frequencymicrocontroller
I am building a Vehicle diagnostic tracker device. I want to read all the data from different sensor interfaces in the car. The problem I am facing is with reading the RPM data. The data I could get regarding the RPM is :-
How can I read it with my micro-controller. I need to step down the voltage level is one thing. What else I need?
I think the radio part is the least of your problems. To my best knowledge most car openers use a cryoptographic system called "rolling keys", which means that recording what the keyfob sent and replaying it won't work. Microchip has extensive libraries to go with thier chips intende for this purpose, but you need to sign a non-disclosure to get it. Here is a (probably simplified) explanation.
A simple resistive voltage divider will achieve what you want.
The formula to calculate the output voltage is:
So if we assume your input voltage ranges from 0-50V, we need to divide it by 10 to achieve 0-5V. If we also assume we want to load the input voltage with 100kΩ, then the calculations would something like:
Vout / Vin = R2 / 100kΩ
0.1 = R2 / 100kΩ -> R2 = 10kΩ
R1 = 100kΩ - R2 = 90kΩ
So R1 = 90kΩ and R2 = 10kΩ
For an ADC requiring a maximum source impedance, you must make sure the voltage divider impedance is below this level. The impedance at the divider can be calculated as R1||R2.
For <2.5kΩ, the above won't meet this requirement as 10kΩ||90kΩ = 9kΩ
If we use 9kΩ and 1kΩ though, we get 1 / (1/1000 + 1/9000) = 900Ω
Bear in mind the lower the resistance the higher the wattage rating resistors you need. 50V / 1k = 50mA -> 50mA * 45V = 2.25W across the top resistor (0.25W across the bottom)
In these cases it's best to use an opamp buffer in between a high resistance divider and the ADC. Or use a 2kΩ and 18kΩ divider, which is not quite as power hungry as the 1k/9k version.
Best Answer
Measuring frequency might seem hard thing to do but probably isn't. Period and frequency are inversely proportional so getting one means you are free to calculate another.
Let's talk about examples. I will use AVR205:Frequency Measurement Made Easy with Atmel tinyAVR and Atmel megaAVR Application Note as a starting point. As you can see it is suggested to use timer and some source to start counting. Count how many peaks there during some time and recalculate. Be aware of tradeoffs - short amount of time makes system fast yet not as precise, also, using external clock would make measurements even better when introducing some small additional cost.