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Remote Control using 300 Mhz RF modules

This project will show you how to build a circuit to control things remotely using the the Basic Stamp II and Ming RF modules. I am making a remote engine starter for my truck, but anything you can turn on and of with a Relay could be used in this project. Without any type of anttena the Modules can get 50 to 100 foot range, which is excellent considering there only 10 bucks each. However with a antenna much better range can be achieved. I got a range of about 200- 300 feet with a 1/4 wave antenna(an antenna that is 1/4 the lenght of the wavelenght of the signal) or 9.36". The data sheet also says a half or full wave antenna can be used for better result, but a lenghy antenna is sometimes hard to conseal. So if you want a full wave antenna just times 9.36" by 4 and that is the lenght of wire you need for a full wave antenna. The frequency of the modules can be also adjusted if needed. There is a variable capacitor on each board to do so. Tuning is only recommended when greater ditances are required or in the event that a borad is mis-adjusted. Refure to the ming data sheet for instructions on how to adjust the frequency.



Transmitter

Here is a picture of the Ming 300 Mhz Transmitter Module.


The transmitter circuit consist of a Ming TX-99 module, a Holtek HT-12E encoder, few diodes, a resistor, and a switch. By using the switches attached to the data pins(D0 to D3) of the encoder, you can send data in binary format to the receiver. Each transmission is 4 bits or data or 16 different combinations and a 8 bit address. The address is set by applying lows(0) A0 to A7 or just leave them open is a high(1). The address on the encoder and the decoder must be equal or the decoder will not except the data the transmitter has sent it. The purpose of using the address pins, is so you can select different addresses for up to 256 different receivers. So you could have 1 transmitter sending data to 10 receivers, the first receiver could have a address of 00000000 and the second of 00000001 and so on... So one transmission is 12 bits long. Data is only transmitted when the push button switch is pressed. The purpose of the diodes are so when idle there is no ground, so there is no power disspation.

The schematic for the transmitter.

Circuit operation

When the pushbutton is pushed TE(pin 14) goes low on the HT-12E encoder the state of A0 though A7 are sent serial out of Dout(pin 17) to the Ming transmitter, then the state of D0 to D3 are sent out of Dout to the transmitter. One transmittion is 12 bits long and consists of 8 bit address(A0 to A7) and 4 bits(D0 to D3) of data. The data will correspond to the state of D0 to D3 on the encoder chip set by the switches.




Receiver

Here is a picture of the Ming 300 Mhz Reciever Module.


The reciever circuit consist of a Ming RE-99 module and a Holtek Ht-12D decoder and a few resistors. The Ming reciever, recievers the serial signal transmitted by the transmitter. It is then send serial out of the data pin on the reciever module and goes into the Data in(pin 14) on the decoder chip The decoder compares the eight bit code appering on inputs A0 though A7 with the first 8 bits send to the module. If the 8 bits are the same, a valid transmittion has occured, and the next four bits of data are latched to D0 though D3. To indicate valid transmittion VT(pin 17) will go high. So the use of this pin is to tell the micro weather to record the data at D0 to D3 as valid or not. Transmission rate of 1200 bps can be acheived using these Ming RF modules.

Circuit operation

When a transmission is made the decoder compares the eight bit code appering on inputs A0 though A7 with the first eight bits send to the module. If they are equal VT(pin 17) is then pulled high and the next 4 bits of data sent to the decoders data pin are latched to D0 to D3. The Stamp will monitor the 5 inputs, and if they are all eqaul to 1 then P5 will go high(1). When P5 goes high it then activates the transistor which in turn activates the coil of the relay, turning on the internal switch in the relay.




Downloads

0 Kb BASIC Stamp II Code
0 KbSchemtic for Ming reciever circuit
0 KbSchemtic for Ming transmitter circuit


Parts Needed

Component Manufacture Price Seller Part Number
Basic Stamp II Parallax Inc. $43 US Parallax Inc. BS2
Data Encoder Holtek $1.7 US Digikey HT-12E
Data Decoder Holtek $1.7 US Digikey HT-12D
Ming 300Mhz Transmitter Ming Sicrosystems $10 US Digikey TX-99
Ming 300Mhz Reciever Ming Microsystems $11.68 US Digikey RE-99




Conclusion

There are many different applications for this project and can be expanded to control up to 16 different relays. My application for this project was a engine starter for my truck, but that is only one out of 16 things it could turn on in your vehical remotely. You could also have remote locks, trunk, and much more.. Another consideration for this project is use it for remotly controlling a robot. The 4 switches on the transmitter could be used to set the direction you want you robot to go in. An H-bridge could be connected to the outputs of the decoder to control the direction of the motors.




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Last update: March 17/2001
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