Scope image taken again as the original ones were wrong - not sure why but I suspect something wasn't connected properly. At least this is more credible. As before blue is before diodes and red is after.
Yes I see that - but look at the attached scope traces on post #34. If you look at the red trace which is the BC547 collector output you can see that without the diodes in place, the transistor switches on much more slowly than when the diodes are in place. This has the effect of not allowing the input signal (blue trace) to pin 2 of the 555 to drop low enough, (because the edge trigger starts to raise the signal before it has been able to drop low enough to trigger the monostable). When the diodes are added, the transistor switches off sharply before the edge trigger has time to do its thing, and consequently the signal drops low enough to trigger the monostable. This is the bit I don't understand, i.e. why does adding the diodes cause the transistor to switch off more sharply? Is it because of the 1.2v lower input to the 1K resistor and its base? And, why does increasing the engine RPM gradually make it switch off more sharply, so that by the time it gets to around 2500RPM, it is triggering the monostable?
And thanks for your schematic. I will try that at some point and see how it works. I think I'll stick with the 555 for this rev counter, but I do have another that needs the same so I'll have a play with that one when I get some time.
Yes I see that - but look at the attached scope traces on post #34. If you look at the red trace which is the BC547 collector output you can see that without the diodes in place, the transistor switches on much more slowly than when the diodes are in place. This has the effect of not allowing the input signal (blue trace) to pin 2 of the 555 to drop low enough, (because the edge trigger starts to raise the signal before it has been able to drop low enough to trigger the monostable).
When the diodes are added, the transistor switches off sharply before the edge trigger has time to do its thing, and consequently the signal drops low enough to trigger the monostable. This is the bit I don't understand, i.e. why does adding the diodes cause the transistor to switch off more sharply? Is it because of the 1.2v lower input to the 1K resistor and its base? And, why does increasing the engine RPM gradually make it switch off more sharply, so that by the time it gets to around 2500RPM, it is triggering the monostable?
Ok I'll try this when I get back to the car, but...won't this have the opposite effect? The problem is the transistor is slow switching on and going into saturation, i.e. the base voltage is slow rising.After thinking about this, you might try lowering the value of the transistors' base pull-down resistor from 47k to maybe 22k
Ok I'll try this when I get back to the car, but...won't this have the opposite effect? The problem is the transistor is slow switching on and going into saturation, i.e. the base voltage is slow rising.
Oops! Yes it is definitely connected to ground - I can't see it working if it wasn't. I've amended the schematic.BTW- your schematic shows the transistor emitter disconnected. I hope its really connected to ground.
I can do better than that - I've laid down a schematic on paper. I did that first when I was trying to get a useable signal for the 555 and the LM2917 but wasn't successful. Let me see if I can find it later today and come back with it.Hi
When you get a chance can you post a photo of the top and bottom of the original tach circuit board? I'd like to look at the components connecting the chip. Especially the tach input circuit.
I tried what you suggested in post #47. Firstly I have to admit to another mistake - the base resistor was not 47K but 4.7K. I realised this when I went back to change it to a 22K as you said. I also removed the diodes from the input line and inserted a 0.1μf cap instead. What I found now was that under 1500 RPM the needle was very erratic, and I took some scope pics to show the problem. The first image is the tach signal that comes from the ignition system. This was taken with a 20:1 attenuator so the initial spike is around 160V. I took this to show you the kind of signal we have to deal with. When you look at the next two at idle RPM you can see that some of the coil oscillations are also causing the transistor to switch on, and we get additional triggers of the monostable when we don't want them, hence the erratic needle. At higher RPM (3000 in this case) this does not seem to happen - the transistor is switching on and off cleanly.
Actually I just realised why I got that wrong. Your schematic in post 47 doesn't show a diode - I was following that when I tested it. But it's no problem, I'll go back to it later this week.1. The input needs at least one diode. Did you test as I described in post 51?
2. The car has electronic ignition, currently running on the later Lucas AB14 amplifier, and the tach signal comes from the amplifier. At some point I plan to go back to the original Lucas OPUS system if I can find one that actually works!?! But my signal looks like it is coming from the coil (-). When I go back to the OPUS system, that does have a ballast block, one of which is for the tach.
I'm not clear though on how your schematic is working. Looks like the tach signal peaks at 160 volts...are there any zener diodes being used?
I'm still curious about where the tach signal source is connected.
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