# Looking for an RC Transmitter & Receiver

#### xxJaRxx

Oct 16, 2011
34
Sorry Dave, I did read your post about transmitting voltages and I did understand it, what I said earlier about transmitting 4.5V, I didn't mean exactly 4.5V.. Still not sure I've grasped the concept on how/what data gets sent though.

I thought that if I connected the input to the transmitter to ground, the output from the receiver would be very low e.g. <0.6V. Similarly if I connected the input to the transmitter to Vcc (4.5V) I would get a higher output from the receiver e.g. > 3.6V.

This is the transmitter I used along with the data sheet:

This is the receiver I used along with the data sheet:

Btw, what you said about a string of pulses, that makes a lot of sense to me and I plan to implement something similar to that once I understand how the trans/rec works!

I did actually draw a quick circuit diagram last night, then realised I couldn't upload it straight off my hard drive!

#### BobK

Jan 5, 2010
7,682
Right so I've got my components, I've wired up the transmitter (correctly) and the receiver (correctly) and wired a momentry push button to the input of the transmitter, so that when pressed, it inputs Vcc (4.5V) into the transmitter. I have also wired an LED to the output of the receiver.

I figured that if I pressed the switch, 4.5V would be transmitted to the receiver and the LED would turn on (and off when not pressed).

This doesn't happen! The LED is just constantly on and I can't seem to logically figure out why! I don't yet have a multi meter as it's in the post so can't look at different voltage levels..

Anyone have any advice on what could be wrong? Or are my assumptions on how the system should work wrong?
Can you give us a link to the transmitter and receiver you got?

Bob

#### xxJaRxx

Oct 16, 2011
34
Hi Bob, I have the links in the above post

#### BobK

Jan 5, 2010
7,682
Sorry, I did not see the links.

These modules are intended to transmit serial data, and they list the min baud rate at 50. So it is not possible to just send a steady state. The output will be high when there is no data coming in, so that is probably what you are seeing.

Try putting an audio frequecy square wave on the input pin then connect the output pin to a speaker through a 100 Ohm resistor and see if you hear the tone on the speaker.

If you look at the circuits in the datasheets, they include an encoder and decoder chip that will give you 4 channels of binary control signals. Isn't this what you want?

Bob

#### xxJaRxx

Oct 16, 2011
34
Right so, I need to send a square wave through? Of a high enough frequency?

I have got a small speaker here, so I'll try inputing a varying frequency square wave to the transmitter and I should get a varying tone on the speaker I guess. And if I turn the frequency down too low it will cut off (because of the min baud rate)?

I'll try that later today, thanks for the info Bob, really useful! Received my multi-meter today as well so good news there!

#### davenn

Moderator
Sep 5, 2009
14,125
I did actually draw a quick circuit diagram last night, then realised I couldn't upload it straight off my hard drive!

Yes you can thats how its normally done just make sure you resize the pic appropriately ie. a max of 800 x 600, preferably a little smaller. And also compress it a bit that is .... it may have been a 500 kB or more originally, with a bit of compression get it down below 100kb

when you are doing a post, you will see 2 buttons below the text box "Quick Reply"
and " Go Advanced" click on the "Go Advanced" button and a bit below that text box there is a "Manage Attachments" button from there you can upload files from your harddrive

cheers
Dave

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#### new_hope

Oct 22, 2011
18
You can buy an RF transmitter and receiver from sparkfun. The pair will run you about 9$for both and you dont need multi channel RF links if you are sending byte or binary commands. But you will need a microncroller to send the commands and receive the commands. You can use avr. You will need two. I recommend the ATmega328 as it is only 4$. You will need two.

Unfortunately you cannot just buy a car and think you are going to use all the parts to make your own. The MCU on the car has been programmed with firmware (a hex file) that is only compatible with the transmitter. You could not customize anything. You would be better off learning to program an MCU such as the avr (its programmers are cheaper and it has built in ADC and PMW)

you use PMW (pulse width modulation) to control motors of all types. ADC is just analog to digital conversion. an avr programmer can run you 34$plus shipping from amtel. Or you could use the teensy 16$ and then also reuse it as programmable HID (human interface device) and mass storge device....which will essintially negate your need to program the serial port or use a max232 circuit. You could make a joystick to control your car from your computer using the teensy.

From there you can hook up your transmitter to your computer via rs232 using a max232 converter chip to convert the 3-25V logic to TTL (0 and 5V) logic (which is negated if you have the teensy that i mentioned earlier).

Alternitavely you could purchase a gsm module and breakout board for 70$and skip the RF links altogether. Then you could control your car via gsm pretty much anywhere in the world. sadly if you want to make your own car you will have to learn to program mcu's (what im currently doing....see my post regarding my gsm module). And if you want to control it from your computer or the internet at large you will have to learn how to program in c++. you will also need to know hex and binary. If you have any other questions just post them. I'll be glad to help you out as im doing much the same thing and many of the principles and techniques that are applied to one project can be implemented rather easily in another project. I know a thing or two...not much though using one mcu you could control all the motors. If your motors on your rc car require more then 5v...say they are 1HP...then you will need a mosfet (also available at sparkfun.com for less then 1$). This will allow you to control the larger power of the motor for your autonomous car (thats if you decide to make an autonomous car).

The idea is you are sending bytes continously to receiver. Receiver sends them to MCU and mcu interrupts program flow to execute command whenever byte has been received.

you could have single bytes represent commands so when the mcu receives them it will go through a swtich statement. This will go through all the cases and compare it to the byte received. if you want your code to go foward to be "0b00000000" (this is just 0 in binary) then you would send it out. Then you would have your receiving mcu interpret it. I can provide you with code to control an entire car for the avr....But if i do any chance you would be willing to donate a couple dollars (5 dollars to be precise) to my paypal through a website?

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#### xxJaRxx

Oct 16, 2011
34
Wow, a lot of really useful information there! Thanks! I'm sure I'll have loads more questions as I'm going along but for starters, you mentioned;

You can buy an RF transmitter and receiver from sparkfun. The pair will run you about 9\$ for both and you dont need multi channel RF links if you are sending byte or binary commands. But you will need a microncroller to send the commands and receive the commands.

Does this mean that I don't have to use PWM? If so can you post a link to one so I can check it out? Does it need to be interfaced with serially?

I am using two PIC microcontrollers, the 16F648A, and a PICKIT3 debugger, unfortunately I need a header to interface between the two so I'm going to order another set of micros instead which don't require any additional hardware to programme!

#### new_hope

Oct 22, 2011
18
Wow, a lot of really useful information there! Thanks! I'm sure I'll have loads more questions as I'm going along but for starters, you mentioned;

Does this mean that I don't have to use PWM? If so can you post a link to one so I can check it out? Does it need to be interfaced with serially?

I am using two PIC microcontrollers, the 16F648A, and a PICKIT3 debugger, unfortunately I need a header to interface between the two so I'm going to order another set of micros instead which don't require any additional hardware to programme!

You interface them serially. And conviently enough the links in the product description on sparkfun have links to how to wire them up schematically (but for an AVR). You still have to use PMW. I'm not sure what i posted that made you think otherwise. PMW is just ouputting high logic on one of the GPIO (general purpose input output pins).

Here is an RF transmitter.
http://www.sparkfun.com/products/10535

Its 315Mhz but its the only one they had in stock at the time i am making this post.

here is an RF reciever.
http://www.sparkfun.com/products/10533

It is also 325 MHz so it is compatible with the transmitter.

I dont know the PIC microcontroller architecture yet so i couldn't tell you how to program it to do what you want.

Here is the article that shows how to wire the RF transmitter and reciever up for the AVR.
http://winavr.scienceprog.com/examp...433-rf-modules-with-avr-microcontrollers.html

I currently know little about the PIC family.

As for the code all you do is write to the serial port and whatever you write will be sent. On the recieving end whatever is received will be sent to serial pin and cause interrupt. With the avr you can change the number of bits that are sent from 7 bits up to 10. You could also send more bytes to represent values.

For example. You send a byte that will tell the receiving mcu to get ready to go foward. Then you send another byte representing an integer speed value. Or for turning you send another byte representing the angle for the wheels to be turned to. I can put up some code because i read through how to program the serial port of the avr and it wasn't confusing me and i didn't have questions. But you would have to be able to program an avr.

#### new_hope

Oct 22, 2011
18
did you typo ?

315 and 325 not the same

Dave

lol i musta hit the two instead of the one. They are the same when you click the links. Also sorry for the late response in getting back to you.

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