OBDII Breakout Box With LED Indicator

Thread Starter

s200bym

Joined Aug 9, 2017
76
Hi All,

I'm going to build an OBDII breakout box but I want to add 3v 20mA LED's to show what pins are active. Would I need to do a circuit like the one in the picture?

Cheers,
Mike.
Data LED.jpg
 

dl324

Joined Mar 30, 2015
10,462
Just because many LEDs are spec'ed for 20mA continuous, that doesn't mean you need to operate them at that current when they're used as indicators. A few mA may be sufficient.

What is the current sourcing capability of DATA without signal level being degraded to the point of being useless? How much capacitance can they drive?

Why not opt for something that doesn't load signals? Using comparators or CMOS buffers to drive the LEDs will have less impact on the circuit.
 

Thread Starter

s200bym

Joined Aug 9, 2017
76
Just because many LEDs are spec'ed for 20mA continuous, that doesn't mean you need to operate them at that current when they're used as indicators. A few mA may be sufficient.

What is the current sourcing capability of DATA without signal level being degraded to the point of being useless? How much capacitance can they drive?

Why not opt for something that doesn't load signals? Using comparators or CMOS buffers to drive the LEDs will have less impact on the circuit.
@dl324 Thank you for your reply.

I had a search on google and found a schematic using the LM358D which I have copied on Eagle. Is this what you were referring to?

Cheers,
Mike.
Can Bus 2.jpg
 

Thread Starter

s200bym

Joined Aug 9, 2017
76
I was looking at that and may consider it, but I want the fun of making it :) I was thinking of adding grounds for all the pins.
 

dl324

Joined Mar 30, 2015
10,462
Is this what you were referring to?
An opamp can be used as a comparator, but it's not usually recommended.

This is what I had in mind:
upload_2019-1-16_14-0-9.png
Opamp supply connections and decoupling omitted for clarity.

R3 and R4 determine the maximum voltage for a LOW signal level. Whenever the inverting inputs to the comparators are above that voltage, the corresponding LED will be turned on.

Most comparators don't sink a lot of current. IIRC, LM393 are guaranteed to sink around 6mA, but typical is 15mA.

I drew the schematic in Eagle. I printed to BW PDF, because many are of the feeling that color coded schematics aren't useful.

I'm not a fan of using IC pin order symbols in schematics because they make for unreadable circuits.
 

Hymie

Joined Mar 30, 2018
797
My knowledge of OBDII signals is close to zero, but given the bus is carrying digital signals (at reasonable switching speeds) the usefulness of the LEDs must be very limited – cheaper versions of breakout boxes are available without the LEDs.
 

wayneh

Joined Sep 9, 2010
16,384
My OBD II tool was only about $10 and enables my phone to access it over WiFi. I wouldn’t dream of trying to build one. Put the energy into something new.
 

Thread Starter

s200bym

Joined Aug 9, 2017
76
It’s not a dongle type elm tool. It’s a breakout box so you can easily measure each pin with an oscilloscope, DMM, etc.

@dl324 I will have a play around with your circuit thanks.

Mike.
 
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