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Need design for speed switch, please help.

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May 16, 2011
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Hi, I have a diesel pickup with a hall effect sensor in line with the speedometer cable. It originally controlled the activation of an overdrive through the use of a control module (now gone) wired to a solenoid.

I would like a design (i'm sure it will be simpler than I think) for a speed sensing switch that can interpret the data from the current speed sensor, and act as a switch to come on at a certain speed and off at another speed. If both could be adjustable, that would be better. Something similar to this product:

MSD RPM Digital Activated Window Switch [MSD-8969] - $154.30 : Performance Parts - Ford Mustang - Cobra - Dodge Viper - Chevy Corvette - C6 - LS1

I don't necessarily need/want a digital readout, just an adjustable function. Basically, I need a 12v1a signal at 45mph, and off below that. Not sure what frequency that is on the hall effect sensor.

If anyone has any ideas, thank you!

The first thing to do is to turn the output of the hall effect sensor into a decent pulse wave form of stabilised voltage. Depending on the wave form at this point, a simple capacitor/resistor integration circuit would work. Basically every time your pulse comes along , suppose the capacitor charges up by 10mV per pulse , then discharges through the resistor by 8mv in the gaps between the pulses. at low revs you would be getting 800 X 2 mV times a second, at 4000 revs, 4000 X 2 mV times a second. Then you need a circuit to switch at a certain voltage level, to activate the overdrive solenoid. There is a lot of fiddling about to find the pulse width (2 degrees?, 10 degrees?) and the voltage level from the sensor. Then you have to set the CR time constant correctly, then work out a way of putting in some way of getting the overdrive "in" speed and the overdrive "out" speed(when you slow down), correct.
Looks like an LM 2907 chip might do just what you're describing, Frank. Am I right? I think if I add a potmeter in the right places and a mini relay in the output, I can connect to a bigger relay and I've got it. Just need to send a low enough amperage signal through the HES and to the 2907 circuit.

A lm2907/17 is a good idea, you'd need a op amp comparator on the end of it to give you the signal, and you could add a bit of histeresys so your o/d wont be cutting in and out rapidly.

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