Logic Level Converter Only Half the Voltage on HV Side

I am trying to use a logic level converter that is wired up like the attached to use a 3.3V microcontroller to drive a 5V relay. For some reason the voltage on the HV side is only half of the source voltage. I can't seem to figure out why.

What do you mean by "source voltage" ?

The source voltage is 5V and on one of the HV channels when it is “high” I only measure about 2.5V.

My guess is your are trying to drive a relay with this level converter. I don't think the level converter is up to the job. It was designed for low level (aka current) signals.

To run the relay from the 3.3V device you could use a simple transistor with base resistor or get a logic level mosfet.

Look here for more information Driver relay with arduino

The source voltage is 5V and on one of the HV channels when it is "high" I only measure about 2.5V.

Sounds like you need the problematic HV channel as an input ... are you sure its not an output that's trying to drive the HV side low?
What is the HV device?
Looks like you have this product that was inpired by this technical reference.

These are the actual parts I am using:

Relay board:

Level Converter:

I don't think the level converter is driving the relay directly as the relay board requires it's own power connection.

You need to connect the GND to the converter!

The source voltage is 5V and on one of the HV channels when it is "high" I only measure about 2.5V.

If it's being sent by the LV side then the HV side would be the DESTINATION voltage, not source, since the
'source' is the LV side.
So are you referring to an LV signal being received as 2.5V on the output of the shifter ?

Can you post a link for the shifter ?

raschemmel:
If it's being sent by the LV side then the HV side would be the DESTINATION voltage, not source, since the
'source' is the LV side.
So are you referring to an LV signal being received as 2.5V on the output of the shifter ?

Can you post a link for the shifter ?

Here is a link to the shifter:

When I have 3.3V on the LV side of the shifter I only get about 2.5 on the HV side when the relay board is connected. If I don't have the relay board connected it is close to 5V however.

kahombur:
Here is a link to the shifter:
https://www.sparkfun.com/products/12009

I'll say it again:

johndg:
You need to connect the GND to the converter!

It won't work until you do.

When I have 3.3V on the LV side of the shifter I only get about 2.5 on the HV side when the relay board is connected. If I don't have the relay board connected it is close to 5V however.

Probably because it's floating without a common ground from the shifter (both sides) to the common GND
between the 3.3V side and the 5V side.
Level Shifter Schematic showing SEPARATE GNDS THAT MUST BE COMMON WITH BOTH SIDES

Connect the GND on the LV side and the HV GND to the 3.3V device GND and the 5V device GND
and THEN try it again and measure the voltage on the HV side.

The picture is not quite correct. The shifter does have a ground and I have the fround of the 5V and 3.3V sides all connected together.

kahombur:
The picture is not quite correct. The shifter does have a ground and I have the fround of the 5V and 3.3V sides all connected together.

Then please post an actual, correct schematic. One where all the connectors are properly labeled, and matching the actual boards. The one you posted is more than "not quite" correct... it isn't even close. Is there anything else we should know that you haven't told us?

Then please post an actual, correct schematic. Is there anything else we should know that you haven't told us?

Is it working now ?

aarg:
Then please post an actual, correct schematic. One where all the connectors are properly labeled, and matching the actual boards. The one you posted is more than "not quite" correct... it isn't even close. Is there anything else we should know that you haven't told us?

I just saw this link here ------- [u]click here[/u].

Southpark:
Also interesting in that the logic converter board in the original post isn't showing GND pins.

It's not even the same board that the OP linked to.

aarg:
It's not even the same board that the OP linked to.

aarg ...... I just saw the pololu site here ..... [u]click here[/u].

The OP should have provided a reference to it though, instead of us needing to search for information.

On page #2 of that link. Same board.

Let's see if the OP did what rasch mentioned ------ as in to connect the 'GND' pin of the device on the 3.3 volt side to the 'GND' pin of the device on the 5 volt side.

Below is a better sketch of how it is wired.

So?
Is it working that way?

JohnRob:
My guess is your are trying to drive a relay with this level converter. I don't think the level converter is up to the job. It was designed for low level (aka current) signals.
To run the relay from the 3.3V device you could use a simple transistor with base resistor or get a logic level mosfet.

This type of level converter is a strong "pull down" on the HV side, and a weak "pull up". So be sure the relay module is configured for active low ("grounding" the input provides current to the opto, and the relay turns on).