(Since this build I have made another conversion: brushless Syma X5C).

The Syma X5C was the first quadcopter I got to learn to fly with. It is a slow docile flyer which is perfect to learn to control quadcopters. It is alsp nearly indestructable because of its light weight and its frame construction (rigid, but able to withstand shocks from crashes). Lately the Syma has been collecting dust because I moved on to brushless mini-quads. Therefore I thought it would be nice to improve the old X5C a little bit.

The two improvements I really wanted to have were:

• More performance
• Able to fly it in acro or rate mode

Performance

With some good high C rated batteries you can improve the performance of the X5C a whole lot. The Turnigy Nano-tech 750mAh is a good example on how to improve the performance of the Syma over the stock batteries.

But I wanted more so I reckoned I had to go with 2S. With two lipo-cells in series you will be able to generate twice the power (in theory) from each motor. There are implications such as the fact that all subsystems should be able to cope with the increase voltage.

Acro mode

In acro or rate mode you have more freedom to control the movements of your quadcopter. Basically it resembles more the way you control a boat or an airplane. I was already flying the Eachine H8 mini and Floureon H101 in acro mode with the superb https://github.com/silver13/H101-acro firmware so I reckoned that I could lend the flightcontroller out of an H8 mini and transplant it to an X5C frame. That would be the easiest way to get the X5C flying in acro mode.

Off we go, let's build this...

First off, unscrew all of the 24 screws to open the frame. The X5C really does not need those screws because the frame is so brilliantly engineered it will stay together firmly even after removing all of those screws. That must be one of the secrets of its rigidity...

(The original flight controller, we need to get rid of that so screw it out)

Once open we see a small red pcb, which is the flightcontroller. We get rid of that one. Unsolder the wires of the motors and the leds and lift the board out. We now have the empty shell left with only the motors, leds and wiring...

The donor flightcontroller comes from this H8 mini. It was already modified to use the silverxxx firmware, hence the programming pins at the side. The props are Bayangtoys X7 which turn these little quads into rocketships.

We are going to strip the quad until we are left with the bare pcb. This is the flightcontroller we are going to use We want to run this quad on 7.4 volts (2s), therefore we need to make sure the main pcb can handle that voltage. Unfortunately the voltage regulator for the main pcb cannot handle voltages over 6 volts...

Here are the parts needed to get 7.2volts to work. We need a step down converter. You can find them on Ebay for pennies. We are also going to change the motor output FETs for larger current types. Below is a picture of the parts needed:

The IRF7413 FETs are just generic high power FETs I had around. They are 30V, 13A n-fet types so should be good enough to handle the motor current.

First we need to get a decent powersupply for the main board. The main board expects a voltage between 3.7v and 4.2v and regulates it down to 3.0v for the control logic. The onboard LDO only support voltages up to 6V so our 2S lipo is too much for that. For that reason we add a small voltage regulator to provide the flightcontroller with a stabilised voltage of around 3.9V. The inbuilt LVC of the H8 mini will therefore not work but we will add an external LVC to compensate for that.

Flightcontroller

The flightcontroller is an H8mini FCB with the silverxxx firmware flashed. For 2S operation the original motor FETs are desoldered (marked in red below, on the bottom of the board there is a fourth).

We also remove the LEDs as the original Syma LEDs strips will be connected to the FCB.

The IRF7413 mosfets are going to be soldered onto small adapter boards for easier soldering. The gates of these four boards need to be soldered to the gate pins of the original mosfets on the FCB. The drawing below shows the pinouts of the original FETs and the new ones.

(Pinout of the A0SHB (2300) FETs and the replacement IRF7413)

Tip: if one of the 2300 FETs blow out, replace with IRLML2502, pin compatible...

(Schematic of how each motor needs to be connected)

Building it all together

We make a PDB from a piece of protoboard stuck on the back of the 3.9V controller. Best place to keep it is in the battery bay, because it fits snuggly there and the 2S LiPos won't fit there anyway.

(All wired up and ready for testing. The LED strips of the X5C are connected to the H8mini LEDs so you will get the flashing pattern at startup before binding is complete. The FCB is held in place with hotglue, just as the rest of the ESC boards. Power is distributed via a wire harnas connected to the improvised PDB mentioned above.)

Testflight

Below is a small clip with the first testflight in rate mode. Surprisingly enough the default PID settings are working very good. No tuning needed apart from the default settings for yaw, pitch and roll speed (upped to 720 degrees/sec). In this clip the X5C flies with its standard 7mm motors.