 

[scottyboy]

To magic matt

Hi matt sorry for the delay but thankyou very much, thats what im after!
Im at sea and dont have the components to hand and was in a hurry to get this done, thats what i meant by not being able to get them.
Thanks again for your time though