# [SOLVED]Need help building a proportional circuit for my RC tank

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#### RC Dave

##### Newbie level 3
Hello everybody, This is my first post.. So please be kind
I am a novice when It comes to building electronic circuits, but I do know the basics and know a few components used in circuits (Caps, resistors, etc..)

I need help in designing a circuit for my RC tank. I require a circuit that will allow me to run a 5 volt fan at 3.5 volts and have the ability to add voltage to the circuit to combine and increase the voltage to the fan via two variable voltage high speed DC motors. (both run at full 7.2volt) 7.2 volts would be what I would want the fan to run at when motors are running full speed then when the motors have stopped the fan returns back to 3.5 volts.

I hope I explained what I need as best as I can, any help would be appreciated :idea:

-x RC Dave x-

#### chuckey

Feed the fan via two diodes with their cathodes at the motor end. one diode anode, you feed from 3.3V+.8 = 4.1V (or a resistor from the +5V line), the other from the +7.2V motor supply. The highest voltage will power the fan.
Frank

#### RC Dave

##### Newbie level 3
Hi Frank, Thank you for the reply.

I think I get the general idea of what you trying to explain, but is there any chance you could create a circuit diagram so that I can follow it, and understand a bit better?
Thanks again
-x RC Dave x-

#### chuckey

I have re-considered your problem an offer another solution. Use a relay, feed the coil from the motors 7.2V rail, when the relay is at rest feed your fan from a centre contact, and the other made contact from the 3.3V. So your fan will now run until the 7.2V appears, now feed the contact that is feeding the fan from from the 7.2 V.
Frank

RC Dave

### RC Dave

Points: 2

#### RC Dave

##### Newbie level 3
I have re-considered your problem an offer another solution. Use a relay, feed the coil from the motors 7.2V rail, when the relay is at rest feed your fan from a centre contact, and the other made contact from the 3.3V. So your fan will now run until the 7.2V appears, now feed the contact that is feeding the fan from from the 7.2 V.
Frank

Sounds like an interesting concept, I used a circuit simulator to test your theory and it does indeed perform the way I want it to.