Some applications require a small amount of power, brains, and analog filtering to get some data into the computer, and should be able to run in a minimal amount of space, using a bit of power from the computer. If you can minimize it enough, you may be able to do it all inside a DB9 hood (good luck).
Actually, some people are definitely thinking along these lines, but apparently it's quite tough to fit much in a DB9.
Note that a SN75155 (SOIC-8) + 12F683 (SOIC-8) + SOIC clip for in-circuit programming + a SOT-223 regulator plus one small capacitor is the best chance I have of doing this by hand (I think pictures I've seen indicate SOIC-16's fit on one side, so this SHOULD fit? Parts are maybe $10. I bet I could sell these at rally races for like $20?)
Maybe if the above are glued dead-bug style to each half of the hood?
DTR — provides a tiny bit of power, first through a diode and then a 5V voltage regulator. Allows the computer to reset the unit as well. On laptop serial ports that only provide 5 volts, this need to be a LDO 3.3v regulator, with a low-voltage-drop zener diode.
RX — constantly sends data back to the computer, directly connected to a PIC output pin. Some notes:
If your sensor doesn't require feedback from the computer (which really is probably the case, no?), then this is a brilliant minimalization. You don't need extra voltage step-down components to receive data from the computer.
Still, this is exceedingly minimalistic, and might have problems on some serial ports? Some suggested additions, in increasing order of complexity:
a 100ohm resistor in series
a transistor plus one or two biasing resistors to bump 5v up to 12v (which is taken from DTR or RTS)
RS232 requires >+5volts to indicate low, and <-5v to indicate a high, so this isn't quite to spec (not that anything is)
this requires inverted logic, so can't be used with the internal UART. But it seems that so many people use this method, that lots of serial code is available.
Some RS232 transceivers can be similarly hacked:
Transceivers that can handle a power supply from 5v - 12v can be powered from the DTR or RTS like above, to allow it to work on all varieties of RS232 ports (eg. laptops and desktops).
To handle the wide-input-voltage, it will inherently have an internal 5V voltage regulator. Simply tie one of the receiver's inputs low, and the corresponding TTL output will go high, and then you can use it as a constant power supply for the µC. (this uses up one of the receivers of course, but as mentioned above, pure sensors likely don't need any input from the computer). The goal of this, of course, is to remove the need for a separate driver and voltage regulator.
In cases where current from both the power supply and the receiver's logic input are combined to provide power for the receiver TTL output, you can use DTR for one and RTS for the other, which will allow you to not have to add the two diodes that would normally be required when doing this.
SN75155 — only has one receiver, but it's an 8-DIP and seems like the receiver can supply all remaining current (24mA, which you'll never get from a serial port). One optional external resistor. For 5v serial ports, provides 4v @ 10ma max.
Okay, not many RS232 transceivers have built-in regulators. But there's that one anyway.
If you can't use a transceiver with a built-in regulator, then the next-best thing is to use a separate small LDO low-current voltage regulator, along with a transceiver that takes +12v and -12v from DTR+RTS.
Powering the transceiver from +12v and -12v will make it smaller/cheaper, and remove the need for external/internal capacitors and funky power solutions.
Note that if your sensor reads in digital data from the outside, ESPECIALLY if it's in an automotive environment where there's lots of noise, then a RS232 receiver is probably a VERY GOOD thing to filter the external digital lines through first. Note this will let you work with either TTL or 12v logic levels (which is IN ADDITION to the necessary hysteresis, voltage-spike protection, transients, etc etc).
transceivers powered from +12v and -12v: SN75155 (1D/1R, 8-pin)
Any of the DIP versions of FTDI USB<=>UART chips allow for trivial breadboarding and provide a lot of bus-based power (the ravar ones seem to be popular). Yes, it may look odd to have a breadboard sitting on your car's dash, and for one tiny piece of it to cost $33, but given that it takes 5 minutes to put together, is reusable, and supports whatever µC and in-circuit programmer you like best, I think it has a place.
