Hi There,

I have a simple 4-20ma curcuit. Sensor - Cable - LCD display. I would like to remove the cable and use some type of wireless system to transmit the data from the sensor to the LCD. It must be dc powered - 1.5-9v at the sensor end and 1.5-18v at the LCD end. Range of about 30-50m. Does anyone have any ideas. I saw some of the phoenix stuff, but it looks to big and the voltage / current requirements are a worry.

If also at the sensor end there could be the abilty to switch on a relay froma signal generated at the LCD end, that would be great.

Thanks in advance.

Cheers, ben

 

I've been wondering recently if I could use a 'cheapish' 2.4 GHz ISM band AV sender / AV receiver to wirelessly transmit data in some way.

It's possible one could convert a 4-20mA signal to an equivalent frequency modulated audio range signal or perhaps to an equivalent PWM audio signal.

This modulated audio could be transmitted and demodulated at the receiver end back to the 4-20mA equivalent. The trick is working out the audio modulator / demodulator circuitry.

Most consumer electronics stores sell the AV sender units. One can also buy the Tx & Rx modules from electronic component suppliers.

 

putting the 4-20 signal into a precision resistor will give a voltage that could be converted by a voltage to freq circuit and that be used to modulate. then the reverse would be done at the received end. freq to voltage circuit.

 

Thanks very much guys for the info. I had a look at a small RF link or Infrared link which seemed like the most cost effective options. Would you be able to point me in the direction of a suitable Volatge-Freq converter please... I've seen some oout there, but I'm sure its a mine field out there.

 

Probably something like the LM331, AD537 & so on, could be used as the core component of the modulator & demodulator devices. Check the data sheets.

http://www.national.com/mpf/LM/LM331.html#Overview

http://www.analog.com/en/analog-to-...quency-converters/ad537/products/product.html

 

Before you get too far along in your design you should determine how much accuracy you need and whether you can achieve that accuracy with your design. In other words, how accurately will your output agree with your input? How much will the signal strength vary within your defined range and what happens to your data at minimum signal level? What will happen to your data when your battery begins to fail? Will you be able to detect defective data due to interference, weak signal strength or failing batteries, or will you accept any data you receive without question?

 

A further thought / question. Could I send a 1v pk-pk PWM equivalent of the 4-20mA signal via the composite video path of the AV sender / receiver? Let's say its a 5%-95% PWM, 1v pk-pk voltage [derived from a 4-20mA input] at a base frequency of 20kHz.

 

These are all great ideas guys!! Thank you very much. Keep it coming