Category Archives: RENIX Interface

Current Loop Interface

This is a little out of sequence (rather, it’s a lot out of sequence), but it’s today’s project.

My Renault has a serial data output stream that provides telemetry from the engine’s state of operation. It operates at 62500 baud, and that rate across a piece of wire a few feet long is soon interrupted and corrupted by the RF generated by the ignition, alternator, various motors and relays and the like. I tried using twisted pair and the results were better but I still had corrupted data.
Somebody suggested to me to make a current-loop interface rather than reading voltage levels.

Why? It’s fairly easy to induce a voltage in a piece of wire- if I take my voltmeter and don’t connect the leads together or to ground, it’ll read about 0.3V AC. That’s just from the radio energy in the air around this house.
What’s more difficult is to induce a current, especially through a resistance. Therefore, if we use the flow of electricity to indicate a signal rather than the presence of electricity, we should be doing better.

So, enter the schematic.

Schematic, current loop interface TX, RX

This was pilfered from the Internet, from various places. I went ahead and took a trip to Radio Shack and picked up an LM324 quad op-amp and an LM724 op-amp to use for either end of the circuit. The transmitter takes two op-amps, the receiver one.


Threw it all onto breadboard. The transmitter on the left, the receiver on the right, connected by the coiled-up wire in the middle.
With a voltage applied, nothing happens. A small current is passing through the loop.

Lights off

Cut the voltage to the input and a larger current flows, switching on the comparator at the far end. Seeing as I had two spare op-amps inside the chip I decided to put an LED on the near end to show what’s what.

Lights on

It’s a fairly compact circuit but I’ll have to build it into a small box, I think.


I just need to get a P-channel FET now. They are fairly expensive and hard to come by locally. I have a PNP transistor in it’s place right now which means the circuit doesn’t shut off completely. Ultimately the transistor will work just fine but it’ll be better with the FET.

I need to put the circuit through its paces next and see what the highest frequency is that I can pass through.