RC Receiver help

[bfox2]

Hello all, I have a model airplane that I need to add controls too. I just want a simple on/off circuit that will take a receiver command and use that to activate another circuit, such as to use the switch on the airplane remote to turn on a flashlight, etc... I don't want to do this mechanically because don't want to worry about the space/cost/power requirements of a servo.

The receiver output is 3 posts, red,black, yellow. Red and black are used to drive a servo motor and are constant at 5V. Yellow varies based on the switch position, Anywhere from 0 to .3V. I have bought and tried a transistor that is activated by the .3V and then activates the other circuit powered by say a 9V battery. My problem is that the voltage from the receiver is too low and doesn't activate the transistor. I need some kind of device that will activate the transistor/2nd circuit without killing the receiver. It should be simple, but I don't know exactly what circuit to make.

I dont know what kind of current comes out of the reciever, I forgot to measure it. I think some kind of transistor loops, like a Darlington pair or something like it to step up the voltages enough.

 

[Sceadwian]

The voltage isn't varried on the data line, it's a pulse width modulated signal between 1ms and 2ms's long 'center' being 1.5ms and 1 and 2ms ' being the extremes of servo travel (though some servos will read pulses as short/long as .75ms and 2.25ms) If you're looking at it on a multi meter the voltage you're reading is a somewhat randomized average of the voltage on the line and pretty much garbage, the actual voltage on the data line is 0 to whatever the supply is, it's just pulsed really fast. Any circuit you plug into this will never fully turn on or off, it will only turn on for 1-2ms out of roughly ever 20ms's (that's the update rate of a standard radio control setup) You need some way of turning these pulse widths into an on/off voltage. I'll leave it to someone better at raw digital electronics to suggest a circuit/components that could translate a 1ms pulse every 20ms' into a 0 output and a 2ms pulse every 20ms' to a 1. Unfortunatly it's a bit more complicated that just a single transistor or two.

 

[bfox2]

Well thanks then. I dissasembled an old servo the other day and the PCB consisted of some kind of long thin control chip, a voltage potentiometer thing to read gear positions, a couple transistors, resistors, capacitors, etc... I know that prolly doesn't help much, but I dont know anymore than that. If someone else could please let me know a way to go down this path to an on/off switch or another route to take it would be appreciated.

 

[bfox2]

Also would there just be someway to capture the pulses and level them out to create an average but steady voltage?

 

[gramo]

I have done this by assigning one of my PWM outputs on the receiver as a "Control" line with the use of a PIC

Variable = PULSIN Pin , State

Overview
Change the specified pin to input and measure an input pulse.

Operators
Variable is a user defined variable. This may be a word variable with a range of 1 to 65535, or a byte variable with a range of 1 to 255.
Pin is a Port.Pin constant that specifies the I/O pin to use.
State is a constant (0 or 1) or name HIGH - LOW that specifies which edge must occur before beginning the measurement.

eg

Signal = PULSIN PORTA.0, 1

This will start to count the number of uS that a pulse goes for. The only catch is that the value returned is dependant on the clock speed you use. With a 20Mhz crystal, each value in Signal will equal 2uS, but if a 4MHz crystal is used, then each unit is 10us.

If the total time took more than 65535 units, than Signal will = 0

Now I can accurately determine the length of the PWM, and do things accordingly.

The above example is written in Proton+

 

[bfox2]

Thanks for the reply. Ill look around on your website. So pretty much your taking the signal and analyzing via a PIC??? and then controlling another circuit through the pic. How hard is this to do?

Im looking at a circuit here

**broken link removed**

from this website.

**broken link removed**


I think this can work, converting the servo signal into analogue for use without a PIC, which I dont have a programmer for.

 

[Sceadwian]

Micro controllers (PIC's or AVR's) are relativly cheap, (2-5 dollars roughly) programmers can be made with a few resistors and transistor on a parallel port and simple programming software is free. But you have to know or be willing to commit yourself to learning how to program them. The learning curve is steep and difficult but once you know what you're doing you can do MANY things very easily that used to required very complex logic circuits.

 

[bfox2]

Alright well I am willing to learn, I just needed to be able to do this the cheapest and easiest way possible and 4x over for 4 separate RC channels. I think the PIC can handle all four channels just to push an on/off state. Can anyone suggest the best route to go about getting this PIC and programming it for my application?

 

[Sceadwian]

The channels don't all come in at once, they come in sequence with a short delay, so a PIC can handle as many channels as the RC receiver has (assuming it's not a top of the line PCM receiver with synchronous outputs)

And actually a PIC should be able to handle at least 16 if not 256 states on each channel, though starting with on/off is a good idea. It's not easy though.

That link for the servo RX decoder looks pretty decent. Gramo and Nigel should be able to point out some PIC information, I use AVR's myself so I can't really help in the PIC department.

 

[Nigel Goodwin]

There's lots of PIC RC projects on the net, have a look at for some examples.