Simple 4-20ma Current loop RF TX RX

[Dangermike101]

Hello,
So i am looking to create a simple RF Transmitter and Receiver unit for a 4-20ma Current loop sensor.
I know bits and pieces, but the simpler the better! haha

I know i will need a RF transmittern and reciever
and ADC converter to change the Signal from 4-20ma to a 0.25 - 5vdc signal. and then another on the receiver end to change it back.

Would i need an Encoder chip in there as well or is there a way i just connect these 2 and it will work...

 

[danadak]

Google "esp8266 4 20 ma" & "esp32 4 20 ma"


Regards, Dana.

 

[rjenkinsgb]

You definitely need an MCU at each end, optionally including the radio link in the module.

The transmitter end does not need an ADC to convert current to voltage, just an appropriate value load resistor.

A 150 Ohm load would give a signal range from 0.6V at 4mA to 3V at 20mA, so a reasonable range for a 3.3V MCU with an internal ADC.

The receiver would need an analog voltage to current circuit, running from either filtered PWM or a DAC out on an MCU, depending what internal peripherals it has & what response speed and accuracy you need.

 

[crutschow]

That's not a trivial problem.

The question is how to best modulate the RF with the analog info.

A microprocessor with a built-in RF transceiver would seem to be the best way to do that, but that's not particularly simple.

A reasonably simple and moderately accurate analog way, would be to convert the signal to a variable low frequency, such as with the LMx31 IC voltage-to frequency / frequency-to voltage converter, and use that to FM modulate and demodulate the carrier.
With a two point calibration, you should be able to obtain 1% accuracy with such an approach.

What accuracy do you need, and how far do you need to transmit the signal?

 

[Pommie]

Be aware that most frequencies available have a maximum duty cycle of 10% or less.

Mike.

 

[crutschow]

Be aware that most frequencies available have a maximum duty cycle of 10% or less.

Mike.

Frequencies of what?

 

[Pommie]

Frequencies of what?

We appear to be talking about radio.

Mike.

 

[crutschow]

We appear to be talking about radio.

Mike.

It would appear so.
So what frequencies only allow short duty-cycles?

 

[Pommie]

I thought all the frequencies allowed for car remotes etc had such a restriction to stop one blocking another.

Mike.

 

[rjenkinsgb]

So what frequencies only allow short duty-cycles?

I believe he means "frequency bands".

The licence-exempt frequencies are often restricted to low on-air transmission duty cycles, as they are shared and one continuous transmission would block any other users nearby.

Regulations vary from place to place.
The EU/UK limits are given in pages 22-27 of this document:

https://www.etsi.org/deliver/etsi_en/300200_300299/30022002/03.02.01_60/en_30022002v030201p.pdf