Welcome to this page on how to build a single channel global DTMF remote control using a MT8870 Chip.
1). Why I designed the following DTMF remote controls is down to the amount of DTMF decoders on You Tube that don't have any form of switching a load, Most of them use a single MT8870 DTMF chip but just turn on LEDs this simple design will turn on a relay to power high power devises if you are using a Cell phone then it can be operated from any where in the world.
2). I have designed this project with a voltage Regulator so you can connect 12 volts to the circuit with out harm the design is in 3 stages the Regulator also DTMF Decoder and the switching of the load.
3). Now if you are going to use your devise with out the regulator then you must not exceed 5 volts or you will destroy your circuit.
4). The regulator is there so it allows you to run your devise from 12 volts the regulator will drop the 12 volts to 5 volts witch is a safe level for your MT8870 Chip.
5). I have added a simple Diode at the front of the regulator for added protection from reveres polarity the 2 caps are 100uF 25 volts when the devise is switched on the RED LED will light.
6). Now most of the parts you can get from any electronic shop or Store you can get the MT8870 from e-bay here. The Crystal is a 3.579545MHz Quartz Oscillator Crystal click here. The Transistor is a BC 639 order it from here. You can get the 50 Milliampers fuse from Ebay Click here.
Please note it would be a good idea if you also added the fuse in to the Positive + side of your battery back up power to act as a over current circuit breaker.
7). My designs below are simple to follow I have only designed it as a single channel DTMF remote control you can experiment with your own devise as it is a basic design it is very simple to switch on or off from any where in the world.
8). When you have finished your own devise check every thing is connected up right now with a Cell phone or Computer press * the relay should switch on and activate your load to turn it off press # keep with them two buttons.
Also note that more pins on the 8870 will go high or low with the different keys pressed. so stick to * & # that will work every time as a simple on/off switch using DTMF tones.
9). Now if you are wanting to run the devise from your computer down load this app DTMF Program it's free Click here.
10). Now as the devise is very basic in design more numbers will turn it on or off but if you stick to using * to turn it on then # to turn it off again it makes a very simple global remote control and you will all ways know if it is on or off.
11). Now I have added a resister and LED to Pin 15 this is helpful when the DTMF decoder receives a valid tone it flashed the led indicating it is receiving the tones ok it also helps in fault finding it lets you know the Decoder is working fine it is called the Valid Point or VP for short.
In the design above the Relay needs to be a 12 volt version.
Now in the Schematic below I have added a Safety Fuse as the devise draws only 30 Milliampers when relay is off, Now when the relay is turned on it draws a current of 40 Milliampers so the fuse can be 50 Milliampers.
In the Schematic below I have designed a very basic DTMF Decoder with Relay.
In the design below I have left out the voltage Regulator so you must not exceed 5 volts in put see Schematic below.
Now in the Schematic Diagram below I have added a LED in with the Valid Point so as soon as the circuit detects a valid DTMF tone the LED will turn on and the buzzer will sound at the same time only for a few seconds.
Now in the Schematic below I have used a cell phone to send back a notification when the relay switched on.
You can go direct to the page for the schematic above this has more information Click here.
Now in the design below I have designed the Schematic to power the Global DTMF Decoder from the mains with a Battery back up in case the mains power got cut out it will automatically switch to battery power.
Now as you can see I am using a Unregulated power supply so had to build the Regulator into this design as the main DTMF decoder did not have it.
Now in the Schematic below I have used a Unregulated power supply but in this schematic I put in the Regulator into the design so I don't need to build the regulator into my emergency back up power supply.
This can be built into any DTMF devise if using a MT8870 or similar as long as it has the 8870 written on the chip.
In the schematic below I have designed two types of switching circuit the designs will switch the relay on for a few seconds then off again.
In the design above if you want to switch on a relay and have it stay on then connect the Connection from the 2.2k Ohms resister to pin 11 this will latch the relay on .
12). Now to start with I will cover using the DTMF with a Computer program this is all so helpful if you ever need to test the devise to know if it is working fine.
13). Now with your finished devise connect a cable to your computers sound card and the end of the cable to the DTMF decoder input. When you have done that power up your devise the red LED should switch on now go to your Desk top look for this Icon below.
Click on it to start the DTMF program now you should have this on your computer below.
Now Click on the * with your mouse pointer see photo below this should turn the relay on.
Now Click on # this will now turn the relay off again see photo below.
14). Now if you connect a Cell phone to the DTMF Decoder all you need to do is make sure Automatic Answer is turned on the Phone the phone I used when I designed this project was a Nokia 1208 Mobile phone.
15). Now all you have to do is ring the Mobile phone your DTMF devise is connected to and you will hear the dial tones about 4 or so when the dial tones stop the receiving mobile has Auto Answered the incoming call.
16). Now all you have to do to turn on the DTMF Decoders Relay is press * this will switch the relay on from any where in the world.
17). Now if you want to turn it off just ring the phone again and wait for the mobile phone to auto answer the incoming call and when the dial tones stop press # this will now turn the relay off again.
Please note if you have any problems please do e-mail me at Anthony@anthony-dacko.net I am all ways happy to help.