# Electrical Engineering - Increasing the Time an Alarm will Buzz

1. Nov 26, 2012

### Mosaness

1. The problem statement, all variables and given/known data

The problem is to create a small alarm that will buzz approximately 6 seconds before it stops buzzing. I managed to create a working alarm, only problem is mines will buzz for around 1.5 seconds not six.

2. Relevant equations

τ = RC
V = V0e-t/τ

3. The attempt at a solution

Req. = 400Ω + (80Ωll100Ω) ≈ 444.44 Ω
VTH = 6V * (400Ω/480Ω) = 5V

Substituting this into equation #2 above gives 5V = 9V*e-6/τ
Solving for T yields:

Actually, I just lost myself in my work. Can someone help me.

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2. Nov 26, 2012

### Staff: Mentor

Where is the buzzer? How is it turned on and off?

3. Nov 26, 2012

### Mosaness

The buzzer has a resistance of 400 ohms so that's what was used. It is turned on when someone opens the window.

4. Nov 26, 2012

### Staff: Mentor

And how do you intend to connect the buzzer and window switch to your circuit that you've drawn? How does the circuit reset when the window is later closed?

BTW, it would be better to use an NPN transistor as part of the circuit that drives the buzzer. Do you know why?

5. Nov 26, 2012

### Mosaness

Oh...the class I am taking is a basic class. I've never even heard of a NPN transistor. I made a PSPICE design. The switch was connected right next to the 100Ω.

6. Nov 26, 2012

### Mosaness

I did a calculation earlier but I slightly messed up. I obtained Vthevenin by the equation 18V * 80/180 --> which gave me 8V.

I then set that so:

8V = 18V*e^-6/tau and solved for tau, which gave me tau = 7.3989

Using that tau I solved for C, which gave me C = .0166 F

7. Nov 26, 2012

### Staff: Mentor

Why not just use a "normally open" (NO) switch in series with the buzzer, a resistor and a cap? Or even just the switch, buzzer and cap....?

8. Nov 26, 2012

### Mosaness

Well I just went on the way I know a few people did. And that seems to be giving me an appropriate time...

9. Nov 26, 2012

### Staff: Mentor

Req I understand. VTH I don't.

According to your schematic, you are using a supply voltage of 18V. Correct? So where did the "6V" figure come from? What is special about the "5V" figure? Where did the "9V" come from? What operating parameter of the buzzer are you working with, in trying to determine how long it will actually "buzz"?