Is RF the right choice for my project?

[Robert Johnson]

Hi all.

My project is a simple one on paper, but im not sure on the best solution.

In a nutshell, i need a vehicle fitted with a form of transmitter that will send a signal to a stationary object as it moves past (to switch on a light).

The confines i have are:

It needs to activate the light when it comes in range by 10 metres.

Can an RF signal be tuned so i can do this? Or are there better alternatives?

Also needs to be low cost, would a simple RF transmitter/receiver set up be good for this project, or are their better solutions out there?

Thanks for reading and comments.

Rob

 

[Robert Johnson]

Hi, does anyone have an idea on this? Any advice would be very much appreciated.

Thanks.

 

[biff44]

This could easily be done with RF. Basically, RF signal strength dies out the further you get away from a transmitter. You could sense the signal strength of your transmitter, and when it got above a threshold level, you could trigger a comparitor to output a "1".

First, I would choose a frequency to run at. Depending on which country you are in, they differ, but many have "unlicensed" frequency bands that you can operate legally in if the transmit power is low enough. Many of these bands are designated as "ISM" bands.

Pick a frequency that is legal. Ones higher than 300 MHz are probably preferred, so that the antenna does not have to be too big.

I would set up a transmitter, probably at 1 mW of transmit power, and hook it to a an antenna.

I would take another antenna, connect it to an amplifier with perhaps 25 dB of gain, pass the amplifier output through a bandpass filter, and then perhaps another 10-20 dB of RF gain. Then I would drive a diode power detector. The diode power detector would turn the RF envelope into detected DC voltage. I would RC filter out much of the noise on the DC voltage, feed it into a comparator op amp, and set the threshold level so that it tripped when the received RF signal was strong enough (10 feet away).

If you chose 315 MHz, you could use a SAW filter, like Digikey part number 495-2328-2-ND. Digikey has RF amplifiers and antennas too.

Such a system might be susceptible to interference from other systems, so if you are worried about that, you can get more fancy, and transmit a specific address that the receiver is looking for.

You could allow the end user to access a potentiometer to set the threshold on the comparator. That way, the end user could adjust the sensitivity to adjust for varying room RF characteristics.

 

[Robert Johnson]

Hi Biff, thats fantastic thanks very much! Im quite new in this field so its great to have some pointers to go on. Will make a start and post back on the results.

Thanks again.