Tim is building an ICV using a 4cly Toyota Camry engine. For a dash he bought a multifunction LCD unit intended for motorbikes. It was a 'vapor' brand -> see picture below....
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| Vapor LCD Dash Unit - Intended For A Motorbike |
The problem was that the tachometer was not triggering. The vapor dash had an input that could supposedly be hooked up a number of ways. The primary method was to wrap in input wire around the ignition lead, inductively coupling it.
Since the engine in Tim's car uses coil on plug ignition, there was no lead to wrap the line around.
Even though the dash instructions said that the tacho could also be connected to the coil trigger line, this didn't work. The trigger line is 5v, we fixed this with a transistor to get the same signal to 12v, but with no success.
The only time that the dash would just to life was when the line was connected to the power supply line of the coil. When I looked at this line on the oscilloscope I could see a lot of noise related to the coil firing. Each time the coil charged the voltage would drop and then spike to 16v when charging stoped (inductive field collapse). This worked ok at low rpm, but there was too much interference from the other coils at higher rpm causing erratic behaviour.
Interestingly some cheap rpm meters that plug into the cigarete plug use the noise on the power line to detect the coil firing rate.
Since the dash seemed to be triggered better by higher voltages (which would get quite high when inductively coupled with the ignition lead) I build a simple charge pump circuit to increase the system voltage to around 25v. The 5v coil trigger line activates an NPN transistor which is pulled up to the 25v rail. All this gives a higher input voltage signal to the dash, causing it to spring to life.
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| Circuit on proto-board |
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| Circuit neatly mounted in a box |
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| Basic Schematic of circuit |
The only modification we made was to hook the circuit up to two coils, doubling the rpm displayed (this adjustment couldn't be made in the dash). The reason was that the unit was designed for a high reving motorbike and didn't look so good only revving to less than half the full display range.
