Circuit Design Assistance needed!

[tomaeugen]

Hi,

I have the following situation that needs a solution.
I need to come up with a solution to make the following scenarios work:

I have a 12VDC source, one speaker and two switches identified as SW1 and SW2.
The switches are a single pole simple on / off function.

The following is the the intended operational requirement.

When SW1 and SW2 are both Closed (on position), no alarm should come from the speaker.
When SW1 and SW2 are both Open (off position), no alarm should come through the speaker.
When SW1 is closed (on position) and SW2 is Open (off position), no alarm should come from the speaker.
When SW1 is open (off position) and SW2 is closed (on position), Alarm should sound through the speaker.

Can anyone please give ma a hand in designing a small circuit board or any other circuit flow or connection possibilities to achieve the above mentioned required operational scenarios?

Thank you.

[pebe]

A 12VDC source will not cause a speaker to make an alarm sound. Is there an oscillator somewhere?

[tomaeugen]

Thought about that too, then came up with the idea of using a tone generator

[pebe]

There is no satisfactory way of solving your requirements by using only those 5 components.

[tomaeugen]

Well... There can be more components, however I need to have only 2 on off switches a tone generator a small speaker and they need to run on 12vdc!

[pebe]

Is this homework?
If so, what other restrictions are there?

[tomaeugen]

No this is not homework! It's something I'm thinking of building for my home!
There are no restrictions! All it needs to have is as mentioned above and function as described too! So as a bare minimum it has to have the two on off switches, a tone generator, small speaker and it needs to run on 12v. It can have other components to make it work as desired!

[pebe]

OK. The simplest and most basic method would be use an NPN transistor (Tr1). Let its base be driven by the oscillator via a suitable resistor. Connected its emitter to 0V (negative of the battery). SW1 would be wired from base to emitter of the Tr1.

Connect one side of the speaker to the collector of Tr1. Connect the other side of the speaker SW2. Connect the other side of SW2 to +12VDC.

The only time an alarm would be heard is when SW2 closes to provide power for the speaker, and when SW1 is open, removing the s/c between base and emitter.

[tomaeugen]

thanks for the help. Any transistor will do? or will it need o be a specific one? and the oscillator resistor? One last question, can you please post a simple wiring diagram with the components? so that I can make sure that I connect them all correctly? Thank you again...

[pebe]

What speaker are you going to use?
What is its resistance and max power?

[pebe]

Have you got a particular speaker in mind?
I need to know that to complete the circuit.

[tomaeugen]

No no particular speaker in mind, the smaller the better!

[tomaeugen]

Is this what you ment?

[pebe]

I have corrected your circuit. Use a 2N3704 for Tr1 and a 680ohm resistor for R. You have shown no oscillator to generate a tone, so you cannot use a loudspeaker. If you wanted, you could use the two-tone piezo siren shown as item number ST019 on this link, that has its own built-in two-tone oscillator:

http://www.bitsbox.co.uk/index.php?main ... th=302_309

Alternatively, you could use the 2½” 64ohm speaker shown on that page. But that would need a different circuit to include an oscillator (using an LM555). It would also need a series resistor to reduce the voltage applied to it. If you want to go that route, let me know.

[tomaeugen]

Thanks for this correction. I will gather the parts and build a small test version early next week. I'm still waiting for the correct pressure switches to arrive.

On a second note... How hard would it be to add a couple more components to this system? For example:

I was thinking of enhancing its design by adding a 10 second sound activation delay after SW1 has opened and SW2 is still closed, and a Bluetooth transmitter, that would connect to a Bluetooth capable phone. On the phone there would be an app (already started working on the app) that would show a message when SW2 is still closed and SW1 is open.

Really appreciate all your help, thanks!