Encyclopedia of radio electronics and electrical engineering / radio reception

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Equally important are the consumer conveniences of radio equipment, in connection with this, interest in various remote control devices does not weaken. Below is a description of a remote control device for a VHF receiver that allows you to switch pre-set reception frequencies and adjust the volume of the sound. The device is suitable for any radios with electronic tuning and electronic volume control.

The proposed device allows both remote (by means of IR radiation) and push-button control of the VHF receiver, described in the article by M. Shikin "VHF broadcast receiver with double frequency conversion"(" Radio ", 2000, No. 11, pp. 18-20).

The scheme of the control panel is shown in fig. 1. It is made according to the scheme of a number-pulse generator. When one of the buttons SB1 - SB4 is pressed, infrared emitting diodes VD2 and VD3 transmit bursts of pulses, the repetition period of which depends on the time constant of the R1C1 circuit (about 0,5 s). The pulse filling frequency is about 5 kHz (depending on the time constant of the R3C2 circuit), the duty cycle is close to 2 (meander), the number of pulses in a pack is from 5 to 8 (depending on which button on the remote control is pressed).

When the buttons are not pressed, the supply voltage is applied to the inputs of the DD1.2 element through the resistor R2, so the output of the element will be low, blocking the operation of the clock generator assembled on the elements DD1.3 and DD1.4. In this case, the output of the element DD1.4 will be high. Therefore, the transistor VT1 is closed and no current flows through the studying diodes. The current drawn from the power supply in this mode does not exceed a few hundred microamperes.

When you press a button, a high level at the output of element DD1.2 leads to the opening of the diode VD1 and the rapid discharge of capacitor C1. Element DD1.1 inverts the signal, and a low level appears at the input R of the counter DD2 (pin 15), allowing the counting of pulses coming from the output of the clock generator (element DD1.4) to the input CN (pin 14) of the counter DD2. At the same time, a high level at the output of the element DD1.2 allows the operation of the clock generator on the elements DD1.3 and DD1.4.

After the generation of pulses, the number of which depends on which button is pressed, a high level appears at the corresponding output of the counter DD2. A low level at the output of the element DD1.2 stops the clock generator. In this case, the transistor VT1 closes and the diodes VD2 and VD3 stop emitting. The duration of the transmitted message is 1 ... 1,6 ms (at a generator frequency of 5 kHz).

After approximately 0,5 s required to charge the capacitor C1 through the resistor R1, a positive voltage drop at the output of the element DD1.1 resets the counter DD2, and if any of the buttons is held down, the generator outputs IR emitting diodes the next series of pulses, the number of which depends on the number of the held button (number of pulses = button number + 4). The SB1 and SB2 buttons are used to control the volume, and the SB3 and SB4 buttons are used to switch programs.

The remote control receives power from a Krona battery or similar, with an EMF of 9 V.

The diagram of an IR receiver with a decoder is shown in fig. 2. An amplifier is assembled on the DA1 chip, which converts current pulses excited in the VD1 photodiode under the influence of IR flashes of the remote control into voltage pulses, the amplitude of which is sufficient to directly control digital microcircuits.

(click to enlarge)

When the first burst of pulses is received from the remote control, the counter DD2 counts the number of pulses received and a high level appears at its corresponding output. Switching programs is possible only when seven or eight pulses are received, since only in this case, after 10 ms (the time constant of the R3C7 circuit) after the first pulse arrives, a positive voltage drop appears at the counting input of the DD4 microcircuit (pin 15), and at the transfer input (pin 5) DD4 - low level.

Upon receipt of a pack with a number of pulses equal to seven, at the moment of switching the counter DD4, its output 10 will be low. Therefore, the state of the counter DD4 decreases by one every 0 5 s with the arrival of the next burst of pulses. If there are eight pulses in the burst, then the state of the counter DD4 is similarly increased by one.

Upon receipt of bursts with the number of pulses five and six, the state of the counter DD4 does not change, since at the moment of action on its counting input a positive polarity pulse at its transfer input is high. From the moment the burst of pulses arrives, the single vibrator on the elements DD1.1 and DD1.2 generates a negative voltage drop of about 1.1 s at the output of the element DD0,3, after which the differentiating circuit R4C5 generates a reset pulse for the counter DD2, and it is ready for counting pulses in a burst following approximately 0,2 s.

The DD6 decoder with the HG1 indicator displays the number of the selected program (0-9) in a seven-segment code. The DD7 decoder selects fixed tuning elements - variable resistors R10-R19, the voltage from which is fed through the emitter follower on the transistor VT2 to the receiver varicaps. The choice of programs from the control panel is carried out with the buttons SB3-SB6 by the method of parallel loading of the binary code into the counter DD4. Channels 8 and 9 are reserved and can only be activated from the remote control.

Volume control is carried out by changing the channel resistance of the field effect transistor VT1. When you close the latter, the volume increases, and when you open it, it decreases. To the receiver mentioned above (see its diagram), the drain of the transistor is connected to the right (according to the diagram) output of the resistor R13, and R12 is excluded. The channel resistance is controlled by charging and discharging the capacitor C8, connected between the gate and the source of the transistor VT1, through the resistor R5 with electronic keys DD5.1 ​​and DD5.2. The volume can also be adjusted from the control panel using the SB1 and SB2 buttons connected in parallel to the electronic keys.

When mounting an IR receiver, an amplifier on a DA1 chip is placed in a copper or brass shield connected to a common wire. There are no special installation requirements for other elements of the remote control device.

Author: M. Ozolin, Krasny Yar village, Tomsk region.

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