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# Build or buy - mating old sensor to new display - signal conversion

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#### macorro

##### New Member
Thought this might be an interesting first project, but my knowledge is very basic.

A 12v old (1988) boat speed indicator with its paddle wheel sensor. I have a new display which can read the old paddle wheel but interprets the speed at close to twice the proper value. Calibration on the new display is limited to about 15%. I guess the paddle wheel sends a signal each time a paddle induces a current.

I think if I can find a way to lose every other signal from the paddle wheel then I can use the two together. Such a circuit doesn't sound too complex - is this something I can buy? Otherwise could you give me some pointers, please?

I am conscious that was a 2012 thread "Need Help: Speedometer Signal Converter"; that problem involved reducing the signal by 8.

#### ericgibbs

##### Well-Known Member
hi,
This could be achieved by using a bistable IC, [ divide by 2 ].
What's the voltage level of the paddle pulse.?
E

EDIT:
One option

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#### macorro

##### New Member
I only have a multi-meter; 5.04v when not in use; varying around 2.6v to 2.7v when the wheel is in motion.

#### ericgibbs

##### Well-Known Member
hi,
If the paddle pulse is +5V at rest, could you check that the pulse goes to around 0V as the paddle rotates?
You will have to turn the paddle wheel slowly by hand while checking with your meter.

If we can detect a 5V and 0V pulse it will make the circuit simpler, if not and its only 2.5V minimum [ not 0v] you will need a little extra circuitry.
E

#### macorro

##### New Member
Eric, I am near Gloucester - and you? Sorry for the delay in responding about voltage range for the paddle wheel - hope to respond Friday/Saturday.

#### macorro

##### New Member
Eric,
Paddle wheel cycles from 0.01v to 4.98v using my 12.42v dc supply off my motobike battery. Each 360 deg. rotation produces 2 5v signals.

#### macorro

##### New Member
Eric,

Taken a lot of learning, and unlearning too! Gone from a full flashing light version of 34 components to my simplest of 4 components when I understood what an inverter might do. To my total surprise to it works! Thank you so much for pointing me in the right direction.

2 questions:
• I get some flicker often negative voltage in the circuit. Both diode and earth seem to destroy the signal. Would an earthed coaxial cable solve this - current the output signal is in twisted copper only? There is an earth connection availabel to the engine block.
• Concerned about the effect of the marine environment. I have spray back and from with PCB Lacquer - is this the best approach?

#### ericgibbs

##### Well-Known Member
hi,
For the 'noise' try adding the components shown in this marked up image.

The PCB lacquer should be fine.

E

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