I’ve got an odd one, my 1989 Honda prelude had a speedo drive run off of a cable, using a hall effect sensor and a toothed wheel to pick up a certain amount of pulses per km off of the output shaft in the transmission. Output to speedo = ~ 2550 pulse per km @12v
I have replaced this with a device that creates the same pulses at same rate off of GPS and runs the speedo, the problem is that there was a circuit in the original board that also pulses at 5.0V for the ECU and cruise control
I need a device or circuit that can convert a mixed 12v signal <200Hz (13~14.2v) into 5v <200Hz
A slower switching circuit in the realm of 100Hz would probably also be fine as the cruise control is unlikely that necessary at or above 140kph
So a buck converter or similar is unlikely to work as they don’t switch that quickly in my experience? My research has pointed to maybe a level shifter, but I am searching for the best answer if people can help ?
As with all systems that have something dangerous attached to them you should have a level of redundancy and some other inbuilt safety. This is well outside the realm of hobby electronics so might be worth chatting to a mechanic or engineer. If you can think up a way that a system can fail, there will be 5 others you didnt think of.
Hi Wozza
With a couple of transistors or an OP amp or a comparator you can convert pretty much anything to anything within the voltage limitations of the particular devices.
BUT. Please take note of Liam’s reply. When you are interfering with Cruse control and ECU things there will be a safety aspect coming into play and really should be left alone.
That is well said and pretty true. The manufacturer will have carried out sometimes many man/years of testing in an attempt to guarantee his product. They also have the legal expertise to look after that side should something go wrong and someone claims negligence. Most, if not all, hobbyists will not have access to anything like this and could find themselves in all sorts of strife.
With all this in mind I for one would be reluctant to offer you a solution as I feel if you don’t know how to do this already you would be hardly qualified to fiddle with anything that could have safety implications.
Sorry about that but I get a bit nervous making suggestions without having any control over what you actually do.
Cheers Bob
Excellent call there, its one thing tapping into the DE9 connector (EDIT: previously DB9) to get some info and a completely different ball game intercepting signals from the sensors on the car, I’ll chop down my response to remove the key info.
Hi Liam
Sorry for being so picky but the correct term is DE9. But that is another discussion.
For instance what would you call a 15 pin connector in a 9 pin size shell (it is DE15). There is already a 15 pin connector in a 15 pin size shell. “DA15” is the correct term for this one (NOT DB15).
Cheers Bob
No need to appologise at all! Detail is where it matters, I had a quick read over the Wiki page and the differences, quite an interesting read.
For those interested: D-subminiature - Wikipedia
I just remembered this product from a while ago mentioning DB-9.
Should be compulsory reading. When I was working we always used the correct terms mainly to save confusion. We were dealing in both normal and high density. I have never had any dealings with double density. Also had to contend with various “combo” types. Believe me the ones with co-axial contacts could be a fair pain to terminate. Very fiddly sometimes.
Cheers Bob
Edit There is a publication I had to refer to a fair bit when dealing with Mil Spec type connectors. Also interesting reading. “The Connector Cyclopedia”. When I last consulted this it was 3 volumes each some 50mm thick and I think costing up in the $thousand mark. Not for the average hobbyist to be able to purchase.
The vehicle boasted at the time that it had 7 sensors in all its literature, it is not horrifically advanced and there is fail safe upon fail safe built in to the vehicle, often mechanical in nature
The speedo works perfectly as it stands and the odometer remains mechanically driven. The speed signal to the ecu and cruise control are a very basic input at the same rate as the speedometer, just 5v. The cruise control module can be deactivated by brake switching, by clutch etc. And is separate in operation to the ecu. So there is no direct and immediate danger condition, the speed input to the ecu is mainly there for the idle air control valve and to tell it there’s a non zero speed, it is not advanced enough to do much more, or need to on the manual model, the 4 wheel steering is mechanical and not influenced, there is no wheel speed input or abs.
I can convert 12v to 5v and such but I just don’t know of a product off the shelf that will do this in the realm of 100Hz. I’d like to avoid making my own circuit board if possible as there is very limited space on the rear of the cluster and dash, I will be updating the ecu to an aftermarket unit which will happily take a pulsed 12v signal in the future as well.
I admit that I am learning on this smaller end of the scale and have only rudimentary knowledge from mechatronics and circuit design classes in school.
If you have recommended resources to puzzle this together I am more than happy to read.
There are a few comparator boards on the market which would look like they could work, however they all appear to have mechanical relays, that I can see, which likely won’t operate fast enough or make noise.
Hi Wozza
Core have a quad level converter SKU: BOB-12009. No relays and should be easily fast enough. As far as Audio is concerned 100kHz is very high. For anything else it sometimes would be painfully slow so I shouldn’t worry about that. You are right though, most relays take in the order of 10msec to operate and another 10msec to release so that option is out.
This converter is normally used to convert 3.3V signals to 5V and vice versa (it is bi-directional) but I don’t see why it would not operate 5V to 12V and reverse. The only thing is it requires the 2 voltage sources in your case the 12V could be the car battery but you will have to arrange the 5V yourself.
The circuit for this converter is on the Core web site. Delightfully simple.
Be aware that I have not personally used one of these units so could not comment on reliability or MTBF (mean time between failure). That is something you are going to have to determine for yourself. MTBF is a term that seems to be pretty much avoided by manufacturers of this type of thing as if required to quote would increase the object price significantly and really not needed for the target market. BUT for what you are contemplating it could be all important. You can bet your life that for items like your ECU and support components Honda would have looked at this figure very closely.
Wozza you asked to be pointed in a direction for a ready made product as you did not want to make anything yourself. This is the only thing that comes to mind but I have been out of the business for a long time now so there may be other things around.
This is only a suggestion as you requested so I cannot take any responsibility for anything that occurs due to using this in any modification to your vehicle.
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