Two options to mute the microphone. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Telephony

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There are situations when during a telephone conversation it is necessary to turn off the microphone of the telephone set for a short time. You can supplement the device with such a function by assembling one of the proposed nodes.

To avoid some sort of annoyance, it is desirable to be able to mute and unmute the handset microphone as needed. Those phones that already have this feature often have some drawbacks in its implementation. Here are some of them:

If there is no desire or opportunity to install a miniature latching button with good quality contacts, then the microphone mute assembly can be performed as shown in Fig. 1.

This is a simple time relay with a silence delay of about one minute. To install it, you do not need to make major changes to the design of the telephone. The device is designed to work together with an electret microphone, which is supplied with a supply voltage of at least 1,7 V.

When the SB1 button is pressed briefly, the capacitor C2 is charged to a voltage equal to the supply voltage of the microphone installed in the tube. The voltage at the gate of the transistor VT1 relative to its source becomes greater than the threshold, the transistor opens and shunts the microphone VM1 in alternating current. The microphone is "muted". Capacitor C2 is gradually discharged through diodes VD1, VD2, which act as a high-resistance resistor with a resistance of 70 ... 200 MΩ at room temperature. When the gate-source voltage VT1 becomes close to the threshold, the field-effect transistor will start to close, the resistance of its channel will increase rapidly and the signal from the microphone will resume.

Resistors R1 and R2 are designed to eliminate the click when the microphone is muted. Capacitor C1 prevents a possible change in the operating modes of the amplifying cascades of the conversational node, which allows you to freely mount this node in most phones. If you need to turn on the microphone before the capacitor C2 has time to discharge, you need to briefly press the button SB2.

On fig. 2 shows a diagram of a more advanced node for turning off the microphone. Here, only one push-button is required for control. The first time the contacts of the SB1 button are closed, the microphone is turned off, the second time it turns on, and the third time it turns off again. The microphone can be in any of these states for an arbitrarily long time. The fact that the microphone is disabled is indicated by the HL1 LED.

(click to enlarge)

Connecting input D trigger DD1.1 with an inverted output made it possible to implement a frequency divider by two. Capacitor C1 eliminates the consequences of the "bounce" of the button contacts. Elements C2, R2 are designed to set the trigger to the zero state after picking up the handset, which will mean "microphone on".

We will show how to mount this node in a telephone set using the example of a telephone set described in [1]. The negative output of the power supply of the node is connected to the negative output of the BM1 microphone - the common wire of the conversational node. The positive power output must be connected to the positive output of the capacitor C15. Similarly, this microphone mute node can be connected to the microphone amplifier described in [2].

Both nodes can be easily integrated into simple Asian-made telephone sets with a push-button dialer and VEF brand devices: Rita-201, Gunta-202, lnta-203.

Resistors can be used any small, for example, C1-4. All capacitors are ceramic K10-17 or film K73-17. Field-effect transistors KP501V can be replaced with any of this series, as well as with KR1014KT1, K1014KT1. LED HL1 (Fig. 2) red glow has a high light output. Since it operates at low current, it is advisable to use a high-brightness LED instead of the one indicated on the diagram, for example, L1503SRD, L1503SGC, L1513IT. It is more convenient to use small buttons, for example, TD-06XEX SMD. You can adapt a pair of "rubber" buttons or connect one or two of the free ones available in the telephone. Diodes (see Fig. 1) can be replaced with any of the KD105, KD208, KD209 series.

The node assembled according to the scheme in fig. 1, needs to be improved. By selecting the number and type of diodes connected in parallel, the desired exposure time is set. This node can be mounted both in the body of the handset of the wired telephone, and in the body of the telephone itself. The first option will be more convenient. Two holes are drilled in the tube body for miniature buttons. Buttons from the calculator "Electronics MK-52" will look good.

The node according to the diagram in fig. 2 can be mounted on a printed circuit board made of one-sided fiberglass, a sketch of which is shown in fig. 3.

Literature

1. Butov A. Conversation node for a telephone set. - Radio, 2002, No. 3, p. 29.
2. Balakhnichev I. N., Drik A. V. Practical telephony. - M.: DMK, 2000, p. 86-88.

Author: A.Butov, village of Kurba, Yaroslavl region

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