SWervo control help

[Drunkskunk]

I'm trying to build a servo switch, and ran into a unique problem.

I planned to use the board from a servo, and use the motor Wire for the signal. I was going to use forward and reverse of the motor as my high and low for signlaling a transistor hooked to the camera's switch, which was to be powered off my reciever.

Sounds easy, right?

The problem is, once I cut the resistor and motor off the servo board, it suddenly consumes too much power, and builds heat rapidly. with no input, It uses more power than a normal servo under load and moving.

I tried a second servo, this time terminating the resistor wires with 1.5k resistors, equil to the pot's resistance. It still draws the same amount of power. I've attached the motor leads to a resistor and LED, no change in power consumption, but it functions.

I'm not sure whats going on here. why would an open circuit start pulling more power on both servos? What can I do, if anything, to use the servo's board as a high/low signal device? Is there a better way to do this?

[Comatose]

http://www.dimensionengineering.com/PicoSwitch.htm is an easier way to do it, at least.

Do you have any way to measure the output waveforms coming off the motor leads?

[Drunkskunk]

Thats the Idea, Yeah! Only it's 7 grams too heavy. it's actualy heavier than the servo I sacrificed for the board. the Idea is a controller to be less than a gram, and the camera with switch to be under 10 grams.



I don't have access to an O scope, but I do have a frequency scanner, so I can measure the level and frequency of an output.

[bravokilo]

Hi DSkunk, am BravoKilo. Tha sounds unusual, I am not sure I understand what you are doing. However, here are some basics. The Futaba servo amps has 2 ics. the small one going to the motor is merely an H bridge. I shouldn't heat up without load. The other types, NE544, ZN409 use two external transistors. My suspicion is when yo removed the feedback pot. You incoming pulse keeps on triggering the schmitt trigger since there is no matching signal(reverse pulse) since your pot was removed. do not remove the pot and after you cneter your TX gimbal, use the pot to put the outputs on neutral, signal to turn the motor. That is your deadband. I will send you a simpler circuit using a quad op amp. You see two ops make a monostable which produces the local pulse whenever an incoming pulse appears. The two pulss are mixed and the diiference or error signal is stretrched to turn a motoer CW or CCW. You can instead put a relay to fire you camera accessory. Tell me n=more of what you want todo.

[AndyOne]

...The problem is, once I cut the resistor and motor off the servo board, it suddenly consumes too much power, and builds heat rapidly. with no input, It uses more power than a normal servo under load and moving. ...

I'm not sure whats going on here. why would an open circuit start pulling more power on both servos? What can I do, if anything, to use the servo's board as a high/low signal device? Is there a better way to do this?

It sounds like you accidentally shorted something as I can't see any way this would happen otherwise.

Check for shorts.

Andy.

[Drunkskunk]

Hi DSkunk, am BravoKilo. Tha sounds unusual, I am not sure I understand what you are doing. However, here are some basics. The Futaba servo amps has 2 ics. the small one going to the motor is merely an H bridge. I shouldn't heat up without load. The other types, NE544, ZN409 use two external transistors. My suspicion is when yo removed the feedback pot. You incoming pulse keeps on triggering the schmitt trigger since there is no matching signal(reverse pulse) since your pot was removed. do not remove the pot and after you cneter your TX gimbal, use the pot to put the outputs on neutral, signal to turn the motor. That is your deadband. I will send you a simpler circuit using a quad op amp. You see two ops make a monostable which produces the local pulse whenever an incoming pulse appears. The two pulss are mixed and the diiference or error signal is stretrched to turn a motoer CW or CCW. You can instead put a relay to fire you camera accessory. Tell me n=more of what you want todo.

Hello BravoKilo. good to meet you.

It may be those transistors that are making the heat. being that the boards came from a s-75 and another, unlabled but similar sized servo, it's hard to see them. they're tiny specks surrounded by other tiny specks of surface mount parts with the lables scratched off.

What I want to occomplish is using the aux switch on my DX6 spektrum to activate a camera. The only stipulation is the switch, wire, and camera are going to weigh 10 grams. For that to happen, the switch can't weigh more than one gram. that rules out any form of Relay. I had planned on using a 2n 4401 or 4403 surface mount transistor as the switch, but I need a Hi/low signal from the reciever.

And thats were the servo control board comes in. I figured it would be the perfect sorce for a signal, but the heating problem on both servos is preventing that. I can't use the pot from either servo as it's a weight issue, but I replaced one pot with 2 1.5k surface mount 1/8 watt resistors matching the resistance of the pot. because there is no center for a servo on the Aux channel, I figured 2 resistors would be fine.

Any of this making sence? its hard to explain without pictures, and I don't have a camera good enough to take pics of Surface mount boards.

[bravokilo]

DSkunk, ok, you can try the transistor sw if it will do. But if the saturation is poor(low contact, try a mosfet.

I think your problem is in the feedback pot. the pot has three terminals and is ususally 5k. What u should have done is to try the servo when it was still intact on that channel. if your TX has a switch, one pos ition would move the servo one way. You see as if moves say to the right, the pot is oved too so that it stops when the local pulse the ic produces cancels your tx sent pulse. i would go back to the off positin of the TX sw then measure the pot center to the two end terminals. i expect the resistance to be different. Get two resistors matching the measurements and connect them. They will simulate the pot. Two ends and the junction which is the "center". Now, when you turn on the sw, the wires to the motor willcome alive. if you had the motor,it just spin and spin. You probably wired the sw transistor and and it will be on and on...., I mean, permanently as long a the sw is on. Series a 1k for a starter to the motor wire in going to the base of you tansistor switch and try higher and higher in as long as it fires the transistor sw. In oher words, use only the signal your transistor requires and do not overdrive it. next, your transistor should have a current limit resistor at its emitter. Put a 100-500 ohms resistor in series. Now , you have to tell me if your sw is just to fire your camera at an instance. I suppose, you turn the sw on and immediately off anyway if it is just to fire a camera. Two ways you can do it. One do it in the transmitter. Put a button instead of the toggle. You will only heat it up momentarily. the other way is to put a "one shot" in the servo amp. it fires once,like charging a capacitor and sicharging it. But since youhave w8 problem, do it in the TX

What heats up by the way? Is it the transistor that you put in there?. U have a reed switch in a glass tube tiny ones maybe from RS).wind several turns of thin wire and you have a lite relay. Tell me about the camera section and what is deeded to fire it or for that matter,what is it u want to do.

Anytime,

BravoKilo

[Drunkskunk]

Thanks!

The camera should be easy. its a tiny little camera that uses a tiny Lipo for memory, and is ment to be powered and recgarged from a 5 volt USB connector, so I will be using it powered from the recievor with an even smaller Lipo to save weight.

The Camera is triggered by a an input to it's CPU going Low. the button in the camera is wired to the Camera's common ground, so a simple transistor can replace the switch. There is no other effect from holding the button down, so a simple toggel switch on the transmitter should be enough to fire the camera.

As for heating up, I have no Idea exactly. the board is only 4 MilliMeters by 6 Millimeters, and surface mount, the whole board feels hot, but any single componant on it could heat the whole thing.

[roffey]

Just browsing through.... why not use a v.small brushed motor esc.. has mosfet output which pulls to ground. take a wire from the mosfet drain to the camera cpu pull down.
Failing that, a simple comparator cct would do it. R into C from Rx channel, two reference ladder r's and it's sorted. I do know how you feel about using the servo boards though. Gotta be useful for something else.

good luck......


p.s. or get one of these http://www.rc-flysoft.com/ :)

[bravokilo]

DSkunk, have u solved ur problem? i was just thinking of your problem of the servo amp heating up. You have an H bridge in there. What could be happening is that they are shorting each other. An H bridge is atransistor matrix to switch polarity. It's like one PNP and one NPN transistors are in series with their emitters to plus (for the PNP) and to minus(for the NPN). Their collectorsare common and this is swhere one terminal of the motor is connected . There are twopairs. The other one isfor the other terminal of the motor. These transistor pairs chould never be on at the same time. Otherwise, they heat up and burn.

Tell me if you have not solved it. Try google and put in RC switch and see if you will be directed to the swithc circuit. I might have even seen in this forum. I downloaded if you can't find it.

BravoKilo