Two-Transistor Circuit

This is the simplest form of the circuit, and the one that we have chosen to use. We have tested it and it performs quite well in every application where we have used it. This circuit requires that you have a source for the 12V DC power. In our case, this comes from either the cigarette lighter plug or from directly wiring the ALDL cable to a DC to AC Inverter (which pulls its 12V power and ground from the cigarette lighter).

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The transistors used in this circuit are generic NPN transistors. 2N3904, 2N2222, or similar will work just fine. This circuit is not particularly sensitive to the gain of the transistors, they are operating well into saturation, and the baud rate is slow enough that switching times are not particularly important either. We are using surface mount versions of these generic transistors so that everything fits nicely in the DB9 backshell, for your RS-232 port. The diode is also a generic low-power switching diode such as the 1N914 or 1N4148. All the resistors are 10K, and again, the circuit is quite tolerant of component variations. If you have a supply of 5K or 20K resistors, these will work also. Try not to get too far afield from these values or the switching times will suffer.

We have done a CadSoft Eagle circuit board layout for this circuit, and produced these circuit boards in quantity.  You can purchase these from us on our products page.

Two-Transistor, Self-Powered Circuit

This is essentially the same circuit as the Two-transistor circuit above, but it uses a couple of pins from the computers RS-232 serial port as a power source. There is plenty of power on these pins to supply the current needed, however, the pins are normally in a negative voltage state. The software in your PC has to set these pins into a positive voltage state most of the time. If you are writing your own software, then this is not an issue. We have found that using off-the-shelf software and coming up with a source of 12V from the car is the easiest, and most reliable fool-proof approach.

MAX232 Circuit

Maxim (and others) make an RS-232 driver chip that provides an assured drive signal voltage and the proper slew rates. This part requires a +5V DC power supply, so the 12V (or self-powered voltage) has to be regulated down to +5V. The 78L05 regulator series is the least expensive and easiest solution for this. (you can find the regulator at almost any parts supplier, Digikey, Mouser, Newark, Radio Shack, etc.) We have not seen an application where the MAX232 will work and the Two-Transistor circuit will not, but if you see one, let us know.

• View a schematic for this circuit