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Isolating a signal

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zorbzz

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Hi
I made an odometer for my car because the analogue one stopped working.
I got an old pedometer and took out the magnetic switching system and replaced it with an NEC 2505 4 pin single opto isolator.
I made a count to 60 counter by using 2 decade counters, first one counts to 10 then second counts to 6 and resets.
When it resets, 1 ticks over on the pedometer which in my case is actually 10 meters.

Problem is when I hook the opto isolator to my speed sensor signal, the counter works but then the speedo stops working and also my automatic gearbox stays in 3rd.

Is there an easy way to get the signal into the device without taking it away from the speedo etc?

The led in the opto isolator must be doing something to the signal?
Ta
 
You probably have found the problem: the LED is drawing too much from the car's circuits. I would first try to putting a large resistance in series with the LED, because it may still work with a lot less current. Try putting a 1k, 4k7, or even 10k resistor in series with the opto LED.
 
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Ok thanks , will doing that draw less from the original signal?
I'll try it and see how it is.
 
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