Passive Automotive Tachometer
parsko
Posts: 501
Hi all,
I'm working on a passive automotive tachometer. I say passive, since it needs to be able to work on just about anything. The technique I am using is an inductive style pickup. This method uses a wired wrapped around a spark plug wire about 6 times. I have setup and tested the following circuit:
It blinked the LED just fine. The author says that if one wants to hook up to a ucontroller, that one just has to connect the transistor collector to the pin. This is where I get confused. I like to know what I'm doing before I fry any pins on my (original) Prop. So, I went to a transistor calculator page (FOUND HERE), to see what is going on.
I believe the calculator is for an "ideal" transistor. So, when you put values in, as can be seen in the diagram I included, it doesn't work. Specifically, R2, on the emitter, must have a value. Is it safe to put in 1? My values for the rest are: R3 is 9800, R1 is 220, R2 is 220. Vcc=3.3, Vbb=100V (AC with a diode in-line). Is it safe to simply connect Vc directly to the Prop pin?
In the Prop manual v1.05.8, it shows a diagram of the transistor style Serial port connection. It uses a transistor in the Rx pin. This is the same implementation as used in the diagram above. Again, no R2 resistor is used. AND, it seems to use a pull up resistor connector to the Prop pin.
When a transistor sees no current at the base, it is "off". When it is off, would the collector see a negative (ground) connection, meaning that the Prop input pin would be low? Wouldn't the current pass through the pull-up resistor into the Prop pin, and then what happens when the transistor is "open" or "switched on"?
This is where my confusion lies. I am trying to use a transitor as an "on/off" switch for an anonymous signal that will be read by an input pin on the Prop.
One additional unrelated question: I'm using a Protoboard. I want to hook up to a vehicles battery directly (7V during cold cranking, 14.4V during normal running). Initially (for now in my data aquisition setup), I won't be drawing much current, I would say less than 100mA. Would the 5V regulator be able to handle the heat? Aka, is the Proto board capable of dissipating the heat enough? Or, should I add a heat sink to be safe? I have done through the calcs, and I know it will need to dissipate about 0.9W @14.4-100mA. The Voltage regulator should be okay, right?
Please offer any suggestions you may have. I'm a bit stumped, and don't yet have the confidence(knowledge of electronics) to make a proper decision.
Thanks,
-Parsko
I'm working on a passive automotive tachometer. I say passive, since it needs to be able to work on just about anything. The technique I am using is an inductive style pickup. This method uses a wired wrapped around a spark plug wire about 6 times. I have setup and tested the following circuit:
It blinked the LED just fine. The author says that if one wants to hook up to a ucontroller, that one just has to connect the transistor collector to the pin. This is where I get confused. I like to know what I'm doing before I fry any pins on my (original) Prop. So, I went to a transistor calculator page (FOUND HERE), to see what is going on.
I believe the calculator is for an "ideal" transistor. So, when you put values in, as can be seen in the diagram I included, it doesn't work. Specifically, R2, on the emitter, must have a value. Is it safe to put in 1? My values for the rest are: R3 is 9800, R1 is 220, R2 is 220. Vcc=3.3, Vbb=100V (AC with a diode in-line). Is it safe to simply connect Vc directly to the Prop pin?
In the Prop manual v1.05.8, it shows a diagram of the transistor style Serial port connection. It uses a transistor in the Rx pin. This is the same implementation as used in the diagram above. Again, no R2 resistor is used. AND, it seems to use a pull up resistor connector to the Prop pin.
When a transistor sees no current at the base, it is "off". When it is off, would the collector see a negative (ground) connection, meaning that the Prop input pin would be low? Wouldn't the current pass through the pull-up resistor into the Prop pin, and then what happens when the transistor is "open" or "switched on"?
This is where my confusion lies. I am trying to use a transitor as an "on/off" switch for an anonymous signal that will be read by an input pin on the Prop.
One additional unrelated question: I'm using a Protoboard. I want to hook up to a vehicles battery directly (7V during cold cranking, 14.4V during normal running). Initially (for now in my data aquisition setup), I won't be drawing much current, I would say less than 100mA. Would the 5V regulator be able to handle the heat? Aka, is the Proto board capable of dissipating the heat enough? Or, should I add a heat sink to be safe? I have done through the calcs, and I know it will need to dissipate about 0.9W @14.4-100mA. The Voltage regulator should be okay, right?
Please offer any suggestions you may have. I'm a bit stumped, and don't yet have the confidence(knowledge of electronics) to make a proper decision.
Thanks,
-Parsko
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-Parsko