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FM transmission and range limiting control

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Jacob J

New Member

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?

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?

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

New Member
I got an idea. Lets say I build a FM transmitter like this one: 1-Transistor FM Transmitter

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?


Super Moderator
Most Helpful Member
That is a horrible circuit for so many reasons.
Yes you can remove the microphone and it will transmit a constant carrier.
Controlling something based on the signal strength received over the air from a simple oscillator like that is not a recipe for success.
Reflections from surrounding objects will give a varying signal strength and your apparent range will vary.

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