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I am an engineer after a fashion: BS and MS in computer science, UC Berkeley. That means I've had training in digital hardware but not analog electronics. I can (and have) design a computer from scratch but I can't design an amplifier. However, I do know somebody who does know analog electronics who thinks he can make me a digital divide by two circuit. I'll post the results.
...And if my friend's circuit doesn't work I'm tempted to try the following rather messy idea.
1. Use the existing speedo sensor's sine wave signal as input to a digital circuit. Use the rising edge as a clock to trigger a D flip flop with Q-bar output tied to D input. The Q output toggles at 1/2 the input frequency in the digital domain.
2. Use the Q output to gate the speedo sensor's sine wave signal via a transistor to the speedo. When Q is low the signal would be zero. When Q is high it would pass 1 sine wave cycle -- in effect, odd-numbered sine wave cycles go through and the output is at the 0v reference level during the even-numbered sine wave cycles. This produces analog output without requiring things like op amps and phase-locked loops which I don't understand. Using low-resolution ASCII graphics, the signal would look something like this:
_ _ _ _ _
input: ( ) ( ) ( ) ( ) ( ) [imagine this is a sine wave]
0v ref: _) (_) (_) (_) (_) (_
_ _ _
output: ( ) ( ) ( )
0v ref: _) (_____) (_____) (_
If the speedo just senses the rising edge of the wave, this should work. But all bets are off if the speedo does a fast fourier transform or something else looking for the first harmonic...
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