Converting speedometer gear signal

Question

This was a board which taken out from speedometer. At first the speedometer was cable type and the display was in digital. I'm planning to convert the cable type speed into electronic one which consist of 3-wire because the original one had worn out on its cable housing. Looking on the board inside the speedometer itself had 4 point connected into converter circuit.

I already measured between these 4 point which relatively low voltage (1.2 V something) and in ohms there are two points were shorted, assuming it was connected.

As I planned to convert into electronic type instead cable ones, do this possible? Also what kind of component on U2 location? I'm attaching also the electric type of speedo gear that uses wire to send signal instead cable that I'm going to convert.

Answer 1

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. What you've got (probably).

It looks like the old system used a mechanical cable from the wheel to turn a slotted disc in the slotted photo sensor. It consists of an infrared (IR) LED (which is where you measured the 1.2 V) and a photo-transistor. The photo-transistor will turn on when IR light reaches it.

You will need to provide the right voltage to your sensor and connect the correct wire to the PCB on one side or other of Q1.

Your fourth photo (you should give them captions with numbers so we can discuss them without confusion) shows four slots in the rotating disc. That means that you will have gotten four pulses per revolution of the cable. If your new sensor does not give the same number of pulses per revolution then the displayed speed will be wrong.

Unfortunately you have not provided a circuit for the speedo board and have not provided a datasheet for the new sensor so further help is not possible.

Answer 2

U2 has E for "light" Emitter diode and D for phototransistor Detector. It is an optical slot with external rotating path to block IR light. A DMM will show 1.1V for positive Vf on emitter and low resistance with V+ on detector only when 10 mA or so flows thru + on emitter , otherwise high impedance. The Transistor also has a pin called (current) Emitter which is not to be confused with Light Emitter E, nor is the Light Detector D to be confused with the Diode.

These types of logic or transistor optical interrupters in stock are shown here. These all have similar input current to output resistance (Rce=Vce/Ic) specs while the logic output types are CMOS Gates.

Original types used a cable to drive a rotating magnetic field voltage generator, which without a significant load current is kV/RPM and may be scaled by to a DMM to read volts as RPM.

Similar types can use the frequency of a 1 pulse per rev signal to trigger a simple 1-shot RC circuit to generate an average voltage that increases with f and again be measured easily on DMM.

Alternatively you can take a Tach or speedometer from a Motorcycle or car and do the same and mount it on your bike, using recycled parts from the junkyard with any kind of magnetic, optical or mechanical sensor.

From one of my former fave employees, Mr. Whizzard.

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