Fire Alarm Bell Alarm Clock - World’s loudest alarm clock?

2008. 1. 12. 12:57

Overview

Ever sleep-in because your alarm clock wasn’t loud enough? With this design that is guaranteed not to happen! This hack takes a standard cheap alarm clock, some interface components and a fire alarm bell to get the job done right. Normally these small alarm clocks use a piezo buzzer to attempt to wake you up. The alarm signal is hijacked from the clock circuit board and used as an input to the interface board.

From http://hackedgadgets.com/2008/01/11/fire-alarm-bell-alarm-clock-worlds-loudest-alarm-clock/8/
by Alan Parekh

Parts needed and Interface Board Schematic

Here is a list of the parts you need for this hack:

Cheap Alarm Clock
Fire Alarm Bell
PNP Transistor
4.7K ohm resistor
360 ohm resistor (optional)
Blue LED (optional)
Perf Board

Take apart the Clock, see how it works

Taking the clock apart was very simple, a few screws allowed the plastic outer shell to be removed. The actual clock module us held together with some plastic clips. A small flat blade screwdriver makes quick work of them. Be careful when opening the clock module though since the top plastic is used as pivot points for many of the gears that will want to fall out as soon as it is opened and tilted.

I am not very impressed with the solder job, the oscillator is in desperate need of a bit more solder.

The alarm function is activated mechanically by the clock and closes a physical switch. In the below picture have a look at the contact (it is open in the picture) just above the small green circuit board. When this closes the top negative pole is transfered to the tiny controller via the bottom pad.

The On Off switch is mechanical and slides up and down.

Funny how letter O, upside down it looks like a C. I wonder if they ran out of the correct letter and the C looked close enough

When the alarm switch is activated this physical tab wedges itself into the alarm contact preventing it from activating.

Closer look at the alarm contact.

The black blob on the circuit board covers the clock controller.

Measuring from the top of the alarm switch to the positive of the battery.

Take apart the Bell

The fire alarm bell used is a typical modern motor bell. These devices simply spin a cam on a motor. The cam lifts and releases a spring loaded hammer that hits the bell on the inside lip and allows it to sound. Fire alarm bells are 24VDC, the power supply being used is a 9 VDC supply which worked fine.

Mod the Clock to have an Alarm Output

The mod didn’t go very well. The plan was to scrape away the solder mask from the trace going from the bottom alarm switch pad and solder a small wire to hijack the negative signal when in alarm. In the below picture you can see the bare copper after the solder mask was removed. Read below to see what happened when I attempted to do this.

For strain relief some hot glue was used to keep the wire from moving. Have a look at the left of the clock assemble, that is one of the adjustment knobs and the black thing is the plastic guide that normally keeps it in place. Everything is so small and delicate… Some crazy glue solved the minor setback.

This is the test circuit in a breadboard for testing.

Initially the plan was to solder the wire to the positive battery connection and the trace that was exposed earlier. That worked fine for about 5 minutes but before I could employ some hot glue strain relief the tiny trace pulled off the board. The only option was to solder the wire to the copper under the alarm contact. Problem with that is that this would physically interfere with the mechanical alarm On/Off switch. The solution was to rip out the On/Off alarm switch.

Build and install the Interface Board

After testing on the breadboard and confirming proper operation the next step is to solder it to a more permanent perf-board. The board needs to be cut to fit into the bell opening.