servo like switch controller for model - Page 1
Forum section: electronics / projects

author: kokodin
author_rank: Regular Contributor
timestamp: March 22, 2018, 02:33:31 pm
content: helloi was thinking about canibalising one channel in my rc controller to use it as a function switch, for turning lights on and stuff lke that.i don't want to buy any complicated boards to my model unless i know how they work. the problem is i don'twhat i come up with is probablt silly. since we deal with a square wave with modulated duty cycle as a servo position signal, why not somehow ac couple to the signal like scopes do and drive two transistors in ab configuration to enable things like lights to the common groundit is probably more complicated than it should have, is there any ready chip that could do that for me? based on pwm factor drive different outputs?i recently discovered that my transmiter can be hacked from 2 to 5 channels for a price of 3 switches and connectors, so i would like to use thoseif you know better way i be glad to hear of itthanks

author: kokodin
author_rank: Regular Contributor
timestamp: March 25, 2018, 10:52:04 am
content: i was thinking about that and i come to conclusion i should find some old serwo schematic and tam my outputs where output h bridge is, or find a way to make h bridge in reverse and just connect that to hacked micro servo controller, that would be the cheapest option.question is now how to make anti-h bridge circuit that would detect polarity of the output and signal it by one of 2 signal wiresi need to think about that noweditin a crude way i might use 2 opto couplers with diodes placed in oposite direction and use their transistors on the safe side as switches

author: Ian.M
author_rank: Super Contributor
timestamp: March 25, 2018, 01:03:53 pm
content: You can directly read Futaba single channel servo signals with an Arduino. The function to use to do so is:Code:pulsewidth = pulseIn(PWM_PIN, HIGH);(seehttps://www.arduino.cc/reference/en/language/functions/advanced-io/pulsein/)Compare the result with a list of pulse widths using multiple if() statements and for each band of pulse widths set your outputs appropriately.Use an 8 pin ATtiny chip instead of a whole Arduino board to save weight and space and you can wire its outputs direct to the gates of low Vgs threshold N channel MOSFETs as low side switches to control your loads.If you need high side switches you'll need level shifters to drive P channel MOSFETs. A small signal N-MOSFET driving the P MOSFET gate, with a pullup resistor to +Vbatt will do the job.

author: kokodin
author_rank: Regular Contributor
timestamp: March 25, 2018, 10:08:25 pm
content: Yup that would be an option too. I didn't even take into consideration attiny to be honest, because i always worked with atmega 8 in dill packages. (too big for the model) I made starting signals with light and sound few years ago based on that.Back to the controller i don't think my loads will need mosfets. My model is 1/21 scale fiat 500l, the old one. so entire project will be around 15 centimeters long, the only loads i will be driving would be 555 timer output to 4 or 6 orange diodes 2 red and 2 white ledswith 2 attiny (one for lights, one for turning signals) when i think about it i could even drive that directly from the potrs of that controller and even eliminate 555 for turn signals by that. i will need to seriously think about it.thanks for the sugestion

author: jpanhalt
author_rank: Super Contributor
timestamp: March 25, 2018, 10:16:00 pm
content: Very well know function for an RC channel. Some examples: retractable gear, glider release, and ignition kill switch.Some are as simple a servo activating a micro/lever switch. Some are more electronic. What do you want?

author: cstratton
author_rank: Regular Contributor
timestamp: March 25, 2018, 10:25:35 pm
content: These days the more compact / inexpensive solutions don't even have a distinct "receiver" - rather the RF chip sits on the same PCB as the main MCU (sometimes in the same IC package) along with LED drivers (often directly off MCU I/O pins), and NFETs or bridges for small motor loads.If you need electromechanical servos those would be driven from there as well.Typically only for larger/brushless motors or more modular legacy configurations do you need off-board speed controllers, and sometimes not even then.

author: C
author_rank: Super Contributor
timestamp: March 25, 2018, 11:26:22 pm
content: The main channels of RC need to be fast so you have control of model.If you can control the input of a channel you could then feed many slower channels over that one channel and get many slower responding outputs.For normal old RC format the signal transmitted was a sync width pulse followed by channels. Receiver locks on to sync pulse to decode the channels.This can be very easy if input circuit can know when that input is scanned.If you do not know this then the slow sub channel has to get slower so that each part of digital stream takes many transmission times of main channels.

author: KD4PBS
author_rank: Contributor
timestamp: March 27, 2018, 05:00:11 pm
content: Seems like integrating the PWM signal with a resistor and a diode, then using a comparator or some such thing such as simply relying on the fact that a relay requires a minimum amount of voltage (current) to pull in would suffice, if you're just wanting a switch.