To build a Voltage Sensor

[beirnes]

Please excuse my electrical handicaps
I need to build a small and inexpensive voltage sensor, that will be able to detect the presence of a preferably low alternating voltage in a wire. The sensor will be mounted approximately 1.5 inches from the wire. The sensor will be mounted on a moving object and acting as a line follower.
The output from the sensor will only need to be determinable as on or off.
I have been told that I should be able to do this using "an induction coil and a couple of transistors" This is more than likely true, but I wouldn't know where to start.

Many thanks in advance to anybody who can help me.

 

[buzzp]

I am not sure about the moving object part, but you can use a current transformer to measure the current through the wire to determine if it is on or not. I would use a standard CT, for example 50:5 means 50 amps through the wire will give you 5 amps out of the secondary. You could place a resistor divider across this to change the current to voltage. This voltage could be used to turn on a transistor when it sees current, thereby turning on a relay or LED. Or just use the LED and skip the transistor. Good luck.

 

[Lewish]

Your 1.5 inch standoff from the wire is going to make this quite difficult. Particularly so if it is low voltage. Otherwise, you could use a hall-effect current sensor.

 

[GLANGWELL]

The difficulty level in doing what you propose is dependant on quite a few factors such as:
1. frequency of the AC current, or signal
2. the amplitude (or voltage level) and current of the signal
3. whether or not the cable carrying it is sheilded
(these are just some factors - not all)
The next, and perhaps biggest hurdle, will be to design enough filtering into the design, in order to reject radio frequency interference, and perhaps even common power line interference, depending on the above factors & more.
It could be a bigger job than you expect because of the above factors. Doing it with "a few transisitors" unfortunately does not take account of real-world conditions, where all sorts of interference to low level signals exists. The gap of 1.5 inches, and a "low alternating voltage in a wire" is what makes this look like a low signal will be involved.
Not encouraging news, but hope it helps. Better to learn of
possible design problems early.
Hope that helps.
GL

 

[CarlPugh]

If I understand your question correctly, you want to follow a wire using a voltage applied to the wire. Hall effect devices and current transformers both require a current flow in the wire and will not work if there is no current flow.
Depending on the setup, voltage and frequency, it may be possible to use a voltmeter to detect the voltage. If a voltmeter is unable to detect the wire, then an amplifier of some sort and or a filter should be used.
Good Luck
Carl

 

[ve7brz]

I agree with Carl that you'll require a complete loop in order to use a sensor such as a Hall Effect sensor to detect the magnetic field created by the current flow through the wire.

On the other hand in your initial post you mentioned an induction coil, so I suspect the approach intended was to use a radio receiver to detect an RF signal on the wire. This would be quite feasable. With an antenna about 1.5 in. from the wire the signal could be very weak indeed. You would have to be certain that you weren't creating RF interference as defined by Code.

This method would be quite easy to implement as you only need to monitor the signal strength at the receiver. The simplest system would use one reciever and have the 'tracker' 'hunt' or search from side to side as it travelled, seeking the strongest signal. A more sophisticated approach would use two antennas and receivers to keep the signal centered between them.

A commercial system which uses a closed loop transmitting antenna and a radio receiver is the 'invisible fence' used for pet containment. In these systems the transmitting antenna is buried around the periphery of your property and your dog wears the receiver on his collar. As he approaches the boundary the collar warns him to turn back.

I hope these give you some ideas.

 

[ve7brz]

Further to the above, RF signal tracking is common for this type of application. At the power level you would use (a few µW) you probably wouldn't be violating any rules regarding spurious or nuisance emissions regardless of the band you were operating in. This can be done with an induction coil, and all you need for a receiver is a few in. of wire and a gemanium diode.

However, I would suggest using tuned circuits for immunity to outside 'interference'. I'm not sure of the legalities involved but if you were to 'transmit' a 600 kHz (sine wave) carrier at a few µW you could use a $5 crystal radio as your receiver.

From there you buffer the (rectified) signal with a voltage follower and measure it with a ADC and send the digitized information to your µProcessor. Alternately you can process the signal via a Voltage to Frequency converter and monitor the frequency change. If you wanted to you could control your tracking with digital logic without the need to use a µP, although that would be simplest.

If your uncertain as to the sensitivity of such a system, bear in mind that signal strength varies as the inverse of the square of the distance. What this means here is that the signal you receive at 3 in. is one quarter the strength of that at 1.5 in..

 

[peebee]

Why don't you buy one instead of trying to make it?