# electrical engineering circuit design help

Background
I got my bachelor’s degree in electrical engineering about 5 years ago and I haven’t done this sort of thing since 1995. This is not for homework help, I’m just a bit rusty.

My 2 year old is getting out of his room at night while my wife and I are asleep. He’s a rather bright kid and has been able to foil all types of toddler proofing except for the plastic things that go in the outlets. We’re worried that he will get hurt wandering the house unsupervised.

I want to make an alarm that attaches to his bedroom door that will wake us up when he leaves his bedroom. I’ve searched the net and everything I’ve found is either rather expensive or has an alarm so loud that it will cause your ears to bleed. Besides, this sounds like fun. (Really, I’m NOT a geek).

Design
I have the basic design pretty much figured out. I want it to act much like the light in the fridge, as long as a little button is pushed in, the light is off, but when the door is opened, the pushbutton closes the circuit and the light goes on. In my application, the light will be replaced with a buzzer. The unit will be mounted on the outside of the doorway in the hall. The door opens into the room. Therefore, when the door is closed, the pushbutton is pushed in. When the door is opened, the button will spring back out. Here is an attempt at a circuit diagram:

``````

-----SW----------------------
|             |             |
|             |             PB
|             |             |
BAT           R1            R2
|             |             |
|            LED           BUZ
|             |             |
-----------------------------

``````

BAT is a 9V battery. SW is the on/off switch so we can turn it off during the day. LED is an LED that is lit when the device is on. PB is the pushbutton that will make contact with the door. When it is pushed in, that part of the circuit will be open and the buzzer, BUZ, will be off. When the pushbutton is released, the buzzer will sound.

According to the documentation, the LED operates at 2.25 volts with a max of 28mA. The buzzer operates between 1.5 and 3 volts (I chose 2.25V to match the LED) and a max of 15mA. I downgraded the current to 25mA and 13mA respectively as to not blow out the LED or buzzer. That means R1 is 270 Ohms and R2 is approx 520 Ohms.

Question
With this design, when the door is openned, the buzzer will sound continuously. How do I get the buzzer to sound intermittenly. In other words, I want “buzz buzz buzz buzz” rather than “buzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz”

A few suggestions:

1. If you’re going to use an LED, I would use a “high efficiency” one. I believe these draw around 2 mA. To save even more energy, you could use a 555 timer to flash the LED with a 10% duty cycle (like what some smoke alarms do).

2. To make a pulsating buzzer, you could trigger a 555 timer configured as an astable multivibrator. The buzzer could be switched using a TIP transistor or FET.