I'm attempting to design a simple circuit that will control an impossible to find worm servo for the landing gear of a discontinued drone.

I'm only a hobbyist, so I'm hoping to replace a complicated circuit board (see pic) that has burned out with a more dumbed down circuit that I can easily solder. By looking at it, I'm sure this burned out board performed other functions like checking if the landing gear is already up or down, but those additional functions are not needed.

The circuit I want to create should be simple, but I'm stumped on how to reverse the servo's micro motor when there's a voltage change sent from the drone's transmitter.

One side of the board has two smd detector switches (Panasonic ESE-18R11C) that are triggered when the micro motor turns a screw gear that moves a metal bar from one side of the board to the other, making contact with the detector switch. When the detector switch is pressed, the motor stops until it receives a signal from the transmitter, then the motor reverses sending the metal bar back to make contact with the other switch on the opposite side, resetting the circuit. There are 3 wires that power the circuit (+3v (red), GND (black), and (white) which switches voltage from 1.5v to 3v when the transmitter's toggle switch is flipped.

I'm not sure how to tell the circuit to reverse the motor when the voltage from the WHITE wire changes from 1.5v to 3v. I imagine a 555 would be able to handle this, but I don't have clue where to start. Plus I'm not sure if I'll need a second 555 to reverse the motor or if one 555 can perform both tasks.

Any help with this would be greatly appreciated.

 

One) Use a microprocessor. An Arduino Nano is fairly small and with a minimal amount of coding, can do what you want. Or with the addition of a programmer from Sparkfun, you could use a single chip microprocessor, such as the ATTiny85.

Two) Use a 555 based servo tester circuit, replacing the pot with two fixed resistors (calculated by first using a pot to set the two limits). Use a window comparator to detect the two voltages and switch each resistor into the circuit with transistors.

But even better is this solution I found, based on 6x Schmitt Inverters (74HC14).

 

A "standard" RC servo command signal is a bit more complex, as I recall. A variable duty cycle sets the servo position, someplace between the limits. the result being position control. And if the drone is battery powered then wasting power on an arduino is a poor choice.
I am thinking that the 1.5 and 3 volts mentioned in post #1 are actually 50% and Max% duty cycles on that white wire. This means a bit of investigating is required. If another RC servo is available that can be connected and check to see what it does when the landing gear switch is operated then knowing the actual signal will allow developing a suitable circuit.

 

One) Use a microprocessor. An Arduino Nano is fairly small and with a minimal amount of coding, can do what you want. Or with the addition of a programmer from Sparkfun, you could use a single chip microprocessor, such as the ATTiny85.

Two) Use a 555 based servo tester circuit, replacing the pot with two fixed resistors (calculated by first using a pot to set the two limits). Use a window comparator to detect the two voltages and switch each resistor into the circuit with transistors.

But even better is this solution I found, based on 6x Schmitt Inverters (74HC14).

View attachment 255587

Thanks for the quick reply, djsfantasi. I like the idea of the Arduino Nano but it's too big for the servo case and the Schmitt inverter solution may be too complicated for me. I've been looking at the 555 tester circuits and that seems to be the right direction, though I'll have to educate myself on using a window comparator, which is new to me.

Thanks again.

 

A "standard" RC servo command signal is a bit more complex, as I recall. A variable duty cycle sets the servo position, someplace between the limits. the result being position control. And if the drone is battery powered then wasting power on an arduino is a poor choice.
I am thinking that the 1.5 and 3 volts mentioned in post #1 are actually 50% and Max% duty cycles on that white wire. This means a bit of investigating is required. If another RC servo is available that can be connected and check to see what it does when the landing gear switch is operated then knowing the actual signal will allow developing a suitable circuit.

Understood. I was hoping this would be a simple circuit (switch turns motor off / motor reverses / repeat) but it looks like it's going to be more complicated than that. I like the idea of djsfantasi's 555/window comparator solution so I guess I'll have to experiment. Thanks.

 

Is that really a servo? It looks like a permanent magnet motor using limit switches for position.

 

Is that really a servo? It looks like a permanent magnet motor using limit switches for position.

I wouldn't call it a servo. It's a micro motor that turns a geared screw that moves a bar back and forth that triggers 2 switches. (see pic) Once a switch is triggered, the motor shuts off until the white wire changes voltage and the motor reverses and the second switch is triggered (lather-rinse-repeat). So it's like a servo but the position is irrelevant and the voltage is constant depending on the transmitters toggle switch position. The white signal wire acts more like a 0 or 1 instead of setting the position like a real servo. 0 = move the motor till switch one is hit, 1= reverse the motor till switch 2 is hit. You'd think this circuit would be simple.

 

Well, I’ll be hornswoggled. 27 posts to find out the TS isn’t asking about a servo at all. I’m embarrassed at the effort I’ve expended on the TS original, incorrect problem.

 

Well, I’ll be hornswoggled. 27 posts to find out the TS isn’t asking about a servo at all. I’m embarrassed at the effort I’ve expended on the TS original, incorrect problem.

Hardly anything to be embarrassed about. Completely my bad for not being clear. If I were to order this part (which I can't) it would be called a "Landing Gear Worm Servo". Your post did send me in the right direction. I'm curious to experiment with a 555 and a window comparator, unless of course someone comes up with a better idea. Plus you're dealing with a noob who knows just enough about electronics to blow himself up.

 

The device has three input connections just like a model servo, and it connects to an RC receiver. So it certainly seems like it has an rc servo interface. No, not the same a a rudder servo, but definitely plug-in compatible. What I don't know about model RC servos is where the signal stops being PWM and becomes analog DC. Then look at the first photo in post#1: That is certainly a complex PCB, easily enough to be a full servo, except with switches for the two positions instead of an analog feedback pot.
And open up a more typical RC servo and you will see a micro-motor turning gears , the big difference is a position feedback pot instead of two switches.

 

The device has three input connections just like a model servo, and it connects to an RC receiver. So it certainly seems like it has an rc servo interface. No, not the same a a rudder servo, but definitely plug-in compatible. What I don't know about model RC servos is where the signal stops being PWM and becomes analog DC. Then look at the first photo in post#1: That is certainly a complex PCB, easily enough to be a full servo, except with switches for the two positions instead of an analog feedback pot.
And open up a more typical RC servo and you will see a micro-motor turning gears , the big difference is a position feedback pot instead of two switches.

Yes, but he is wanting to replace the internal circuitry of the "servo", so the motor he is trying to drive is just a motor. I may be mistaken but it appeared people were treating the motor itself as a servomotor with the drive requirements of one where in practice what seems to be needed is a method of powering the motor in response to the signals which may well be intended to appear to be an ordinary hobby servo.

If the signals were destined for the input to a servomotor assembly, it would make sense to treat it as one but it seems that what is being driven is a DC motor, forward and reverse, in response to a signal meant to be sent to a servomotor. In other words, the new electronics has to turn a DC motor into something that looks to the controller like a servo but the signals to the motor won't be ones that a servo would expect, the signals to the circuit would be.

I could misunderstand what is going on here, but it seems like a simple H bridge switched by an MCU or window comparator is what is called for. It would drive the DC motor forward to the limit switch, then stop; and reverse it to the limit switch on the contrary signal. If this is correct, the suggestion of an ATTiny85 seems really good.

If I understand this whole thing incorrectly, I'm sorry for causing more confusion.

 

The device has three input connections just like a model servo, and it connects to an RC receiver. So it certainly seems like it has an rc servo interface. No, not the same a a rudder servo, but definitely plug-in compatible. What I don't know about model RC servos is where the signal stops being PWM and becomes analog DC. Then look at the first photo in post#1: That is certainly a complex PCB, easily enough to be a full servo, except with switches for the two positions instead of an analog feedback pot.

Yes, that complicated PCB is a head scratcher. It's certainly doing more than moving a motor, which is why for my purposes it could be dumbed down. As I said, this PCB is irreplaceable. This drone was made in 2014 and every person who owns one would give their left nut to get their landing gear working again. It's the first thing to go if you have a hard landing or run into a tree.

The landing gear needs to be down so there's clearance for the camera and gimbal while on the ground. Then the landing gear needs to swing up so it doesn't block the camera. Without the landing gear, you'd have to place the drone on a bucket or something before take off. That's a pretty sad workaround for a $1,200 drone.

 

Yes, but he is wanting to replace the internal circuitry of the "servo", so the motor he is trying to drive is just a motor. I may be mistaken but it appeared people were treating the motor itself as a servomotor with the drive requirements of one where in practice what seems to be needed is a method of powering the motor in response to the signals which may well be intended to appear to be an ordinary hobby servo.

If the signals were destined for the input to a servomotor assembly, it would make sense to treat it as one but it seems that what is being driven is a DC motor, forward and reverse, in response to a signal meant to be sent to a servomotor. In other words, the new electronics has to turn a DC motor into something that looks to the controller like a servo but the signals to the motor won't be ones that a servo would expect, the signals to the circuit would be.

I could misunderstand what is going on here, but it seems like a simple H bridge switched by an MCU or window comparator is what is called for. It would drive the DC motor forward to the limit switch, then stop; and reverse it to the limit switch on the contrary signal. If this is correct, the suggestion of an ATTiny85 seems really good.

If I understand this whole thing incorrectly, I'm sorry for causing more confusion.

I'll look into the ATTiny85 option as well. I've been soldering since I was a kid so that's a no brainer, but the circuit design is gonna be hard for me, which is why I'm reaching out to the pros. I can follow a schematic, but I'm restricted to a 1" x 1/4" PCB in a tight space, so I'll be working with SMD ICs, resistors, capacitors, etc. I'm sure I'll screw it up at first, but I love the process.

 

I'll look into the ATTiny85 option as well. I've been soldering since I was a kid so that's a no brainer, but the circuit design is gonna be hard for me, which is why I'm reaching out to the pros. I can follow a schematic, but I'm restricted to a 1" x 1/4" PCB in a tight space, so I'll be working with SMD ICs, resistors, capacitors, etc. I'm sure I'll screw it up at first, but I love the process.

If you go the ATTiny85 route, solder a socket to your PCB. You’ll be putting the microprocessor in and out of the board while you develop the software.

 

A tiny DC motor is what we find in almost ALL hobby RC servos, this one was set up to provide more force and it only needed to move to two positions, hence using switches instead of a linear sensor. IF a small enough servo can be found, then that servo can operate a switch to drive the motor in this application using that worm drive system. But the simple ? fix is to replace the failed board that presently was driving that motor. THAT is far from a trivial task. One option would be to locate a translator module that converts a servo position control to an on/off drive control and then a simple circuit to drive that same motor.

 

I'll look into the ATTiny85 option as well. I've been soldering since I was a kid so that's a no brainer, but the circuit design is gonna be hard for me, which is why I'm reaching out to the pros. I can follow a schematic, but I'm restricted to a 1" x 1/4" PCB in a tight space, so I'll be working with SMD ICs, resistors, capacitors, etc. I'm sure I'll screw it up at first, but I love the process.

Really, the work-around will be to learn to fly the drone in a direction different from the traditional straight ahead scheme, so that the whole thing can point in one direction while flying in a different direction. Some helicopter pilots could do it, so that a gunner could soot forward out the side. In a fixed wing plane it is called "yawing" and id difficult and not efficient. Doing that would allow leaving the landing gear down.

 

Really, the work-around will be to learn to fly the drone in a direction different from the traditional straight ahead scheme, so that the whole thing can point in one direction while flying in a different direction. Some helicopter pilots could do it, so that a gunner could soot forward out the side. In a fixed wing plane it is called "yawing" and id difficult and not efficient. Doing that would allow leaving the landing gear down.

Unfortunately it's impossible to get good video with the landing gear down unless the camera in pointing straight down. Currently the landing gear is permanently up because one of the "servo" PCBs is fried. The drone needs to be launched from a bucket if the camera is going to be connected. I've tried every conceivable workaround for this situation for years now. Redesigning the PCB is the only option left but I had no idea that reversing a motor using a voltage change would be so complicated.

But I do appreciate the quick replies and effort from everyone here. I'm going to order some 555s, window comparators and a bunch of SMDs and start experimenting. Who knows. Maybe I'll have a breakthrough.