Looking for a little advice. Here's some background on my project.....
I like restoring old Automotive Test Equipment. The project I'm currently working with is an old Allen Electric Distributor Test Stand. You mount an automotive distributor in a chuck, spin it, and you get a strobe indicating when the distributor triggers the ignition system and also meters display rotations per minute and distributor "cam angle" or "dwell". A little more on that in a minute....
Both original meters were broken and I'm replacing them with Adafruit 3990's which are 0-5vDC voltmeters that accept PWM input at 1 kHz or better. Therefore, the RPM (rotations per minute) function should be quite simple, as a magnetic proximity switch is used to count the number of times the iron core passes the sensor (once per revolution). The "dwell" function is a bit more difficult.
So, I'm thinking that, for the RPM function, it should be as simple as inputting the signal from the proximity switch using an ANALOG PIN, and writing PWM output (one wave per revolution) to a DIGITAL PIN, and out to the meter, whose scale will read zero to 5,000.
The DWELL function is a bit more difficult.... the output, of course, would write to a DIGITAL PIN, the INPUT would be analog, and would come from a circuit that varies between OPEN and CLOSED states, and the PWM output would be the percentage of time the circuit was CLOSED over the sum of both CLOSED and OPEN time. For example, if the CLOSED and OPEN "cycle" takes 1/100 of a second, and the circuit is CLOSED for 1/300 of a second, the percentage would be 33.333% to drive the meter, whose scale will read zero to 50 degrees. Normal analog circuit voltage will vary between 6v open and approximately 2v closed. I'm guessing I'll need to toss in some resistance before it hits the Arduino's ANALOG pin so if I toss in a 100 ohm resistor I should have voltage the UNO can deal with and then it's a matter of a simple math calculation, right?
If I'm "barking up the wrong tree", please don't hesitate to tell me.
Interesting project, I need to get around to rewiring my old Radio Shack unit.
I have one query - will the volt meters handle the 7v coming from the ignition coil (in Australia they used 12 volt coils with a "Ballast Resistor from 1963 until the mid 1980's).
mick in glen innes
No idea why you would use "analog" input to measure the dwell.
Signal from the points would be either closed or open, assuming a small calculation for switch bounce.
Same here. In the case of the test stands (Sun and Allen) they used an ordinary 6v ignition coil, with 1.5 ohm impedance. a transformer & rectifier to step down the voltage from 115vAC to 6vDC.
Since my meters use PWM input and are 0-5v I'm using the digital output from the Arduino. However, since the board will only map input voltages from 0-5v my analog input to the board, from the ignition coil primary, will include a 100 ohm resistor to reduce the input voltage.
Correct me if I'm wrong, but isn't a "digital input" 0,1 (off, on)? How can you input a waveform like the one shown as a digital input without an additional converter?
True. Tachometers and dwell meters also predate transistors. My rationale behind using one was to utilize the accuracy and flexibility of using a PWM signal to drive the meters. Since RPM is being acquired from a proximity switch based on distributor shaft rotation and not breaker point or solid-state ignition trigger (HEI, Duraspark, MSD, Accel, etc.) it should be a simple matter of counting the positive (or negative) pulses and outputting a PWM signal. For dwell, the input being the opening and closing frequency of the breaker points (dwell signal not input for solid-state triggers as it is typically not adjustable), it should be a calculation based on the amount of time the circuit is closed (low voltage) as a percentage of the total circuit cycle time (closed + open) which can be converted, by the UNO, into a PWM output to the dwell meter. This also bypasses the typical issue with analog dwell meters where the number of cylinders must be selected to properly fix the output signal voltage. A third factor could be considered which is that I already have the UNO, I don't have to purchase 2 frequency to voltage converters, resistors, cap and pots to build the circuitry for each which, in itself, isn't a big deal plus it gives me some experience working with the Arduino programming logic.
