S800G Helicopter Mod

2013-04-06 20:42 - Making

As I posted briefly earlier, I've gotten into flying miniature helicopters recently. It started with a cheap impulse purchase. And I had fun so I kept going. The V911 model in the linked article was broken, and I'm waiting for parts to arrive to repair it. Along the way of ordering those parts, I also picked up a Syma S800G helicopter, for under $20 from Amazon. I didn't realize at the time, but it came through a seller from China. It just arrived today after about three weeks.

The Syma S800G micro helicopter.

If you look for reviews online, they'll be right. The controls aren't very good, there's a wide dead area in the center, and a small active area at the edge. Since I already have a V911, I know that this copter's controls aren't great. They're great for a $20 toy, but clearly not as good. So if you want to get into the hobby, I still suggest this as a first buy. It's so cheap that you don't have to worry about breaking it at all, it's a great trainer.

So here's where I geek out talking about how helicopters work. The first cheap helicopter I got was a co-axial model like this: two blades, which rotate in opposite directions to counteract each other's rotational torque. Changing the relative speed of the two blades lets the copter turn (yaw). Changing the speed of both lets it climb. Finally a vertically oriented tail rotor will pitch the copter, allowing flight forwards and backwards. Common simple three channel models use this scheme.

This copter is different. It uses a swashplate to move by using "cyclic control". It alters the pitch of the blades as they rotate (cycle) around the main rotor, changing when they offer lift, and thus the pitch and roll. You can still control the speed of each rotor, and thus you get a lot of control added with the fourth channel. The V911 I've already had fun with uses "mode 2" controls: both cyclic (forward/back or pitch, left/right or roll) controls on the right stick, throttle and rudder on the left. The S800G is "mode 4", however, and thus the sticks have different meanings. It's tough to learn how to fly while also switching between controller modes.

I heard that it's possible to modify the S800G transmitter to do mode 2 style control, both through posts on rcgroups.com, one from user FlyingTape and the other from user Kurt. It was enough for me to go on. But I recorded my progress with bigger/more clear pictures.

The back of the circuit board of the S800G transmitter before changes, with key areas highlighted.

That's the back of the un-modified circuit board inside the transmitter. Click full size to see the four highlighted areas. Since we're looking at the back, flipped twice it's worth mentioning: area number 1 on the left is the horizontal axis of the right stick, it connects to area 2 which is both pin 6 of the IC and connected to an un-used pad right next to it. Area 3 is the horizontal axis of the right stick, connected directly to the nearby IC on pin 14.

The modified S800G transmitter, now mode 2 by switching the connection of the horizontal sticks.

Since throttle is already on the left stick, and pitch already on the right, all we need to do is swap the rudder and roll. Do this by cutting the traces that connect area 1 and 2, and the same between area 3 and 4. Be careful! Circuit boards are delicate, and it's easy to cut the wrong part. I used a Dremel tool on low speed with the narrow grinding bit, taking off very small sections and going slowly. I verified with a multimeter before that the pins I expected did connect together, and that they did not after I was done. Be sure to check, so that you don't short both controls together.

Then solder wires in to re-connect each stick to the opposite pin of the IC that it used to connect with. In this case I sent the right stick straight to the IC pin 1; being on the edge it was easy enough to reach. And the left stick to the mentioned unused pad which already connects to the correct IC pin. Note that there are a pair of pins from each stick which are already shorted, so you don't need to be too careful on that side. But don't short anything that shouldn't be: especially those close-up pins on the IC!


A third helpful post
2013-04-07 10:52 - arantius

I forgot about this third post by talktodan which I also used as a reference.

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