RC circuit question

[si_rcproject]

Hi,

I'm a complete novice at electronics and have just started to build a remote control hovercraft.

I'm using a ready made circuit taken from another RC product and that's going to provide the main body of the circuit. It has three 2.4V motors that can all be controlled seperately, with two of them being able to reverse direction.

However, this circuit only operates at 2.4V and I'd like to use it to operate 3 11V motors in the same fashion that the 3 2.4V motors work at the moment.

I was thinking of using transistors but the motor needs to be able to switch direction, and I haven't been able to find any relays that turn on at a voltage as low at around 2V. (porbably because I don't know what I'm looking for).

What would be the simplist way to make this work? Component codes would be great because I can't decifer what I need at the moment.

Thanks for any help.

 

[ahydra]

Hi,

I'm a complete novice at electronics and have just started to build a remote control hovercraft.

I'm using a ready made circuit taken from another RC product and that's going to provide the main body of the circuit. It has three 2.4V motors that can all be controlled seperately, with two of them being able to reverse direction.

However, this circuit only operates at 2.4V and I'd like to use it to operate 3 11V motors in the same fashion that the 3 2.4V motors work at the moment.

I was thinking of using transistors but the motor needs to be able to switch direction, and I haven't been able to find any relays that turn on at a voltage as low at around 2V. (porbably because I don't know what I'm looking for).

What would be the simplist way to make this work? Component codes would be great because I can't decifer what I need at the moment.

Thanks for any help.

Not sure what to recommend here - most devices don't turn on at voltages as low as 2.4V.

I'm assuming since you mentioned relays you're only looking for on/off, not analogue voltage control. If this is the case I have a crazy idea involving using an opamp as a comparator. Supply it with a voltage >1.25x what you need to switch on a relay (since you're using motors, you will need relays to provide enough current), then wire up a 2.0V supply or similar to the inverting input and the circuit output to the non-inverting input:

                |\
           +2V--|-\
                |  -->|-- to relay
circuit output--|+/
                |/

>| = diode (1N4148 or similar)

The diode prevents the voltage to the relay going below zero. This circuit will turn the relay on when the "circuit output" voltage rises above 2V and you can connect your motor to the other side of the relay.

Please note that I'm not very wise in the ways of opamps and relays so this may well not work , other people's input would be useful.

ahydra

 

[si_rcproject]

That sounds like a good idea! Would you be able to post a quick sketch of what the complete circuit should look like please?
I've tried, but I'm fairly clueless at the moment :S
Once I know how it all fits together I should be able to test it on my computer to make sure it works.

Thanks

 

[si_rcproject]

I've drawn up how I think the circuit you suggested should go. I'm not at all sure what I'm doing though does this seem right?

 

[ahydra]

I don't think that's quite right. I'm not sure if you've got your opamp power supply wired up backwards (there's a + on the bit connected to 0V). Anyway the rest of the opamp is wired incorrectly. I assume the SPST2 switch is just to represent the RC circuit input. If so then you need to wire this to the + input of the opamp as in my diagram above. You need a separate 2V supply wired to the - input. (in practice you can probably do this using 2 batteries (3V) and a potential divider)

Make sure you are using a 5V relay (the voltage will be about 6V when it's turned on, so anything much higher than 5V won't work)

The last thing is, I'm not sure if the motor will pull so much power that it will affect the opamp's operation. You may have to power the opamp from a separate supply if it turns out the voltage drop is too high.

Hope that helps, sorry I didn't get back to you with a diagram but actually it was useful to see the kind of setup you have.

ahydra

 

[audioguru]

the 2V reference for the opamp can be made with a two-resistors voltage divider from the 7.4V battery.

Why use an opamp? Use a transistor with a series resistor to its base instead. When the 2.4V occurs then the transistor turns on and turns on the motor or an h-bridge that drives the motor instead of a relay. The transistor can drive the relay if you want.

 

[ahydra]

the 2V reference for the opamp can be made with a two-resistors voltage divider from the 7.4V battery.

Why use an opamp? Use a transistor with a series resistor to its base instead. When the 2.4V occurs then the transistor turns on and turns on the motor or an h-bridge that drives the motor instead of a relay. The transistor can drive the relay if you want.

Sounds like a plan. I wasn't aware transistors turned on at voltages that low, but if they do then that's a much better idea.

ahydra

 

[audioguru]

The base voltage of a transistor is from 0.4V to 1.5V depending on the current for them to turn on. The required base current might be pretty high.

 

[si_rcproject]

Thanks a lot for the input.

I've attached a rough drawing to show the setup I'm looking for. One of the requirements I need is for the voltage direction to be able to reverse (to reverse the motor's direction), would this be possible with transistors?

I've also attached a photo of the circuit and the small motors I obtained from a remote controlled rat of all things! I believe that this circuit is only able to run off a max of 2.4V, which is not enough considering I need 7.4V to run the motors. I basically need a method of amplifying the 2.4V signal that the RC circuit sends out to 7.4V, as well as being able to switch this amplifyed voltage circuit when needed.

And, because it's a remote controlled hovercraft, it has to be as light as possible.

So would a transistor switch be able to work in this senario?

 

[audioguru]

Series resistors do not limit voltage, they limit current. The current is not constant so with resistors limiting the current then the voltage will vary all over the place. You need a 2.4V voltage regulator.

Look in Google for H-bridge Circuit. It allows a motor's speed and direction to be controlled.

 

[si_rcproject]

Thanks audioguru,

The original circuit (from the rc rat) already reverses the voltage polarity when needed, so would this circuit operate correctly if the input voltage was switching?

Thanks