TD27T tacho adaptor curcuit

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Adrian585

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Ever since I fitted the TD27T to my D21, the tacho has not worked. I finally decided to look at it to see why.

There are two different tachos for the D21. These see different signals. If the needle goes past the red line on idle it needs a sine wave. If the needle doesn't move at all it needs a square wave.

I have surmised that the TD27T pickup sends out a sine wave at 1Hz per 1RPM, So 1000Hz = 1000RPM. However the standard tacho sees 50Hz as 1000RPM. So the signal needs to be divided by 20.

Also the pickup sends out no more than 1.7 volts.

My tacho needs a square wave. The way I achieved this was to amplify the signal using an audio amplifier, converted it to a square wave then went through a frequency divider to the tacho.

I have been trying to figure out how to convert the square wave back to a sine wave at 1.5 volts maximum.

Here is the schematic for my circuit.
 

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  • D21 TD27T  adaptor.jpg
    D21 TD27T adaptor.jpg
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