ELM327
Example Applications (continued)
and the J1850 Bus+ circuitry, but the majority of the
rest has been eliminated. The J1850 voltage switching
circuitry has been reduced to a single 8V regulator as
well, since there will be no need to switch to 5V. Note
that pin 3 has been intentionally left open-circuited as
it is not required by the voltage regulator.
J1850 VPW only application (protocol 2) then you
should issue the command AT SP 2 the very first time
that the circuit is powered. From that point on, the
ELM327 will remain set for protocol 2, whether it fails
to make a connection or not.
The USB interface in Figure 11 uses a different
product than that of Figure 9, just to show how simple
it is to interface other products. This one is the
CP2102 which is produced by Silicon Laboratories
(www.silabs.com).
If using the CP2102, we do caution that it is very
small and difficult to solder by hand, so be prepared
for that. Also, if you provide protection on the data
lines with transient voltage suppressors (TVS’s), be
careful of which ones you choose, as some exhibit a
very large capacitance and will affect the transmission
of the USB data.
The circuits of Figures 9 and 11 will both power up
and begin communicating at 38400 bps. You will need
to set your terminal or OBD software to this same rate
in order to communicate with them. Should you want
to experiment with other rates, you will need to change
The first time that this circuit is used, it will likely be
set to protocol 0, which is the default ‘automatic
search’ setting when shipped from the factory. When
you connect it to a J1850 VPW vehicle, it will then
automatically detect the protocol, and if the memory is
enabled (it is as shown), J1850 VPW will then become
the new default, with no action required by you. This
will work well for most applications, but if the circuit is
used on a vehicle with the key off, for example, then it
will again go searching for a new protocol. In general,
you do not want this to happen every time (simply
because the search may take several seconds). It may
be only a minor inconvenience to have to wait while
the ELM327 determines that it is ‘UNABLE TO
CONNECT’, but why go through it if you do not have
to? If you know that you will be using the circuit in a
Semiconductors
Resistors (1/8W or greater, except as noted)
R22, R23 = 100 W
D1 = 1N4001
D2, D3, D5 = 1N4148
R3, R5, R27, R28, R29, R30, R31 = 470 W
R17, R19 = 510 W 1/2W
D4 = 1N5232B or SA5.0AG TVS
L1, L2, L3, L4 = Yellow LED
L5 = Green LED
R2, R4, R16, R18 = 2.2 KW
R6, R7, R14, R15, R24, R32 = 4.7 KW
R8, R9, R11, R13, R26 = 10 KW
R1, R10 = 22 KW
Q1, Q3, Q5, Q6, Q7 = 2N3904 (NPN)
Q2, Q4 = 2N3906 (PNP)
U1 = ELM327
R21 = 33KW
U2 = MCP2551
R12, R20, R25 = 47 KW
U3 = LP2950 (5V 100 mA regulator)
U4 = 317L (100 mA adjustable regulator)
U5 = FTDI DBP-USB-D5-F usb module
Capacitors (16V or greater, except as noted)
C3, C4 = 27pF
C8, C9 = 560pF 50V
C1, C2 = 0.1uF
Misc
X1 = 4.000MHz crystal
C6 = 0.1uF 50V
DB9M connector for OBD cable?
IC Socket = 28 pin 0.3" wide (or 2 x 14pin)
C5 = 2.2µF 50V
C7 = 10µF 10V
Figure 10. Parts List for Figure 9
ELM327DSI
Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
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