ELM634
Example Applications (continued)
78L05
Battery
Positive
0.01µF
0.1µF
470Ω
+5V
‘Power On’
LED
+5V
22KΩ
Notes: - the PNP transistor is
a 2N3906 or similar
- Diodes are 1N4148,
1N914, 1N4001, etc.
LIN Data
10KΩ
RS232
Interface
3 (RxD)
1
20
19
18
17
16
Signal
Ground
2
4.7KΩ
3
8
7
6
5
4
5
LIN
Interface
634
+5V
1
2
3
4
+5V
6
7
8
15
14
13
12
11
7 (SG)
2 (TxD)
3.58MHz
9
10
27pF
27pF
Figure 2. LIN to RS232 Interface
regulator shown, and filtered with the capacitors. An
LED provides visual feedback to show that the supply
is available.
The PNP transistor that is shown connected to the
LIN input (pin 7) serves mainly to level-shift the input
signal, and to invert it. A typical CMOS input will switch
state at about half the supply level (2.5V), which might
cause problems with noise in an automotive
environment. By providing a transistor buffer, the input
threshold is effectively raised to about 4V, increasing
the noise immunity while adding amplification, signal
clamping, and inversion. Note the use of the diode on
ELM634DSB
the input. This prevents possibly damaging backfeeds
from entering the circuit, protecting the transistor and
the other components.
Circuit timing is maintained by the crystal shown
connected between pins 2 and 3. It is a common TV
type that can be easily and inexpensively obtained.
The 27pF crystal loading capacitors shown are only
typical, so you may have to select other values
depending on what is specified for the crystal you
choose.
The circuit of Figure 3 provides an alternative to
the previous one. It is just as functional and does not
9 of 11
Elm Electronics – Circuits for the Hobbyist
ELM [ ELM ELECTRONICS ]
