# FM transmission and range limiting control

#### Jacob J

Joined Jun 18, 2009
159
Hello

I want to make a device wich can do the following:

I have two DC motors in a modelship. I want to make a little boks, wich I can place in the middel of a pool or something similar, and I want my boat to be sailing within an radius at 20 feet or so (maybe if I can work something with a variable radius out, that could be cool). But basicly, when I turn on my modelship and my center-device, the boat sails away and when it gets out of the radius of 20 feet it makes a turn so it points towards my center-device. It sails now towards my center-device and slides past it, when it hits the radius limit it turns, and the whole thing begins over again.

Now the idea I got, but the knowlegde to make a reciever and a transmitter, wich can do this is out of my league, so I ask you guys for help.

I want to have the motor that drives the boat forward at constant speed, so in fact its only the second motor I want to control by this center-device.

What do I need of circuits for this? I guess I will be needing some sort of radio/FM transmitter and reciever. I guess I will be needing some sort of range limiting circuit too, or how will the boat figure out when it is in or out of my 20 feet radius?

Hope you can help me like you did last time with the PWM-module, wich is up and running and attached to my modelship.

/Jacob J

#### Jacob J

Joined Jun 18, 2009
159
Do I maybe have to use the Inverse Square Law? If I measure the voltage my FM transmitter sends out at the antenna and then use the Inverse Square Law to deturment the voltage I will have at a specified distance, then I can maybe build some trigger circuit, that starts my second motor and puts it back in neutral, when the voltage is at it highest again?

#### Jacob J

Joined Jun 18, 2009
159
Doesnt anyone have a clue how to make this?

#### Jacob J

Joined Jun 18, 2009
159
I got an idea. Lets say I build a FM transmitter like this one: http://www.geocities.com/tomzi.geo/1-transistor/1-transistor.htm

Then I take the microphone component out of the circuit. Then the transmitter will send out a straight signal, right? Then I have a receiver in the other end and it receives the constant signal, holding a switch together on my servo-motor for my steering. The steering is connected odd, and by that I mean, that the motor will be on and holding the boat in a straight forward possition. As the boat comes out of the transmitters range, there will be no signal and the switch to the motor will go off, and the servo-motor turns off, making the boats odd steering turn, and the boat turns. When it gets in the range of the signal again, the servo-motor turns on again, and the boat sails straight forward.

What do you think and do you have a nice schematics for an receiver and can I remove the microphone part in the circuit above and then get a constant ON signal send out?

#### Audioguru

Joined Dec 20, 2007
11,251
The single transistor FM transmitter circuit is garbage. It says so.
Its frequency changes if something gets near its antenna or moves away because the antenna is directly connected to its oscillator's tuned circuit.
The frequency also changes as the battery runs down because the circuit is missing a voltage regulator.

An AM radio cares about signal strength because it works with Amplitude, an FM radio does not care about signal strength. But you can add a signal strength indicator circuit to an FM radio if you want.

#### Jacob J

Joined Jun 18, 2009
159
So you say I should find an AM circuit insted? how do I get the function I need in such a circuit, all the circuits I could find were with microphones, wich I dont want. But I just want to send out a constant signal, when the signal dies out, the motor shall turn off.. It sounds easy, but I just cant figure it out..

#### Audioguru

Joined Dec 20, 2007
11,251
The "A" in "AM radio" is Amplitude. The strength of the signal is its amplitude. The audio detector in an AM radio has the amplitude of the signal as its output.
But an AM radio has an automatic-gain-control circuit that makes a weak signal as loud as a strong signal. Disable the AGC circuit.

For a transmitter you need a crystal-controlled one so its frequency doesn't wander all over the place. It is not modulated so it is not AM and it is not FM.