Radio Remote Control Circuit Using DTMF

Posted by Micro System

on Thursday, August 5, 2010

This is a circuit of a remote control unit which makes use of the radio frequency signals to control various electrical appliances. This remote control unit has 4 channels which can be easily extended to 12. This circuit differs from similar circuits in view of its simplicity and a totally different concept of generating the control signals. This is the figure of the circuit;


Here we make use of DTMF (dual-tone multi frequency) signals (used in telephones to dial the digits) as the control codes. The DTMF tones are used for frequency modulation of the carrier. At the receiver unit, these frequency modulated signals are intercepted to obtain DTMF tones at the speaker terminals. This DTMF signal is connected to a DTMF-to-BCD converter whose BCD output is used to switch-on and switch-off various electrical applicances (4 in this case). The remote control transmitter consists of DTMF generator and an FM transmitter circuit. For generating the DTMF frequencies, a dedicated IC UM91214B (which is used as a dialler IC in telephone instruments) is used here. This IC requires 3 volts for its operation. This is provided by a simple zener diode voltage regulator which converts 9 volts into 3 volts for use by this IC. For its time base, it requires a quartz crystal of 3.58 MHz which is easily available from electronic component shops. Pins 1 and 2 are used as chip select and DTMF mode select pins respectively.

When the row and column pins (12 and 15) are shorted to each other, DTMF tones corresponding to digit 1 are output from its pin 7. Similarly, pins 13, 16 and 17 are additionally required to dial digits 2, 4 and 8. Rest of the pins of this IC may be left as they are. The output of IC1 is given to the input of this transmitter circuit which effectively frequency modulates the carrier and transmits it in the air. The carrier frequency is determined by coil L1 and trimmer capacitor VC1 (which may be adjusted for around 100MHz operation). An antenna of 10 to 15 cms (4 to 6 inches) length will be sufficient to provide adequate range. The antenna is also necessary because the transmitter unit has to be housed in a metallic cabinet to protect the frequency drift caused due to stray EM fields. Four key switches (DPST push-to-on spring loaded) are required to transmit the desired DTMF tones.

Simple Ham Radio Circuit Using Filter Audio

Posted by Micro System

on Sunday, July 25, 2010

This is a circuit that simple and easy to build. It uses common parts that can be purchased easily from any electronic shops. At the core of the circuit is a OP AMP where its input is connected to the headphone jack of the receiver. Some of the amplified signal is feedback through potentiometer VR1 to the non inverting input which helps to boosts the gain and hence called positive feedback. Some is passed to the inverting input through a LC circuit. This cancels the gain of the amplifier except at the resonant frequency - high impedance at resonance state. The amount of positive feedback is adjusted using the potentiometer VR1. This is the figure of the circuit;

L choke should be in the region of 5H but this is not critical. Try using the winding of an audio transformer to make this choke. The output of this filter is connected to the earphones. On Switch SW and on the radio. Vary the VR1 until it goes into oscillation and then turn it back a bit. Tune across the code station and you will hear a sharp peak response effectively cutting off other noise and interference. It is sharp at the peak at 20dB at the audio frequency of 800kHz - 900kHz.