Thread: DIY accelerometer bicycle alarm.

Hey there!

I want to add a bit of extra security to my bicycle, so i'm going to build a bike alarm.

Now its easy enough to get a wire and a piezo siren (110db) wired up so that if your lock gets cut the alarm goes off. But thats not INTERESTING! I want my alarm to go off as soon as the bike or lock is moved.

Clearly i need to detect movement and the best way is to use an accelerometer. This will be a 3 axis accellerometer consisting of 3 piezoelectric sounders, clamped at one end and with a weight at the other (this need carful consideration of the resonant frequency). The individual Piezo outputs will be fed into a charge amplifier with adjustable gain and the output stage conected to a darlington array.

Any one of the three accelerometers will triger a 10s siren, while a cutting of the lock cable or tampering with the device will result in a constant siren.

The alarms locking cable wont provide a strong physical lock so i'll always have my usual bike lock as well. The cable is of the three core armored type, its also transparent so it acctualy looks like a normal locking cable rather than a peice of electrical wire.

A tamper sensor will be in the form of a light dependant diode in the casing. The deivce will be turned on with a pushbutton, and turned off with a key switch.

Power consumption will be a real issue as i dont want to recharge this every other week!


Anyway thats the project, i'm sure i'll need some advice soon so i'll keep you all posted.

Thats true but its not as interesting, any monkey can do that. Also the power requirments will be slightly lower with the peizo design i suggest as i will be using less active components and no clocks.

So i've been moving slowly on this one.

If now got my standard PZT transducers, and i've built a charge integrator for them. With a weight at one end and a secure clamp at the other i can generate a decent signal.

I'm going to include an adjustable gain on the integrator to sensitivity can be changed.

Next i'm going to work on the 555 timers, one to drive the PZT siren, the other a monostable to trigger a 30s pulse on the siren when vibration is detected.

I'll also be using some logic chips to detect the lock cable being cut.