Remote controlled helicopters and other toys have become incredibly advanced for a very low price, this is partially thanks to the huge mobile device market, which has pushed technology in small high energy batteries forward and ramped up production of micro motors. A stable micro helicopter which can fly for 5 minutes on a charge can now be gotten for $20 or less, and there are many interesting variants like UFOs and mini quadrocopters.
The main downside of these devices is that they use infrared (IR) remote control rather than radio, this is the same technology used in television remote controls. It works by pulsing an IR LED light, which is invisible to the human eye, and the device being controlled detects the pulses using an IR phototransistor, which changes its resistance when hit with IR light. Because these devices are light dependent, they require a clear line of sight between the transmitting LED and the receiver. Since this is a flying device, if anything, even the body of the helicopter, blocks the path for a moment, it crashes to the ground. It is also very susceptible to interference. Sunlight is full of IR light, which means the helicopters cannot be flown outdoors.
Radio control on the other hand works in any light and does not need a clear line of sight. The power consumption and weight do bring the flight time down a bit, from 5 minutes to 4 in my case, but I am able to fly for the full 4 minutes without any loss of signal, and from very far away, 50 to 100 ft, and outdoors if itís not too windy.
This is my method for making a Syma S107 helicopter work by radio using an ASK transmitter/receiver set, that are sold commonly on Amazon, Ebay and many other sites for around $5, typically in 433 MHz, or 315 MHz.
If you like this, vote for me in the Hardware Hacking Contest.
This video is a little "walkaround" of the modded helicopter:
Here is a video of it flying. It was hard to get clear video of it. This could qualify as one of those blurry UFO videos, blinking lights in the sky and all:
Step 1: Getting the parts
For this I used common through hole resistors. 1k and 10k. You can vary from that a bit if you have something else on hand.
If you are ordering one just for this, go with the 293904.
Pretty much any NPN signal transistor will work, and the package type (the style of case it is built into) is TO-92. The particular one I used was a BF420, but that's just what I had in my drawer, any of the following should work.
Receivers that will work
Make sure you pick a receiver that can operate at the voltage of your helicopter's battery. Most of these batteries are 3.7V lithium polymer. They reliably put out a bit less than that (3.4V or so). and the majority of these receivers are 5V. Sometimes you can get away with under powering a device, but with these receivers I haven't found that to be the case.
This one from amazon lists the voltage as 2.5-5V for $5 free shipping
This is a set, but you could use the transmitter with the above, and you have a spare, it's $1.20 free shipping
Depending on what you find, I might order a few different ones to try and test it on a breadboard.
Step 2: Modifying the transmitter
The main trick to modifying the transmitter is finding the connection points. There are lots of notes in the pictures below, so be sure to take a look.
Here's the tricky one. You don't want to take this off of the trace connected directly to the diode. Look at the pins from the chip on the front. One of them will have a path to the IR LED transmitters at the top of the remote. Probably going through a tiny capacitor and a transistor. (See picture for more notes on this). If you have the exact same controller I do, it's the pin in the upper right when looking at the back of the controller.
For this I used a piece of solid hook up wire because it's stiff. Some thick magnet wire would work well too. For best results the length should resonate with the frequency of the signal. You can find the equations for this on wikipedia, but I just used the website in the picture below. For 433 MHz, the 1/4 length is 6.48 inches.
Once you've found all these it's a matter of connecting the transmitter to them. I think it's best to attach the wires to the transmitter unit first, then attach the ends to the points in the controller. If your transmitter has legs you can just spread them out, spiral wrap around them and solder on, then put some electrical tape between them for good measure, see the next page for more on the lead wrapping technique. If it has holes, like mine did, just twist the stripped wire ends, poke them through and solder. Don't make the bare wire too long, you don't want it shorting against anything.
On the controller side of things It helps if you heavily tin the wires with a big bead of solder first. If you have a point where a lead comes through, solder to that. If not, use a razor knife or a file to clean off some of the green masking and tin it with solder, then attach your wire there.
The positive voltage on the controller PCB goes to VDD on the transmitter module, it may be labeled VDD, +V, +5V, POW depending on model
Ground may be labeled GND, -V, VCC, or the ground symbol, which looks like an upside-down antenna
The signal input may say DATA, or INPUT. If there is one labeled TE it's not that.
Step 3: Adding a NOT gate signal conditioner to the receiver
This step is all explanation, you can jump ahead if you just want to build it.
It is necessary to invert the logic on the receiver. This is because the IR receiver was designed to use a photo-diode, which blocks current when it is struck by enough IR light. So the chain is:
With IR transmission
It also helps to amp up the signal a bit coming from the receiver.
Don't worry, this can all be done with one transistor and two resistors. We have to keep it simple and light after all, the helicopter will be carrying it.
A NOT gate is nothing more than a switch which turns off when it receives a high signal and on when it receives a low signal. Configured as a very simple inverting amplifier an NPN (negative positive negative) transistor will do just this.
Step 4: Building the signal conditioner for the receiver
The first image below is the whole circuit. To make it compact were going to put the resistors in line with the legs of the transistor and lay it down against the board.
Figuring out the legs (E, B, C)
First, make sure you identify your transistors legs by looking up the appropriate data sheet. It should give you the Collector, Base and Emitter relative to the flat side of the transistor. I think on most of the ones I listed if you hold it with the flat side facing you, the left to right order is E,B,C. For example here's the datasheet for the 2N3904. If you got a different one, a internet search for the part number + datatsheet should come up with what you need.
(first picture of build) I like to do a spiral wrap for these connections, it's easiest with the cheap resistors which have really thin legs. Once that's done it will stay in place and you can solder it up.
(picture 2) The device with all the wires and resistors attached in a similar manner.
(picture 3) Put some shrink tube on the middle if you've got it, otherwise use some electrical tape. Leave the leads long, we'll customize the length on install.
Step 5: Installing the modded receiver
The first picture is of my original build. I laid the conditioner down on the board, and cut the leads to length so they would match up cleanly. Remember to add wires to go to your helicopters control board. I find it easiest to just twist both together and put them through the hole, then solder both.
You need to be gentle with the top of your helicopter when you pull it off to get to the board, there are some very tiny wires there. Luckily none of mine have broken any time I've done this. You'll need to swing the main board aside and desolder the IR phototransistor. If you have desoldering tools this should be easy, otherwise, clip it off, take some small pliers, and grab the remaining pin bits one at a time, heating the solder holding them and extracting them.
(middle picture) After that you'll want to feed the wires from your receiver through the hole that the IR phototransistor used to pass through, and like the receiver, twist and thread the stripped ends through the hole. Be careful not to over solder onto the adjacent pins.
The next picture shows it all assembled. Put it all back together with the radio receiver hanging out of the back, then fold it over so it sits under the belly of the helicopter. I taped mine into place, but maybe a zip tie would be better.
As with any hack, this might take some tweaking and retrying depending on what you have. I'll be watching and answering questions on this over the next few weeks. If you liked it please vote for me in the Hardware Hacking Contest.