Question for electronics peoples

[Adoom]

I am trying to get my Link G4 to drive a toyota electronic speedo.
But the general consensus is that the PWM signal from the link is not strong/powerful enough.
After some research I came up with this idea.
I am only suggesting a darlington drive because I already have some.


This should work, right?

[Snaps]

What's the model code of the IC you're thinking of using?

Sounds like what you're trying to do would most easily be done with an Op-Amp Buffer (/Unity Gain Amplifier). The basic idea of this is that it produces the same voltage that's coming in, but allows a higher current output without being limited/breaking anything...

But then, I'm not entirely sure what you're trying to do; how is the link's signal "not strong enough" - ie. can it not produce enough current, or does the voltage need to be amplified?

[Adoom]

But then, I'm not entirely sure what you're trying to do; how is the link's signal "not strong enough" - ie. can it not produce enough current, or does the voltage need to be amplified?

I am not too sure myself.
It makes more sense that the current should be higher rather than the voltage. I think my idea only increases the voltage.

Normally the speedo is connected directly to the speed sensor. and the factory ecu just leeches off the signal. I don't have a wiring diagram that show exactly how it is connected.

You can alternatively have the speed sensor as a digital input on the link. And use an auxiliary output on the link to drive the speedo. This allows you to log the vehicle speed and also to send the correct signal to the speedo.
I need to do it this way because I have a complete mix match of parts.

Geoff, the electrical guy at Shred. Was relatively sure I needed to boost the signal from the link to the speedo. But it is not something he is really familiar with.

[DexGT]

I have just made my G4 drive my toyota rev counter and all I had to do was change a resistor on the rev counter board , I wonder if there is something similar you could do to the speedo ?

[gasman]

i would have thought that the output from the link would be more than strong enough to drive the speedo.

what outputs have you got free on the link?

you need the speed sensor feeding into the link and the link to add or minus a certain percentage to get correct speed reading at the dash?

could just setup a suitable transistor that opens and closes the circuit just as the standard speed sensor does from factory, just that instead of the magnet spinning and switching the transistor on/off, you have the link switching it on/off.

[Snaps]

It's very hard to know how to fix it without knowing exactly what the problem is:

- If the Link doesn't output enough voltage, then you need to amplify the signal to achieve a higher voltage.
- If the Voltage is ok, but the speedo isn't getting enough current then you need to reduce the resistance of the circuit (like DexGT said) to allow more current through.
-If the voltage and current is ok, but the current that is flowing would max out something that the link has (ie. a maximum output current), then you'd need a buffer.

Very hard to tell which one it is without having a good look at the specs of the link

[Adoom]

i would have thought that the output from the link would be more than strong enough to drive the speedo.

I can't find it right now, but apparently some speedos require the siganl to be boosted over what the link can provide. Boosted how, I haven't figured out yet...

what outputs have you got free on the link?

Aux 7 and 2 of the injector drives which can be configured as outputs.

you need the speed sensor feeding into the link and the link to add or minus a certain percentage to get correct speed reading at the dash?

Basically, yes. Speedo is AE101. Speed sensor is S14. Gearbox is S12. Diff is T-series 3.9 or 4.1, I forget. Tyres are 195X50X14. So it is really unlikely to be correct.

could just setup a suitable transistor that opens and closes the circuit just as the standard speed sensor does from factory, just that instead of the magnet spinning and switching the transistor on/off, you have the link switching it on/off.

That could work.

[Adoom]

Okay, so now I have a nissan speedo.
I wired the speed sensor directly to the speedo.
One wire to ground and the other to the signal terminal. Then connected the positive and ground terminals on the speedo.
I used the drill to turn the speed sensor.
Using the drill, I could get the speedo needle to sweep from 0-180kmh.
(I tried the same thing on the Toyota speedo, nothing happened.)

Okay, so I moved it to the car. (It's on axle stands with no wheels on the back.)
Speed sensor is in gearbox.
Speedo connected to power and ground.
Speed sensor is connected to ground and to the signal terminal on on the speedo.
Start car and put it in 2nd gear.
Speedo goes up to 30kmh when idling. Seems too fast.
Rev engine, speedo goes up and down as expected.
Connect the digital-in wire on the Link to the signal wire from the sensor(the speedo is still attached).
Speedo says 30kmh. Link says 1654kmh.....
Forgot to calibrate the digital input(number of pulses per 100m).
NEK MINIT!
I worked out it is about 1018 pulses per 100m.
Now the speedo reads 30kmh, the link reads 14kmh(which is about right).
Rev engine, speedo goes up, Link speed reading goes up.
Put gearbox in 5th, rev to 3000rpm. Link reads 100kmh(which is about right). Speedo goes well past 180kmh.

Connect Speedo signal wire to Link Aux 7(set to speedo out).
It is set to "sweep" to 200Hz on key on. This doesn't happen.
Engine running at idle. 2nd gear. Link reads 14kmh. Speedo reads 0kmh.

Aux 7 can produce a PWM signal up to 300Hz.

What is the difference between the signal from the speed sensor and the pwm signal from Aux 7?
I don't have anything as fancy as an oscilloscope(or know how to use one) to look at the two signals.
If anyone can answer this, is there a circuit I can use that will convert the pwm output from the link into the same kind of signal from the speed sensor? This would be the preferred solution.

If there are 1018 pulses per 100m. Then does that make it 2.8Hz per kmh?
If I can only produce 300Hz, doesn't that mean the Link can only make the speedo go up to 107kmh?

It works if I have the link and the speedo both attached to the speed sensor. But, the speedo reads WAAAAAAAY too fast. WOF guy won't like this.
Is there a circuit I can use that will manipulate the signal from the speed sensor to the speedo that will reduce it?

[Adoom]

Here is the picture version of my last post.

[Adoom]

I think I can use this http://www.jaycar.co.nz/productView.asp?ID=KC5435&form=CAT2&SUBCATID=965#11
to correct the speedo when both the speedo and the link and connected to the speed sensor!

[matt dunn]

Hmm,


problem I see is what do you think PWM is?

Pulse Width Modulation,
which usually puts out the same frequency and alters the width of the pules,

toyota speedo are frequency modulation,

Are you sure you are on the right track. I think the link will do both?

I have a simple circuit we use all the time for boosing speedo signals to run tripmeters in roading contractors cars,
as most new factory stuff does not have enough current supply
or dont earth switch properly to trip the Monit's.

I only have it on paper though, would have to scan it in somewhow.

[Adoom]

Hmm,


problem I see is what do you think PWM is?

Pulse Width Modulation,

YES!

which usually puts out the same frequency and alters the width of the pules,

Right...

toyota speedo are frequency modulation,

You've lost me.. Can you draw a picture?

Are you sure you are on the right track.

Not entirely

I think the link will do both?

From the Link Help:"They are digital outputs and therefore only have an on and off state". That implies "no", doesn't it?

I have a simple circuit we use all the time for boosing speedo signals to run tripmeters in roading contractors cars,
as most new factory stuff does not have enough current supply
or dont earth switch properly to trip the Monit's.

I only have it on paper though, would have to scan it in somewhow.

Please do!

[blindnz]

Frequency modulation



PWM.

[Adoom]

Frequency modulation

Can a hall effect speed sensor produce this kind of signal? Actually, I don't know if it is a hall effect type sensor. How the hell can I tell?
FYI: I am using a Nissan 2 wire speed sensor and a Nissan speedo.
I believe the Toyota speed sensor is quite different to the nissan one and has extra wires.

[Adoom]

So according to the manual. You test the Speed Sensor by putting a multimeter, measuring AC volts, across the terminals. When you spin it, it should produce 0.5volts AC.

Would that mean it uses frequency modulation?

[Adoom]

So if it uses frequency modulation, that's a sine wave, right?
Can I use a low pass filter circuit to make my PWM wave shaped a bit more like a sine wave?
Anyone know how to calculate the resistor and capacitor values?

[matt dunn]

Basically,
Frequency mudulation is counted in pulses per minute,
and
PWM is measured in how long each pulse is.


Frequency modulation is usually to determine speed,
and is what a speed sensor usually puts out,
and how long each pulse is doent really matter.

PWM is usually at a set frequency although the actual frequency doesn't matter, as is not really dependant on how many pulse there are.
It is usually for things like boost control solenoids, vvt solenoids.

[Adoom]

So my only option is to get one of those Jaycar sppedo correctors and put it between the speed sensor and the speedo, but after the digital input to the link?
Like so:

[Grrrrrrr!]

So if it uses frequency modulation, that's a sine wave, right?

not nessecarily, could be square wave.

Can I use a low pass filter circuit to make my PWM wave shaped a bit more like a sine wave?
Anyone know how to calculate the resistor and capacitor values?

Turning a fixed frequently square wave into a sine wave still isn't frequency modulation.

http://www.linkecu.com/forums/G4Forum/885457179

[sergei]

Think of the speed sensor basically a switch that switches once (or twice) per rotation.
Normally speed sensor in the gearbox drives the speedo directly (older types were reed switches). Then the speedo feeds "normalized" speed signal to ECU.
I am not familiar with nissan speed sensors, but Toyota speed sensor is fed by 12V, and it generates a pulse every 180' degrees (it pulses twice over one rotation). The width does not matter. All you need is count rising edges of the pulses. The frequency of pulses will increase if you increase the speed and this is how it reads the speed. Think of it as counting how many pulses it outputs every second.
It is very similar to how CAM/Crank angle sensor outputs (it generates N-pulses every rotation). It does not really matter what type of sensor (optical, hall or simple reed switch), as those type of sensors will generate pulses per time type of output, where frequency of pulses will be dependant on rotational speed.
PWM is incorrect approach.