Louder Carbon Monoxide Alarm

Pete brought me a challenge from Sarah's house - make their carbon monoxide alarm louder. A malfunctioning boiler and landlord means they suffer frequent CO problems. I suggested that Pete buy a personal attack alarm from the SUSU shop whilst I figured out how to get a signal out of the CO alarm.

When he returned, I cracked open the attack alarm. It's pretty cool - three 1.5V alkaline button cells drive a piezo disk with a resonant cavity on the front, generating a staggeringly loud noise for only £3. The driver circuit is a really simple DC-DC which generates about 190Vpp. I disabled the pull-cord switch and put an n-channel FET in the ground connection to the  driver board. A pull-down resistor ensured it didn't deafen me during the mod! I fed ground and the gate signal out of the case. 

On the other end, I found a signal that pulsed 5V when the CO alarm was triggered. I connected it to the new attack alarm control signal. It triggered the alarm ok, but it wasn't completely deafening.The duty cycle of the CO alarm's weedy beep-beep sounder was too low. So I added a simple diode demodulator to stretch the signal out. The pull-down in the attack alarm fortuitously formed half of the RC circuit. I picked the values for a time constant of a few seconds. The result was a truly face-melting piezo assault duet!

Here's a schematic of the mod. Blue highlights my additions.

...frankalarm before final sewing. Note the attack alarm cable tied to the back of the CO alarm case.

 

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