Designing a overspeed alarm

[joza18]

Hello, I would like to do it a speed alarm/limiter for my scooter, because I have a young kids.
I have Idea to mount a magnet on rear wheel, and install a some sensor or reed switch on frame, and then built a electronic who will count a wheel revolutions, and if wheel increase for example 3 revs in second, electronic will activate relay and it will turn off ignition if wheel revs are >3 rpm/s.

Please do you have any suggestion, and what do you think about this:
https://www.homemade-circuits.com/2013/05/make-this-car-over-speed-limit-warning.html


Thank you!

 

[Tony Stewart]

Any Tach uses a 1 shot circuit to generate a known fixed voltage and fixed pulse width. From edge trigger.

Then an average voltage sense or low pass filter converts to DC and a precision Comparator with a precision reference voltage to create a logic level out.

The logic level out can drive a piezo alarm to serve your purpose or drive extremely low resistance power switches.


However, since 3 pps is quite slow, the reduce ripple of the DC result by x10, the LPF must be around 0.3Hz which means some delay will occur <1 s.

Another method is a digital counter and simple clock that detects time interval < 1/3 s or for example , checks that counter has not overflowed between pulses, such as 8 bit counter at 760Hz or 256 pulses from 760Hz RC clock in CD4060 counter/clk between scooter pulses to reset counter . Overflow or output from any Qx out indicates safe speed between scooter pulses of RPM.

Of course if your pulse fails , so will your speed detector, and if there is noise, it will always fail overspeed.

A 31kHz watch crystal will give more accuracy than a cap RC and then choose appropriate output for time interval of 333ms with a latch

 

[joza18]

Hmm, this is maybe too much for me. Please could you tell me, wich type od sensor is best practical for this operation? And do you have a shematic with simple pulse comparator, who will give a V out when some (manualy adjusted) number of pulses is exceed.

 

[Tony Stewart]

comparators can drive a piezo alarm load from low side.from 9-14V for example.

Choose Hall Switch, not just sensor

Then use Analog timer method, which means precision one shot.
If trigger limit is > 3 pps then a 333 one shot is used.
Hall switch triggers one shot and if timer is active then trigger 2nd one shot for minimum alarm duration e.g. 1 sec.

Timer high AND Sensor pulse = trigger Alarm

Search "google images" for ideas. Using keywords HALL SWITCH, ONESHOT ,CMOS, SCHEMATIC, ALARM

Eg switch https://media.digikey.com/pdf/Data Sheets/Honeywell Sensing & Control PDFs/SM35xLT_Nanopower_Series.pdf