Amplifying pulses.

[Dave Plowman (News)]

The rev counter on my old SD1 was connected directly to the coil LT negative. I've changed the ignition system to EDIS which is wasted spark with 4 coils, and the common way to feed the rev counter off this is via four diodes - one from each coil - and a zener and resistor to limit the peak voltage.

The EDIS ECU has a diagnostic output which will drive a sensitive enough rev-counter - but it is a nominal 5 volt pulse output, and my rev counter needs approx 15 volts.

A quick lash up using an emitter follower and a 1:3 audio transformer got it working just fine - but wondered if there was a more sanitary way to do things?

Only reason I'd rather do this is neatness. I can hide an amp at the rev counter end - rather than have all the extra wiring on show for the diodes trick.

[Tabby]

I know about amplifiers, but little about the pulses/impedances you have in mind.

NT

[Andy Dingley]

Buy yourself a stepper motor, an Arduino and a glue-gun. You'll get a tacho built and you'll find yet another new hobby 8-)

[Peter Scott]

Just a thought that might be nonsense. RS232 drivers work down to about

3V and produce up to about 15V. Could you use one of these to convert the voltage? Can't give you a reference number off-hand but should not be difficult to find.

[The Natural Philosopher]

well there are more ways to skin this cat...BUT the basic problem is where to get 15V without having to generate it. The car supply with the engine running will be about 14.4v..

The original tacho would have run off the coil primary - 200V or so. If its the same tacho that used to be fitted to Spridgets and the like, it is in fact a 0-10mA moving cold meter so you can gut it and build your own pulse counter.

Or you can make a mini ignition coil and use the flyback with a high voltage switching transistor.

Audio transformers can be pretty small as well, you know.

You dont want an emitter follower either. No voltage gain.

What occurs to me is taking the diode bridge, smoothing it, and using a standard inverter from the ECU to get a couple of hundred volts pulse at the collector.

Or run it off 12V with an inductor in the collector.

[Tim Streater]

My 68k SBC appears to use MAX232 chips:

just google for max232.

[John Rumm]

One of the MAX232 charge pump TTL to RS232C line drivers would do quite nicely. They work off single sided supply rails. Only thing to watch is there can be ENC susceptibility issues with charge pump circuits IIRC.

[Man at B&Q]

Make sure the existing tcho is happy with negative inputs, or clamp the RS232 levels to avoid them.

MBQ

[Man at B&Q]

I think we've been told that the tacho requires 15V.

MBQ

[The Natural Philosopher]

No, we haven't. We've been told that it will *work* off 15V.

How accurate it is at that voltage, is another matter.

[Tabby]

Yes, if it works on 15v its likely to work on 14v too. But why spend a bunch of time & error on soemthing that works fine already (the transformer option), even if it is historic technology.

NT

[The Natural Philosopher]

I think he explained that: to make it small enough to fit in the dashboard.

[Jules Richardson]

I'm having a right ol' thicky day today, but for my addled brain, why

15V? Why isn't it just "whatever the alternator is putting out", which will vary a lot between cars and at different loads, but might be anywhere down to 11V or so and with 15V as a probably upper design limit?

Or have you fitted something that wasn't originally intended for car use?

cheers

Jules

[Dave Plowman (News)]

I was also hoping to offer it as an option for others with a similar problem. I had a suitable ratio audio tranny lying around - but they're quite expensive. A small PCB could be fitted inside the actual rev counter.

[Dave Plowman (News)]

I'd like the rev counter to work under all possible conditions. So lets say just over 10 volts or so - when cranking on a cold day.

It's already got one built in. But they're delicate things to start hacking around. The present PCB is glued in place.

It buffers the ECU output. The transformer provides the voltage gain.

The diodes as suggested work just fine. But require a connection to each of the four coils. Which will make my loom look a bit untidy. One wire from the dedicated output on the ECU, not.

[The Natural Philosopher]

Coil primary starts at +12v and 0v, top end normally held to +12v but bottom end swings up to about 200-300V when the points open,.

[The Natural Philosopher]

all you need to do is make a 'little' ignition coil and drive it from a

400v capable transistor

wind a few hundred turns on any old lump of ferrite or similar.

[Dave Plowman (News)]

The pulse counter in the tacho is connected to the car's 12v supply, but the trigger feed to that comes from the coil negative. Which produces a pulse of many volts as the flux collapses. Tests on the tacho shows this must be a minimum of about 15 volts to trigger the pulse counter. The output I'd like to use is a diagnostic one and limited to about 5 volts.

[Dave Plowman (News)]

Think you're missing the point...

The transformer I'm using at the moment works just fine. A single IC would be even better.

[The Natural Philosopher]

Right,. You need an inductor in the mix somewhere then. It doesn't need to be MUCH of an inductor. Years ago those little transistor output transformers..would do. Microphone transformer secondary might do as well.

Blimey. Amazon does audio output transformers for 3 quid.

That's just the ticket.

You can simply drive the primary from a transistor collector, and drive the base from your 5v pulse via a suitable resistors. Collector should have a diode to earth, reverse biassed.

Collector output should be a 50v+ spike.