DIYPnP VSS circuit

[GraemeD]

I am going to try and build a VSS circuit on the proto area of my v1.5 DIYPnP. I plan to use the circuit described Here
http://msextra.com/doc/ms1extra/antirev.html.

I have been looking for part numbers but I am not sure about the type of capacitors and diodes. If anyone could help fill in the blanks, it would be appreciated.

[SymTech Laboratories]

Per the schematic, C2 should be a 2.2uF tantalum capacitor. These are polarized, so pay attention to how you connect it. If you look a little below the schematic, there's a note explaining that C1 should either be a 3.3nF, 6.8nF, or 15nF capacitor, depending on how many pulses your sensor generates.

The 1N4148 diode is pretty popular and easy to find, but any small signal diode should work. The 1N4735 is a pretty popular 6.2V Zener diode, but again, just about any 6.2V Zener diode should work.

[GraemeD]

Thanks, by time you have to filter down with all the details like leg spacing temp rating it gets a little thick.

Thanks again
Graeme

[GraemeD]

well, I built the circuit on a test board, and I can't seem to get it to work.
In a Miata, the speedometer provides a pulsed ground, I am assuming the ECU provides the pulls up. the FSM calls for about 7v while driving, so if it is a 50% duty thats about 14V and at 60 mph the frquency is about 68Hz.

I am using a signal generator to simulate the speed signal. but the Vout is about 1.5V no matter what the frquency. Adjusting R1 does not effect the output until it is bottomed out (zero ohms) and then the output jumps to 5.4v

Has anyone built this or a similar circuit that might have some tips for me?

Thanks,
Graeme

[GraemeD]

I got the circuit to work, must have been my test build. I Built another on a breadboard and it worked as it should. I have a question, the output can go as high as 6.24v, is that too much for the spareAD2 ? If it is, I am going to try to make it run on 5v.

Thanks,
Graeme

[SymTech Laboratories]

I got the circuit to work, must have been my test build. I Built another on a breadboard and it worked as it should. I have a question, the output can go as high as 6.24v, is that too much for the spareAD2 ? If it is, I am going to try to make it run on 5v.

According to the datasheet, the chip can withstand that much voltage, but it won't be able to read it. You should adjust VR1 so the voltage is 2.5V at half your maximum speed.

[GraemeD]

I got the circuit to work, must have been my test build. I Built another on a breadboard and it worked as it should. I have a question, the output can go as high as 6.24v, is that too much for the spareAD2 ? If it is, I am going to try to make it run on 5v.

According to the datasheet, the chip can withstand that much voltage, but it won't be able to read it. You should adjust VR1 so the voltage is 2.5V at half your maximum speed.

Thanks for the reply,
I know the LM2917 chip is okay with it, but the MS chip is the one I am worried about. If there was a failure in the VSS circuit and the voltage went high(6.2v) will that damage the MS processor?

[jsmcortina]

Do not apply more than 5V to the Megasquirt processor.

Your design should operate over the 0-5V range. If a greater voltage can be generated in fault conditions then arrange a voltage and current limiting clamp circuit.
(e.g. a 10k series resistor and a diode to 5V.)

James

[SymTech Laboratories]

I know the LM2917 chip is okay with it, but the MS chip is the one I am worried about. If there was a failure in the VSS circuit and the voltage went high(6.2v) will that damage the MS processor?

We were referring to the MS chip, not the LM2917. We wouldn't recommend testing the absolute maximum rating, though! Like James said, if you think it's possible more than 5V can be generated, you should take measures to limit the voltage.

[GraemeD]

Thanks,
I will try it using 5v as the supply. it might be easier and use a couple of less resistors.
Or does the "de-coupler" accomplish this limiting?

[SymTech Laboratories]

I will try it using 5v as the supply. it might be easier and use a couple of less resistors.
Or does the "de-coupler" accomplish this limiting?

The de-coupler circuit won't limit the voltage to 5V, and supplying the LM2917 with 5V isn't a good idea. The chip probably can't operate too close to the rails, so supplying it with 5V will limit its output range.

[mac1]

Do not apply more than 5V to the Megasquirt processor.

Your design should operate over the 0-5V range. If a greater voltage can be generated in fault conditions then arrange a voltage and current limiting clamp circuit.
(e.g. a 10k series resistor and a diode to 5V.)

James

Like so ?