Hi

I have a 4 channel relay box that uses an RF signal (I think) from a key fob to activate circuits. http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=250650687475&ssPageName=STRK:MEWNX:IT

I'm using it to provide a remote "kill" switch for my child's electric car, so that when she's about to crash or do something daft I can kill the power. The box is all connected up and functioning in that respect.

When the car is not moving the key fob can activate the relays from about 35ft. However when the car is moving the range drops to less than 10ft.

The receiving Aeriel is basically a length of insulated wire wound into a short coil inside the plastic box housing the circuit.

Any ideas what causes the reduction in range of the signal? Could it be some sort of interference from the drive motors?

What "simple" things (if any) could I do to improve it? If I straightened the coil aerial and fed it outside the housing would that make a difference?

Thanks for your help.

Nick.

 

Hello,

As far as I can see the circuit works on 315 MHz.
There is a power limit to use it on that frequency.
Straightning the antenna will help to increase the sensitivity.
You could also connect a coax-cable (the shield should go to ground) at the antenna-position
and move the antenna to an other place for better reception.

Bertus

 

Hi Bertus,

Thanks for the response.

Replacing the wire antenna is going to be more difficult that I thought as the daughter board is soldered in place (initially I thought it was a push on connector).

Any idea how sensitive the components are to heat from a soldering iron?

For now I'll try straightening the antenna and making a hole in the case to move it outside and see what effect that has.

Thanks.

Nick.

 

Most components will be able to withstand the rigours of soldering and desoldering if the right equipment and time. DO NOT uses 80W plumbers soldering irons. 20W would be the biggest I uses and temperature controlled.

With regards to you statement about the range dropping from 35ft to 10 ft when it is moving I would put that down to interference from the electric motor. Not a lot you can do about that without some time and money.

PS. Not wishing harm on any child but it sounds like you are trying to deprive the would of a "Funny home video".

 

How much range do you need? Check the motor(s) and see whether they have noise suppressing capacitors installed. These will be little capacitors, probably ceramic disk type, from the motor leads to ground at the motor. The idea is that they reduce the RF from arcing of the brushes.

Older electric models had the same problem. That is, those models that used brushed motors. If you have any friends who fly electric model airplanes or have electric cars, they can show you what they look like. Modern brushless motors don't have them.

John

 

Thanks for the replies.

When stationary, straightening the antenna etc, increases the range to about 50m. I'll have to test with the motors under load later once the weather clears up.

My soldering iron is a basic 30w one, and my soldering skills are sometimes best described as interesting!

Its not about depriving funny home videos's we have plenty of those , but some of the kids that have a go deliberately try to crash into concrete posts and other such stunts (long story and yes they get told off!). Also when its capable of doing 5+mph its far too much work to keep up

I have come across a car aerial in the garage which I could mount; would that offer any opportunities; presumably the coax that Bertus talked about could be connected.

Once again thanks for putting up with my naivety on this.

Nick.

 

Hi

There are no capacitors on the motors; unless they are inside the housing. However there are some plastic blocks on each of the motor wires which presumably containing something that is supposed to do a similar job.

Another observation from this afternoon is that if the vehicle is moving away from me, the switch works over a shorter distance than if the vehicle is coming towards me; presumably because the signal has to pass through the interference field.....

@windoze killa

This is presumably what you like to see

Nick.

 

What a cuttie!

Find some 0.1 uF ceramic disk capacitors (or 0.01 if you can't find 0.1). Connect one lead to a motor terminal and the other to the motor case. Sometimes, you can solder to the case; otherwise just use a mounting screw. Connect a capacitor to each terminal. It might help.

John

 

Hi


@windoze killa

This is presumably what you like to see

Nick.

Cute.

I reckon you could make a real big improvement to that car.......

Blades on the bottom......

That yard is a jungle

Now seriously. If you could put some small capacitors as the others suggested would be a good idea. Nip down to Maplins, they should have something.

Where is the receiver mounted with regards to the motor and the antenna?

 

Hi

Thats not actually my yard Wish it was though. Its a large area of open space for common use which makes a great jeep driving area.

The receiver box is mounted just inside the bonnet/hood at the front of the jeep. The antenna is about 20cm long and comes out of the receiver box. The antenna looks to be a piece of single core insulated wire (very thin).

Bit confused on the capacitors as the previous thread suggests connecting between a terminal and the motor case and then connect to each terminal. So does that mean two capacitors for each motor; one across the two terminals and the other between a terminal and the case?

Nick.

PS. I have a list of "modifications" to do over time all for my daughters benefit of course.

 

Re: previous thread. That was talking about using a voltage regulator to get 4.5V for her music box, if I found the right one. The capacitors in that case were for a different reason.

In the case of the motor, arcing at the brushes produces RF energy -- just like the old arc transmitters that were used by Marconi et al. It is broad spectrum and may swap the receiver, thus reducing range.

The purpose of the capacitors is to shunt that RF to ground as close to its source (i.e, the motor) as possible. As you know, capacitors block DC, but let the effects of AC pass. The resistance to AC is called reactance and is expressed in the same units as resistance, namely ohms. It is frequency dependent. With DC, it is like an open circuit. For a 0.1 uF capacitor at 1000 Hz, it is 1600 ohm; at 20 KHz, it is 80 ohm; and at 1 MHz, it is 1.6 ohm. It is obvious how an RF frequency in the MHz range would be shorted to ground.

Now, your electric car has a little twist. I suspect the motors are reversible and have some sort of speed controller associated with them. (If not, don't worry.) The speed controller probably works by PWM. That is, it converts the battery power to a type of AC, which is easily controlled. The frequency is not 60 Hz, but is something higher. We don't know how much higher it is, though. If it is operating around 1000 HZ or so, the reactance of a 0.1 uF capacitor is so high, there is little to worry about. But, some controllers work at much higher frequencies, such as 20 KHz. 80 ohm reactance will result in about 150 mA at 12V, which is about 0.27 watt. That is enough for each capacitor to get a little warm.

Does the motor/controller make a real noticeable squeal when it operates? If so, the the frequency may be at the lower end of the range (maybe 2400 to 3600 Hz), and the the reactance of a 0.1 uF capacitor is quite high. However, if the controller is quiet, then it may be operating above our hearing range, and the lower reactance could lead to heating. I like empirical science, so I would just try it and see what happens. If they get too hot, then switch to 0.047 uF (169 ohm at 20KHz) or 0.01 uF (800 ohm at 20 KHz).

As for where to connect them, remember, the purpose is to shunt RF produced by the brushes to ground. I would put one from each motor terminal to the motor case. Putting one between the terminals will also work, but since the case is a good conductor and each terminal is connected to the case, putting a capacitor between the terminals and form each terminal to the case is a bit redundant. The case may be independently grounded to the battery. In brief, I would use one capacitor between the terminals or one capacitor from each terminal to the case, but not both arrangements.

John

 

Hi,

Yes you got it right on the previous thread stuff.

The car is pretty simple electrically. 12v battery, and two motors; which do run in reverse. The car has two speeds so no controller and the two speeds are obtained by changing the motors to running in serial or parallel. All the switching is managed by DPDT switches (speed and reverse) and SPDT (start/stop).

Thanks for the background on capacitors, I wasn't aware of the purpose; although I have noticed capacitors on motor in lots of different devices.

I'll pick some up this week and put them in at the weekend.

Thanks.

Nick.

 

Have a look at the motors. If one of the terminals is directly connected to the case of the motor then you will only need one capacitor for the terminal that isn't. But i am guessing that neither is connected to the case so you would need to put one on each terminal of each motor. You can solder them to the case or attach them to a screw that is holding them in so long as it makes contact with the case.

I would also run a reasonably thick wire from the cases of the motors to the ground side of the receiver.

Depending on how it is wired (Schematic would be nice, any chance of sketching one up) you could also connect it to the negative of the battery.

 

Have a look at the motors. If one of the terminals is directly connected to the case of the motor then you will only need one capacitor for the terminal that isn't. But i am guessing that neither is connected to the case so you would need to put one on each terminal of each motor. You can solder them to the case or attach them to a screw that is holding them in so long as it makes contact with the case.

I would also run a reasonably thick wire from the cases of the motors to the ground side of the receiver.

I am sure you mean that advice about the wire from the motor case to the receiver ground (or battery negative) only if one of the leads is not connected to the case of the motor. I concur that that connection is unlikely in this situation, but it is worth checking out before potentially causing a dead short.

John

 

Hi

I finally got round to putting in the capacitors for the motor terminals and the casings. Its made a reasonable distance (25m) and is certainly something I can live with until I increase the battery voltage

Many thanks for all your help in cracking this.

Attached is a jpg of the wiring as it is today. It will probably change as I get more "ideas" for enhancing the driving experience for my daughter

Nick.

 

Well done. Glad we could help.

Hi

Attached is a jpg of the wiring as it is today. It will probably change as I get more "ideas" for enhancing the driving experience for my daughter

Nick.

V8 would be a good option for the next mod.

 

Hi

More like V18....

Once the current battery dies; it will be 12v + 6v via turbo switch

I'm sure these cars are designed more for dads to have fun that the kids

Nick.

 

Surely the next project should be full remote control. I can imagine that being funny.

 

Hi

<snipped>

Another observation from this afternoon is that if the vehicle is moving away from me, the switch works over a shorter distance than if the vehicle is coming towards me; presumably because the signal has to pass through the interference field.....

<snipped>

Nick.

It's probably just that the driver and any metal parts in the vehicle are causing RF "shadowing" of the antenna, when they are between your transmitter and the antenna. So the received signal level would be lower, which would cut the range by itself. But, in your case, it also allows the interference-to-signal ratio to be higher, possibly making it even worse.

You might also want to make sure that the receiver board has enough bypass capacitance across the DC input terminals. Adding some might help. But you'd probably have to experiment with the values. Some small ones to try to drain away high frequencies might help. Adding an actual RF filter might help, if you could tolerate a small resistor or inductor in series with the DC supply. Do you have an oscilloscope capable of looking at 315 MHz?

I might also help if you shielded the receiver board by putting it inside a grounded metal box perhaps (with the antenna outside the box). But killing the interfering RF at its source is usually best. So, as others have suggested, I'd try various small-ish capacitances across the power leads of each motor (and maybe elsewhere also, such as at the battery). You could also try either shielding or tightly twisting together (or both) each pair of conductors, everywhere.