Circuit Design Assistance needed!

[pebe]

I can give you a circuit, if you want, that replaces TR1 with a 555 timer to give 10secs delay.

As for Bluetooth, it uses its own protocol and I am not familiar with it. Most devices are dongles for specific applications. If you could detect when the dongle has power supplied to it, then it would be easy.

[tomaeugen]

yes I would like the 10sec delay incorporated in the circuit...

As for BT, I've seen so far that a BT has 4 connection points: Power, GND, TX, and RX. I assume that the power, ground and RX are always connected to provide connectivity, as for the TX, I assume it would be connected to the same part of the circuit as the speaker, technically when the speaker is producing sound, so would the BT, transmit to the connected device.

In theory I think that this is how it is supposed to work...
Anyone else in this thread with BT experience?

[pebe]

OK. Here is the modified circuit. It uses an LM555 as a bistable switch whose output can drive the buzzer directly. With a 12V supply, when pin 6 goes above 8V the output goes low. When pin 2 goes below 4V the output goes high.

When first switched on, C1 is discharged so pin 6 is at 12V. That ensures that the output is low and the buzzer is off. As long as SW1 is closed, it will stay that way.

If SW1 opens and SW2 closes, C1 will start to charge via R1. After about 10secs the cap will have charged so that pin2 will have gone down to its 4V threshold. Pin 3 will then go high turning on the buzzer. The charging time equals C1 x R1 = 10 x 1 = 10secs. If it is not what you want, alter R1. Use a bead tantalum for C1 for consistent timing.

When SW1 closes, C1 discharges and the buzzer is turned off. When SW2 opens, the buzzer is turned off, but this takes several seconds because C1 has to discharge via R1 and R2 until pin 6 gets up to 8V. If you want a faster recovery time, fit a 1N4148 diode across R1, with its cathode to pin 2.