Operation Manual

Resistors (1/8W or greater, except as noted)
R22, R23 = 100 Ω
R3, R5, R27, R28, R29, R30, R31 = 470 Ω
R17, R19 = 510 Ω 1/2W
R2, R4, R16, R18 = 2.2 KΩ
R1, R7, R14, R15, R24, R32 = 4.7 KΩ
R6, R9, R11, R13, R26 = 10 KΩ
R10, R8 = 22 KΩ
R21 = 33KΩ
R12, R20, R25 = 47 KΩ
Capacitors (16V or greater, except as noted)
C3, C4 = 27pF
C7, C8 = 560pF 50V
C1 = 0.1uF 50V
C2, C5, C6 = 0.1uF
Semiconductors
D1 = 1N4001
D2, D3, D4 = 1N4148
L1, L2, L3, L4 = Yellow LED
L5 = Green LED
Q1, Q3, Q5, Q6, Q7 = 2N3904 (NPN)
Q2, Q4 = 2N3906 (PNP)
U1 = ELM327
U2 = MCP2551 or MCP2561
U3 = 7805 regulator (5V 1A)
U4 = 317L adjustable regulator (100 mA)
U5 = FTDI DBP-USB-D5-F usb module
Misc
X1 = 4.000MHz crystal
DB9M connector for OBD cable?
IC Socket = 28 pin 0.3" wide (or 2 x 14pin)
Figure 10. Parts List for Figure 9
Example Applications (continued)
sub-circuits of Figures 11 and 12.
Figure 11 shows a discrete RS232 interface, that
may be connected directly to the ELM327. This circuit
uses a resistor, diode, and capacitor between the two
RS232 signal lines to ‘steal’ power from the host
computer. In this way, the required RS232 negative
voltage is obtained without adding a complicated
power supply to the ELM327 circuit. The RS232
connections shown are for a standard 9 pin connector.
If you are using a 25 pin one, you will need to
compensate for the differences. This circuit works well
at baud rates of 57600 bps or less, but begins to show
some errors at rates of 115200 bps and above.
The circuit of Figure 12 offers another RS232
solution that works well at higher baud rates. It uses a
Maxim product (the MAX3222E) that is capable of
operating at up to a 250 kbps rate (be sure to visit
www.maximintegrated.com for more information).
The MAX3222E RS232 transceiver contains
internal charge pump circuitry that generates the
voltages required for RS232 communications, in
addition to the analog interface circuitry needed. All
you have to do is provide a few capacitors, and it does
ELM327
81 of 94ELM327DSJ Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
Figure 11. A Low Speed
RS232 Interface (≤ 57.6 kbps)
2 (RxD)
RS232
Interface
(DB9F)
+5V
+5V
3 (TxD)
5 (SG)
1 (DCD)
4 (DTR)
6 (DSR)
7 (RTS)
8 (CTS)
4.7KΩ
10KΩ
10KΩ
0.1µF
4.7KΩ
10KΩ
+5V
15
16
17
18
19
327
2N3904
2N3906